Oxygen luminescence from UV-excited water (H2O and D2O) ices

J. Phys. Chem. 1993,97, 10539-10553

10539

02 Luminescence from UV-Excited H2O and D2O Ices A. J. Matich, M. G. Bakker,? D. Lennon, and T. I. Quickenden’ Department of Chemistry, The University of Western Australia, Nedlands, W.A.,6009, Australia

C. G. Freeman Department of Chemistry, University of Canterbury, Christchurch, 1, New Zealand Received: October 19, 1992; In Final Form: July 16, 1993”

The luminescences from H20 and D20 ices excited with 260-nm light a t 77 K have been resolved into a long-lived (71/2 = 1.3 s) component and a short-lived component. The latter contains a number of bands having maxima a t 350.5, 369.5, 386.6, and 416.5 nm. The band positions and separations are consistent with those for the Herzberg I A’2: X 3 2 ; system or the Herzberg I11 C A , , -,X 3 2 ; system of excited 0 2 . The excited 0 2 is formed chemically and not by optical excitation of ground-state 0 2 produced in the ice lattice. The possible mechanisms for the formation of the excited 0 2 have been reduced to two alternatives on energetic grounds. In the first mechanism, mobile 0 atoms react with accumulated H202 molecules trapped in the ice lattice. In the second mechanism, mobile 0 atoms react with accumulated 0 atoms trapped in the ice lattice. The latter mechanism is shown to be the most likely source of the oxygen emission.

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Introduction Ice Luminescence. Previous communicationslv2 from this laboratory have reported broad, UV-visible emission bands from pure HzO ice irradiated with UV light. The luminescence is brightest at temperatures below 120 K but is still detectableZa t 260 K. A brief study1 of the excitation and luminescence spectra a t 88 K showed excitation bands around 220 and 260 nm and emission peaks around 340 and 420 nm. It was tentatively suggested that thematrixshifted AzZ+-SII transitionof excited OH radicals in either substitutional or interstitial sites, might be responsible for the respective short- and long-wavelength features. More frequent reports of UV-excited luminescence from strongly alkaline or acidic HzO ices are available in the literature.’-5 However, no consensus has been reached as to the identity of the emitting species in these systems and suggestions include excited’ OH, excited4OH-, and matrix-broadened Balmer emission from5 excited, hydrated H atoms. Luminescence emitted by pure ice excited with high-energy electron beams is well e ~ t a b l i s h e dand ~ , ~has been assigned7 to the CIB1 AIB1excimer transition of the water molecule. Although this assignment to a transition between upper levels of the water molecule is strongly supported in the case of high-energy electron excitation, it is not appropriate for excitation by UV light, in the 200-300-nm (6.2-4.1 eV) range, as a threshold energy7,*of ca. 11 eV is required for ground-state H20 to be raised to the CIBI state. The present study separates the UV excited ice luminescence into long-lived and short-lived components and uses the spectral series of emission bands observed in the latter in order to identify the emitting species. By combining this spectral information with the effect of isotopic substitution of D for H on the peak positions and with the help of energetic considerations, it is shown that the band system in the short-lived luminescence arises from either the Herzberg I A’Z: X’Z;or the Herzberg I11 P A u X’Z;series of emissions from excited 0 2 . Herzberg I and 111 Band Systems of 0 2 . In 1931 Herzberg observed a series of absorption bands in molecular oxygen between 240 and 210 nm which he assigned to the A’2: - X’2; band

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t Present address: Department of Chemistry,The University of Alabama, Tuscaloosa, AL 35478-0336. To whom correspondence should be addressed. *Abstract published in Aduance ACS Abstracts. September 1, 1993.

0022-3654/93/2097-10539$04.00/0

~ y s t e m The . ~ bands in this system arise from symmetry forbidden transitions and are henceof very low intensity. Additional, longer wavelength bands belonging to this series were subsequently observed by Hermanlo and Chalonge and Vassey.Il In 1941 DufayIz proposed that some spectral features of the UV night sky emission were due to this band system. However, it was not until 1952 that Herzbergl’ obtained a detailed analysis of oxygen absorption fine structure, a t a pressure of 1 atm, using multiple reflection path lengths of up to 800 m and an exposure time of 24 h. The weaker bands required higher oxygen pressures of up to 2-3 atm, but higher pressures were not feasible because of band broadening. Herzberg measured ten absorption bands which he tentatively identified as the Y’ = 0 Y” = 0 to Y’ = 10 Y” = 0 transitions. A Deslandres array of predicted band origins for this system was then prepared from the molecular constants so obtained. Observations reported by ChamberlainI4confirmed that part of the UV night sky spectrum is due to this system, and in 1954 Broida and Gaydonls produced emission bands belonging to this system during laboratory afterglow experiments. However, these workers also detected hitherto unknown bands which showed that Herzberg’s original assignment of v’ in 1952 was incorrect and should be increased by one unit. Subsequently, Borrell et a1.I6 from the same laboratory reexamined the absorption spectrum using longer absorption paths and higher resolution. In this work the 0-0 absorption band was reported for the first time and the positions of the bands these workers detected differed from Herzberg’s values by no more than a few wavenumbers. The Herzberg 111 QA,, X’Z,system of 02 was first clearly identified by Herzberg17 in 1953. These extremely weak lines were observed on the same photographic plates used by Herzberg” to analyze the Herzberg I system in 1952. However, only fragmentary data were obtained, and thus a full analysis of this system was not then possible. Lines from the emission spectrum of this system were later detected by Slanger,18 who studied emissions resulting from the recombination of 0 atoms. Then in 1986 Coquart and Ramsaylg used longer absorption path lengths and a higher resolving power than previously used by Herzbergl’ to detect a further 10 absorption bands in this system and thus effected a detailed rotational analysis. These workers also analyzed the diffuse bands which were detected at higher 0 2 pressures and assigned them to the ( 0 2 ) ~complex in which one of the 0 2 molecules is excited to the QA,, state.

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0 1993 American Chemical Society

Matich et al.

10540 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993

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This paper assigns a small series of ice luminescence bands to one or another of the above Herzberg I and I11 systems. At first sight this assignment may seem surprising because of the low intensities of these band series in the gas phase. The gas-phase emission intensities observed by Broida and Gaydon's for the Herzberg I series were very weak and required photographic exposure times of up to 7 h for their detection. However, the introduction20-23of an inert gas matrix around the 0 2 molecules appears to have substantial effects on the Franck-Condon factors for the Herzberg emissions and increases the 0 2 emission intensity. In addition, a study by Richards and JohnsonZoof the effect of temperature and isotopic substitution on the matrix 0 2 emission spectrum and a study by Goodman and Brus21of the fine structure in the correspondingexcitation spectrum show that the Herzberg I11 (QA,,-X32i) series of bands dominates the matrix 0 2 emission rather than the Herzberg I (A3Zz-X32,) series which dominates in the gas phase. In addition, these workers also observed another emission from the matrix 0 2 which was attributed to the c'Z;-alAg series. Similar results and assignments have been reported by Okada et a1.22and Brooks.23

Experimental Section The equipment used is outlined in Figure 1 and comprises a high vacuum chamber maintained at a pressure of 98.5%. After the copper tray containing the ice sample had been transferred rapidly from the evacuated preparation chamber to the evacuated irradiation chamber, the ice sample was maintained at the required temperature by a pair of electrical cartridge heaters. These heaters moderated the cooling effect of the liquid nitrogen, and their output was controlled by a temperature controller which utilized an AD 595 chip (uiz.,a thermocouple amplifier with cold junction compensation).

Results and Discussion Three-Dimensional Spectral Presentation. Figure 3 shows a three-dimensionalassembly of the excitation and emission spectra

UV-Excited H20 and D20 Ices

The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10541

TABLE I: Absorption Maxima of Species Produced Photolytically and/or Radiolytically in Solid, Liquid, and Gaseous Media and Which Show Absorption Maxima at Wavelengths between 200 and 300 nm XIIl*,/nm 230 230 240 245 220-245

Figure 3. Three-dimensional presentation of the excitation and emission spectra of UV irradiated H20 ice at 88 K. These spectra are the means of three sets of data, each obtained after 7 h of continuousprior irradiation with 260-nm light. All bandpasses were 5 nm.

255 260

system

assignment

ref

irradiated water irradiated ice at 77 K irradiated water NaCl, KCI, and KBr crystals grown in an 0 2 atmosphere solid O3and 0 3 dissolved in 0 2 at 77 K gas-phase ozone ozone in liquid water

HO2 HO2

Bielski et a1.O Taub and Eibenb Bielski et a1.O Rolfec and Rolfe et a1.d

flash-photolyzed water flash-photolyzed ice at 263 K irradiated ice at 77 K irradiated ice at 77 K

230 245

0202-

0 3 0 3 0 3

OHk OHk

Vaida et a1.e Vaida et al.c Alder and Hill/ Taubs Boyle et al.! Ghormley and Hochanadel' Taub and Eibenb Ghormley and Steward

of the UV-excited H20 ice. The dominant features are a strong OHk 280 f 5 OHk excitation band around 235 nm which gives rise to two lumi280 nescence bands around 340 and 420 nm. At longer excitation wavelengths the 340-nm luminescence band becomes less sigReference 28. Reference 29. Reference 30. Reference 3 1. Refnificant compared with the 420-nm band. These broad features erence 32. /Reference 33. g Reference 34. Reference 35. ' Reference 36. J Reference 37. The speculative nature of these particular assigncorrespond to the conclusions made in an earlier and more limited ments has been discussed in Appendix A. study from this laboratory' in which two two-dimensional cross sections of Figure 3 were obtained. However, the present threeTABLE II: Tentative Assignments of the Excitation Peaks dimensionalpresentation provides more spectral information than from UV-Excited Ice to Absorption by Species Which Can Be was then available. Produced Radiolytically or Photolytically in Aqueous Systems Excitation Features. It is clear that there are a number of absorption peaks of species excitation features which are observed for all emission waveproduced photolytically or lengths. These are the intense excitation feature at ca. 235 nm positions of radiolytically in ice or water excitation Deaks from and the other less intense excitation peaks centered around 245, UV-irradiaGd ice, X/nm species absorption peak/nm 260, 275, and 285 nm. As excitation peaks occur at the same 230 235 HO2 (in water) wavelengths as absorption peaks, it is useful to compare the 230 H 0 2 (in ice) positions of the excitation peaks with known absorption wave240 245 0 2 - (in water) lengths in ice. 245 0 2 - (in NaCl, KBr However, it should be noted that fluorescence excitation and KCl crystals) spectroscopy is a much more sensitive technique than absorption 255 260 0 3 (gas phase) 260 0 3 (in water) spectroscopy. Therefore, although the excitation peaks of low220-245 0 3 (solid and in 0 2 concentration species may well be observable, the concentrations at 77 K) of these species may be too low for absorption spectra to bedirectly 280 270 OH (in ice at 77 K) measured. It is thus possible that some of the excitation peaks 280 OH (in ice at 77 K) 280 in Figure 3 may arise from minority species which have never been identified in ice using absorption spectroscopy. Nevertheless, Either of the excitation peaks occurring at 275 and 285 nm in a comparison between the two types of measurement is a useful Figure 3 might correspond to the well e~tablished29.3~ 280-nm ice starting point for species identification. Table I shows the absorption peak, which Taub and Eiben29 assign to the x2II (Y" wavelengths at which H2O fragment species are known to produce = 0) A2Z+ (Y' = 0) transition of OH, broadened and blue absorption maxima in water and ice. As the literature on which shifted from its gas-phase wavelength of 306.4 nm. Such a Table I is based contains a mixture of speculative and wellwavelength shift is quite reasonable, as the absorption spectrum established assignments, an assessment of their reliability is of H20 ice is blue shifted41by ca. 20 nm from that of H20 vapor. contained in Appendix A. An examination of Table I suggests that the 235-nm ice The excitation peak at ca. 260 nm in Figure 3 has not been excitation peak in Figure 3 could be the absorption peak at 230 observed as an absorption feature in either photolyzed or nm shifted by some 5 nm. This absorption peak, which has been radiolyzed water or ice but might correspond to absorption by observed in both ice and water, was assigned to the HO2 radical ozone which in the gas phase has a peak32 at ca. 254 nm. The by Taub and Eiben29and Bielski et a1.28 The excitation peak at value of A, for ozone is solvent dependent, and solid 0 3 or O3 245 nm may correspond to the 245-nm absorption observed by dissolved in 0 2 is blue ~ h i f t e d ~to~220-250 ~ ~ 2 nm, while in water Ghormley and Hochanade13'jin flash-photolyzedice at 263 K. As at room t e m p e r a t ~ r eit~is~ red ~ ~ shifted ~ to ca. 260 nm. Table pointed out in Appendix A, although there is no doubt that this I1 summarizes the above tentative assignments of the excitation absorption peak is real, there is doubt as to whether it arises from peaks. the absorption by OH radicals as suggested by these workers. Emission Features. Because higher spectral resolution was It is also possible that the 245-nm excitation peak observed in achieved in the present work, the emission fine structure shown the present work might instead correspond to the absorption peak of 0 2 - which is centered at 240 nm in irradiated ~ a t e r ~ and ~ 9 3 ~ in Figure 3 is more detailed than observed in our earlier study.' The persistence of a number of these small features across a at CQ. 245 nm39 in crystals of NaC1, KCl, and KBr (at 77 K) number of excitation wavelengths suggests that they are indeed which had been grown in an oxygen a t m o ~ p h e r e . ~ ~The - ~ l02real, and their regularity suggests that they may form an absorption peak has not been reported in irradiated or photolyzed spectral series. To sharpen up the emission peaks ices, but its ESR signal has been detected by Knowles et ~ 1in . ~ identifiable ~ prior to spectral analysis, the temperature was reduced from 88 the products of enzymic and chemical reactions in rapidly frozen aqueous systems at around 210 K. to 77 K and luminescence spectra were then obtained by multiple ____

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Matich et al.

10542 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 1.0 h

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Wavelength / n m Figure 4. Luminescence spectra of H20 and D20 ices irradiated with 260-nm light at 77 K. Ice samples were irradiated for 7 h before the spectra were measured. Each spectrum is the average of 18 replicate spectra, three from each of six separate ice samples. Excitation and emission bandpasses were 10 and 5 nm, respectively. scanning at a single excitation wavelength43 of 260 nm. The emission peak quality was improved by increasing the excitation bandpass from 5 to 10 nm, resulting in increased emission intensity and thus a better signal-to-noise ratio. It was found that narrowing the bandpass of the detection monochromator to 2 nm did not significantly increase the resolution of the observed emission bands but substantially decreased the luminescence intensity. Consequently, the emission bandpass was maintained at 5 nm. The H20 and D 2 0 luminescence spectra in Figure 4 were obtained using the above procedures and each is the mean of 18 replicate emission spectra, three being from each of six separate ice samples. It is clear that a series of four luminescence bands at ca. 351,369,393, and 417 nm appear from both HzO and D2O ices. Time-Resolved Subdivision of the Emission into Two Components. A brief examination of the decay kinetics of the four emission bands in Figure 4 revealed that the 351-, 369-, and 393-nm bands had half-lives which were shorter than could be measured on the present system which had a minimum sampling time of 0.03 s. In contrast, the 417-nm band decayed with a half life of ca. 1 s which was well within the time-resolving capability of the system. The three short-lived bands correspond to the broad short-lived 340-nm emission band observed in the lower resolution study by Litjens et al.l.2 The long-lived 417-nm band corresponds to the long-lived 420-nm emission also observed in that study.’J The existence of two time domains for the luminescence decay indicates that at least two emitting species are responsible for the emissions. Furthermore, this observation provides the means of deconvoluting the existing spectrum into two separate spectra which can then be used to assist in the identification of the species responsible for the emissions. Figure 5 shows the separated, long-lived and short-lived luminescence spectra of H20 and DzO ices. For each of the two ices, the long-lived and short-lived spectra have been deconvoluted from each of the measured spectra presented in Figure 4. In each case, the spectrum of the long-lived emission was obtained by multiplying its fractional intensity, at each wavelength, by the intensity of the luminescence spectrum in Figure 4. A Gaussian curve was fitted to the region around the peak of the long-lived emission band obtained by this deconvolution method. The position of the peak was thus found to be 427.1 f 0.4 nm for H20 ice and 425.6 f 0.3 nm for D 2 0 ice. A Student t test performed on the difference between these peak positions gave t = 2.230 which is slightly larger than the 95% confidence level value o f t = 2.228. Therefore this peak is subject to a small but detectable isotopic shift. A more lengthy deconvolution procedure was necessary to find the precise positions of the short-lived emission bands because of the effects of the overlap of adjacent bands. At the very short

Wavelength

/

nm

Figure 5. Separated long-lived and short-lived luminescence bands from H20 and D20 ices at 77 K. Spectra (a) and (c) are the short-lived

components of the luminescence from H20 and D20 ices, respectively, and spectra (b) and (d) are the respective long-lived components. For each of the two ices, the respective spectrum in Figure 4 was multiplied by the fractionalintensity of the long-lived component at each wavelength. This product gave the spectra ((b) and (d)) of the long-lived emission band. Each spectrumwas then subtracted from the relevant total emission spectrum, and the differences so obtained yielded the spectra ((a) and (c)) of the short-lived emission bands.

Wavelength / n m Figure 6. Sums of four Gaussian curves fitted to the short-lived luminescence bands from D20 and H20 ices at 77 K.

wavelengths in spectra (a) and (c) in Figure 5, there appears to be the tail of a large, broad band which must also be accounted for in the fitting procedure as it may well have a significant effect on the position of the peak at ca. 350.5 nm. Therefore, as a first step, a Gaussian curve was fitted to the steep, short-wavelength tail, and this was then subtracted from the total spectrum. Although there are only three clearly established bands in spectra (a) and (c) in Figure 5 , there is also a small hump at ca. 415 nm in each case. When Gaussian curves were fitted simultaneously to the three most prominent bands and then subtracted from the original spectrum, the small peak became more clearly defined in this region. Therefore, following the subtraction of the steep, shortwavelength tail from each spectrum in Figure 5, four Gaussian curves were fitted to each spectrum of the short-lived emissions. Figure 6 shows the deconvoluted spectra of the short-lived emissions which resulted, along with the sum of the four Gaussian curves which were fitted simultaneously to each spectrum. The deconvoluted peak positions and their associated errors (as determined by the fitting program) are listed in Table 111.

UV-Excited H20 and D 2 0 Ices


TABLE 111: Peak Positions of the Four Spectral Bands Obtained by Separation of the Short-Lived Luminescences into Four Gaussian Fits' t Values (for the differences % confidence level peak positions/nm between the HzO (at which the band H,O Ice D,O Ice and D20 ices) wsitions differ) 350.4f 0.14 369.6f 0.10 386.1 0.21 417.0f 0.30

*

a

350.6f 0.15 369.5f 0.11 387.0f 0.13 416.1 f 0.22

0.64 0.62 1.66 0.948

46 45 87 63

The errors are 50% confidence intervals.

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TABLE I V Comparison between the Band-Head Wavelengths for Several Bands in the AZZ+ x2II System of Gaseous OH and OD emission transition band positions/nm (v' v") OH OD (h(0H) - h(OD))/nm

-

0-0

306.4' 342.8O 384.3"

0-1 0+2

308.4b 333.OC 359.7d

-2.0 9.8 24.6

a Reference 45.* Reference 46. Reference 47. This value was estimated using AVOH/AVOD= 4 2 .

Examination of the spectra in Figure 6 reveals that apart from the four bands to which Gaussian curves have been fitted, there are signs of two more bands at ca. 326 and 338 nm. These two bands were only revealed after the steep tail of the intense emission which extends below 200 nm was subtracted from each shortlived spectrum in Figure 5 . Their positions could not be determined with any reasonable degree of accuracy and so no attempt was made to fit Gaussian curves to them. The t values given for the differences between the H2O and D2O emission bands in Table 111are significant only at confidence levels which are all less than the 95% level, which corresponds to t = 2.228. Therefore, it can be concluded that this experiment has found no evidence of an isotopic shift in peak position for any of the short-lived emission bands, although a shift is observed for the long-lived emission band.Il5 Assignment of the Short-Lived Emission Bands. It has been suggested36~~ that the species formed during the UV photolysis of ice at wavelengths between 200 and 300 nm include OH,HOz, H, Hz, HzO, H 3 0 ,H202,0,and 0 2 . This list could be augmented by adding the visible-absorbing trapped electron, e-"is, and the oxygen-containing species H20+, 03, 0 2 + , and 0 2 - . The arguments for and against each of these species being the emitter of the luminescence are now considered. Unfortunately, kinetic information could not be used to assist with species identification as the time-resolving power of the present equipment was not sufficient to measure the lifetimes of our short-lived emissions. However, the effect of isotopic substitution on the positions of the emission bands provided valuable assistance with species identification. OHas the EmittingSpecies. Unperturbed, gaseous, OH emits with a well-known band-head maximum at 306.4 nm45 due to the A%+ (v' = 0) S I I (Y" = 0) transition. Previous work from this laboratory1q2found a broad emission band a t 340 nm from UV-excited ice, and this was tentatively assigned to the 0 1 transition of this series. The red shift was attributed1J to perturbation of the Franck-Condon factors to weaken the probability of the 0 0 transition and favor the 0 1 transition. The above assignment can now be rejected by comparing the results of the substitution of D for H on the ice luminescence (Table 111) with the effect of the same substitution on the above emission band system for gas-phase OH (Table IV). This table shows an isotopic shift of 9.8 nm for the gas-phase 0 1 transition and an even higher value of 24.6 nm for the 0 2 transition. In comparison, the isotopic shifts for the ice luminescence are either nonexistent or less than 1 nm. This effectively rules out the

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proposed assignment of the ice luminescence to these vibrational transitions of the A2Z+ S I I band system. The measured -2.0-nm isotopic shift for the gas-phase 0 0 transition provides less decisive information but also suggests the rejection of that assignment. Another strong argument against assigning the ice emissions to OH bands is that the gas-phase emissions in Table IV have band spacings of around 3300 cm-l, while those for the ice emissions are approximately 1500 cm-1. H02 as the Emitting Species. Although the HO2 radical in ice has a UV absorption peakz9 a t 230 nm and the gas-phase species has a series of infrared absorption bands$* there are no reports of any emission from HOz in the 300-500-nm-wavelength region, whether the H02 is in the gaseous or condensed phases. Further evidence for discounting HO2 as the emitting species is provided by the temperature dependence work of Litjens and Quickenden,2 who found that the broad 340-nm luminescence band shows a marked decrease in intensity above 125 K, whereas it is k n o w r ~ ~ that ~ - ~ trapped l HO2 does not become mobile in the ice until about 145 K. H a s the Emitting Species. The H atom is an unlikely source of the ice emission for a number of reasons. First, it becomes mobile in ice and disappears by reaction50,sl at 50 K which is well below the temperature (ca. 125 K) at which Litjens and Quickenden2observed a decrease in the ice luminescence intensity. Second, when H atom emission has been observeds2 from irradiated ice, it comprises a weak, narrow, Balmer a-lineemission at ca. 650 nm, which is unshifted from the wavelength for the gaseous atom. This suggests that the excited H atoms have either been ejected from the ice lattice or are in grain boundaries or similarly unperturbed environments. Such a weak and narrow emission has no similarity to the broad-band ice luminescences. Third, there are strong energetic grounds which oppose the production of electronically excited H atoms. The minimum energy required to excite the H atom is ca. 12 eVS3which is much greater than the photon energy of 4.8 eV for the 260-nm exciting light used in the present work. Thus, in order to produce both an H atom from the photolyzed ice and to then excite it, a quite improbable number of four or more sequential photon-absorption steps would be required. Therefore, H cannot be the species which produces the short-lived emission bands from ice. Mathers et ~ lhave . ~attempted to overcome some of these difficulties by suggesting that in the case of UV-excited alkaline ice, the luminescence observed might be a wavelength-broadened emission by an exciplex of the hydrogen atom with water, i.e., (H30*)*or [(HzO)~H*] *. However, their own calculationss of the lowest energy excited (quartet) state of this species suggest that it is dissociative and therefore not an emitter. H2 as the Emitting Species. The Hz molecule is also unlikely to be responsible for the luminescence observed from H2O and DzO ices because the emission observed when an electrical discharge is passed through H2 gas consists of a broad continuum which extends from the UV to the infrared and is most intense at around 630 Furthermore, H2 does not display an absorption peak in the 200-300-nm-wavelength region where the excitation peaks are observed in the present work. H20 or HjO as the Emitting Species. These two species are both unlikely to be sources of the short-lived emissions from UVexcited ice. The gas-phase H20 molecule does emit a broadband excimer luminescence from its CIBI AIBI transitionSSat ca. 440 nm, and there is very good evidence that the 385-nm emission from electron irradiated H20 and DzO ices arises from this transition.' However, the 260-nm (4.8 eV) exciting light used in the present work is not energetic enough to produce the excited (CIBl) state of H 2 0 as this process requires threshold energies of 9.7 eV for gas-phase excitationss and ca. 11.4 eV for excitation in ice.' In addition, the observed accumulation of the luminescence intensity from UV-excited ice would require that

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Matich et al.


bands under consideration have spacings of ca. 1500 cm-I. the concentration of the H20* molecule should accumulate in Therefore, O3is unlikely to be the species responsible for the ice the ice lattice. In ice, the CIBIstate of the water molecule decays emission. in a few microsecond^,^^^ and therefore it cannot be the accumulating, emitting species. Similarly, H 3 0 should not The 02+ ion possesses three systems of emission bands known accumulate because calculations have shown5 it to be an unstable as the Hopfield, the first negative, and the second negative species which dissociates to form H20 and H. ~ystems.~OThe Hopfield system occurs between 194 and 236 nm, the bands in the middle of this range being the most intense, H202as the EmittingSpecies. H202 accumulates in irradiated and having spacings70of ca. 1050 cm-I. In contrast, the emission ice, and its yield has been measured by melting the irradiated ice bands from UV excited ice are quite different, lying between ca. sample and analyzing the H202 content of the resulting water at 350 and 420 nm with spacings of ca. 1500 cm-1. Consequently, room tem~erature.~9 However, because of the stability of H202, the Hopfield system is an unlikely source of the ice emission. its concentration would not be expected to show the marked temperature dependence observed by LitjensZfor the broad-band The first negative 0 2 + system extends from ca. 500 to 850 340-nm luminescence, whence it is unlikely to be the emitting nm7I which is well removed from the wavelengths (350-420 nm) species responsible for this emission. Furthermore, unlike the at which the emissions from UV excited ice are being observed. excitation spectrum of UV-excited ice, which has five peaks, the However, the second negative A 2 n ,-XQ, system ranges from absorption spectrumof H202is without structure. Itsabsorbance 206 to 610 nm and has band spacings which are comparable with increases continuously from ca. 320 nm to the measurement limit those for the emissions from UV-excited ice. Therefore, this of ca. 190 nm, and this unstructured spectrum is observed in the band system is a possible candidate for the UV-excited ice solid,56 liquid,s7 and gase0us5~35~ states. It is w e l l - k n o ~ that n ~ ~ ~ ~ ~luminescence. light absorption by H202 produces a dissociative state which leads The low-temperature fluorescence spectrum of 0 2 - in alkalito the formation of two or more products. It can be concluded metal halide^^^,^' has generally been assigned to transitions in the that the H202 molecule does not itself emit visible or UV A211, Xzn, series. These bands are situated around 300-400 luminescence, although its photolysis product, OH, can be nm with spacings of ca. 1140 cm-1 which is significantly less than formed57.59~60in excited states to give its well-known band theca. 1450 cm-I obtained in the present work, and therefore this emissions. band system is an unlikely contender for the source of the emissions e-,q as the Emitting Species. The visible absorbing electron, from UV excited ice. This conclusion can be confirmed on two e;is, is a well-established radiation product in ice29s6143but its additional grounds. The first of these arises from the energetics characteristic broad absorption band is around 650 nm2996343 of 0 2 - formation. Electrons for the formation of this species whence it is unlikely that e;i, could fluoresce at shorter might conceivably be obtained from 02, which has an ionization wavelengths. In addition, the observed luminescence bands from energy of 12.06 eV,'j60r from H20, which has an ionization energy UV irradiated ice are much narrower (fwhm of ca. 25 nm) than of 12.6 eV.66 However, both of these values are considerably the width of the absorption band of the trapped electron in ice greater than the energy of a photon of exciting light (4.8 eV). The (fwhm of ca. 200 nm).63 Furthermore, calculation^^^ and electron affinity of 0 2 is 0.45 eV,66 and even when this energy experiment@ suggest that the first excited state of the hydrated is combined with the energy of the exciting light, it is still electron is dissociative and therefore not emitting. In addition, insufficient to provide the required ionization energy. Consea search52 for emission from excited hydrated electrons in quently, if 0 2 - is to be formed in ice under the present experimental electron-irradiated ice revealed no emission attributable to this conditions, it would have to arise from a mechanism involving source. several, sequential photon absorptions. Most importantly, in their absorption study of the flash Matrix shift arguments also inveigh against the assignment of photolysis of ice, Ghormley and HochanadeP were not able to the ice emission bands to 0 2 - . Although the gas-phase spectrodetect the formation of e-vis.This is not surprising because the scopic constants are not available for 02-, the band positions can water molecule has an ionization energy of 12.6 e V 6 and thus be calculated using the values30obtained from 02-luminescence 260-nm (4.8 eV) exciting light does not have sufficient energy in NaCl crystals. A study by R 0 1 f e ~of~ the effect of various to cause ionization. alkali-metal halides on YW and the Raman frequency of 02-has shown that both frequencies increase in crystals with larger cationH2O+as the Emitting Species. Because the ionization energy anion interatomic distances. From that study, it was estimated72 of the water molecule is 12.6 eV,66H20+ would not be produced that YW for gas-phase 02-is ca. 25 000 cm-I (400 nm). in ice excited with 260-nm UV light, and thus it is an unlikely emitter. In addition, the H2O+ emission spectrum is much more The value of uw = 27310 cm-1 (ca. 366 nm) in NaCl is blue complex than the spectrum of UV excited ice and the strongest shifted by 2310 cm-I (ca. 34 nm) from the gas-phase value, and bands from H20+ appear in the 550450-nm regionb7 which is R 0 1 f e ~and ~ Holzer et suggest that this shift is caused by far removed from the UV emissions observed in the present repulsion between 0 2 - and its neighbors in the ionic lattice. work. Assignment of the ice emissions in question to 0 2 - requires a minimum blue shift of 48 nm, which is unlikely because emission Oxygen or Its Allotropes as the Emitting Species. As in the case of H atoms, excited 0 atoms are unlikely to be the source bands typically undergo red matrix The anomalous shifts observed by Rolfe72 might possibly be specific to alkaliof the broad ice emissions. However, a number of the molecular and molecular ion species of oxygen are possible candidates. Such metal halides, where perturbation of 02-is caused by the ionic lattice. Furthermore, the most intense 0 2 - bands (at ca. 550 choices are particularly attractive as isotopic substitution of D nm)75observed from alkali-metal halide crystals are far removed for H would clearly have no effect on the emission spectra of from the 350-420-nm ice bands, which adds to the unlikelihood these species. The possible candidate species are 03,02+, 02-, that the ice emissions are attributable to excited 02-.Taken and 0 2 . together, the three arguments provide a strong case against An early worker, Janin,6s reported emission bands from O3in assigning the ice emission to 02-. the 300-450-nm region and found that these were coincident The neutral 02 molecule has several band systems, the most with the ozone absorption bands observed by previous workers. Later, Imre et ~ 7 1studied . ~ ~ the emission bands of gaseous ozone prominent one being the B3Z; - X3Z- Schumann-Runge system between 270 and 340 nm and attributed them to transitions in which extends from 175 to 535 nmS7O+his emission is not observed the Hartley band system. However, these emission spectra are from low-pressure sources such as discharge tubes, but it isdetected more complex than the spectrum from UV excited ice and have from systems at atmospheric or higher pres~ures.~O The emitting band spacings of a few hundred cm-l at most, whereas the ice systems include high-voltage electric arcs, shock tube heated 0 2

-

UV-Excited H20 and D2O Ices

The Journal of Physical Chemistry, Vol. 97, NO. 41, 1993 10545

and air, H2-02 flames, and CO-02 explosion flames. The 260to produce weak, diffuse bands. This band system is observed nm exciting light used in the present work is not energetic enough in the oxygen afterglow and in the chemiluminescence from the to excite the 0 2 molecule to the B state required for this series, reaction of H202 with NaOC1.70,8H2 Pearse and Gaydon70 have whence it can be discounted as the emitting band system. listed some of the substantiated bands which have been observed between 380 and 700 nm from such processes. The shorter 0 2 band systems which can also be discounted because they wavelength bands at 380 and 400 nm possess only 10% of the are at excessively long wavelengths, include the b'Z: - X3Z; intensity of the most intense bands located at 478.4 and 632.0 atmospheric absorption system70 (540-1000 nm) and the dA,nm. These more intense bands are not observed in the present X3Z; infrared absorption ~ystem.~O Broida and Gaydon15 have work, whence it is unlikely that emission from this band system observed a number of emission bands (370-480 nm) in the is being detected. In addition, the formation of the O4complex afterglow from an electrical discharge through gaseous oxygen. from the very dilute 0 2 molecules in the ice matrix is unlikely. They have tentatively assigned's some of these bands to the Summary of Possible Assignments. Of the band systems A3Z: - b'Z: forbidden system of 0 2 , and the shortest wavelength discussed above, the A2Hu XLH, second negative system of (369.7nm) band was assigned to the vf = 8 vf' = 0 transition 02+,the A'Z: X32- Herzberg I, and the P A , X3Z; of this system. However, in solid matrices, radiativede-excitation is from the 'v = 0 vibrational level of the excited ~ t a t e . ~In~ , ~ ~Herzberg I11 systems of 6 2 are the most likely contenders for the emissions from UV-excited ice. Table V gives the band positions this band system, the emission bands due to transitions from this for these systems along with the positions of the emission bands vibrational state would be at wavelengths longer than 500 nm, observed from UV excited H20 and D20 ices in the present work. which is far removed from the emission bands in UV excited ice. In this table, the band positions for the Herzberg I and 111systems The Herzberg systems of 0 2 show particular promise as of 0 2 and the 0 2 + band systems were calculated using the candidates for the ice emission bands observed in the present spectroscopic constants given by Krupenie71 and Borrell et a1.,16 study. However, the relative Franck-Condon factors for the K r ~ p e n i e ,Coquart ~' and Ramsay,lgand Krupenie?' respectively. Herzberg I, 11, and 111 systems seem to be dependent on whether the environment is that of a gas or a solid, as discussed in the Selection of the Preferred Spectral Assignment. There are only four experimental ice luminescence peaks available for Introduction. The Herzberg I (A32: - X3Z;) series is clearly dominant in the gas phase, and as pointed out in the Introduction, comparison with the various spectral series in Table V. However, extends from 250 to 490 nm. The band spacings in this and the their appearance in both D20 and H20 ices does provide additional other Herzberg systems are ca. 1450 cm-I which is similar to the validation. In Table V, for convenience of presentation, the experimental peak positions have been located so that they are band spacings in the present work. It is thus possible that any of these is the band system observed in the emissions from UV aligned with what turns out to be one of the best fits to the excited ice. alternative band series. Thus, they have been aligned with the 0 3 to 0 6 section of the Herzberg I11 series of 0 2 . However, The c'Z; - X3Z; Herzberg I1 system is another series of it should be noted that the fitting procedure used made no such forbidden 0 2 transitions and initially, only two feeble emission prior assumption. That procedure involved plotting (Figure 7) bands from this series were detected in the emission from an 0 2 the wavenumbers of the four H2O ice peaks against the + Ar afterglow and also in the night sky In the gas phase, wavenumbers of all possible sets of four adjacent vibrational this band system is much weaker than the Herzberg I system but transitions, for each oxygen species in Table V. The line of best stronger than the Herzberg I11 system, and the two gas-phase fit for each of the oxygen species was taken to be that which had emission bands have been tentatively assigned70 to the 0 7 a slope closest to unity. Figure 7 shows the line of best fit for (449.1nm) and 0 8 (479.1 nm) vibrational transitions. In each of the species 0 2 and 0 2 + and Table VI validates these line later work, Slanger18has identified six more vibrational transitions choices by showing the slopes obtained for the nearby and less in the gas-phase emission spectrum of the Herzberg I1 system satisfactory lines of best fit. and more recently, R a m ~ a yhas ~ ~detected 10 new vibrational transitions in the absorption spectrum and has thus increased The graphs in Figure 7 do not effectively distinguish between 0 2 (with either its Herzberg I or I11 series) and 0 2 + as candidate Herzberg's17 original numbering by 5 units. However, the species as there are only slight significances in all three cases for Herzberg I1 emission is evidently so weak from 0 2 molecules the slope deviating from unity (Herzberg 11102: 1% probability isolated in rare-gas matrices that it does not appear to have been observed in such situations, even though the Herzberg I11 emission of deviation, t = 0.014;Herzberg I 02:6%probability of deviation, has. 20-23 t = 0.089;0 2 + : 18% probability of deviation, t = 0.263). The symmetry forbidden PA,-X3Z, Herzberg I11 system of Fortunately, the choice of emitting species can be narrowed down further by considering the expected effect of a gaseous, 0 2 is also a possible contender for the ice emission, although in the gas phase it is a weaker band system than the Herzberg I and liquid, or solid matrix on the transitions. Herzbergs4 has pointed 11systems. Nevertheless, both emissionisand absorptionlgbands out that when diatomic gases such as 0 2 are liquefied, solidified, of the Herzberg I11 system have been reported in the gas phase. or dissolved in another liquid, the positions of absorption peaks However, in rare-gas matrices emission bands from the Herzberg change by relatively small amounts. For example, the emission I and I1 systems have not been observed,20-23 while emission from band positions of gaseous O2(lAg) are red shifted by only ca. 3 the Herzberg I11 system has.2G23 As with the Herzberg I system nm in the solid phase.83 However, the spectral shifts for solution of 0 2 , the Herzberg I11 system overlaps with the wavelength species are dependent upon the solvent and can be larger in polar range observed in the ice luminescence studies and the band solvents such as H20. An example of this is the absorption peak spacings are again ca. 1450 cm-1 which fits the ice emission band of Br2 gas a t ca. 24 000 cm-I which is shifteds4 by 1750 cm-1 to spacings just as well as does the Herzberg I series. ca. 25 750 cm-I in aqueous solution. This corresponds to a blue shift of ca. 28 nm whence shifts of this magnitude might not be The tentatively assigned PAu-aiAg Chamberlain airglow unreasonable for 0 2 in an ice matrix. system70~~~ of 0 2 (370-440 nm) is also a forbidden system and occurs under the same conditions as the more intense Herzberg Although direct experimental information about the effect of I system. Again, becauseit is weaker than theHerzberg I system, an ice matrix on the emission spectrum of molecular oxygen is it is unlikely that it is being observed in the present work. not available, there is information about oxygen trapped in other Furthermore, the bands in this system are triple headed, a feature solids. A number of workers have observed emission bands from not exhibited by the ice emissions. the laser21322 and proton-beam23 excitation of solid gas matrices containing small amounts of 0 2 . Some of these bands were Finally, double electronic transitions of the oxygen molecule assigned2l-23 to the 0 3-15 transitions of the Herzberg I11 complex, 04,formed by the collision of 0 2 molecules, are known70

- -

-

- -

-

-

-

-

Matich et al.


TABLE V Measured Wavenumbers (Vacuum Corrected) of the Luminescence Bands from UV-Excited Ice Compared with the Wavenumbers Calculated from the Molecular Constants Given by Borrell et d , 1 6 Coquart and Ramsay,l9 and Krupenie71 for the A3Z: 9 2 ; Herzberg I System and @Au YZ;Herzberg 111 Systems of 0 2 and by Krupenie71 for the A T , SII, Second Negative System of 07 literature values for the positions of emission bands from molecular oxygen positions of emission bands from 0 2 band~l~,~~/cm-' 0 2 band~l~*~l/cm-] UV-excited ice (from the present study) transition (v' Y") (Herzberg I) (Herzberg 111) 0 2 + bands71/cm-' H2O/cm-l D20/cm-l

-

-

-

-

0-0 0-1 0-2 0-3 0-4 0-5 0+6 0-7 0 4 8 0+9 0 - 10 0 - 11 0- 12 0- 13 0 - 14 O + 15

34378.8 32823.2 31292.9

35003.52 33447.13 3 1914.24 30404.49

29788.0 28308.6 26854.5 25425.8

28917.53 27452.93 26010.25 24589.05

24022.6 22644.7 21 292.2 20625.5 19311.1

23188.80 21 808.99 20449.06 19 108.41

40069.20 38196.63 36356.63 34549.19 32774.31 31032.00 29322.25 27645.07 26000.45 24388.39 22808.90

28514 f 12 27057 f 8 25829 f 9 24026 f 13

28528 & 14 27048 i 7 25893 f 14 23975 f 17

21261.96 19747.60 18265.79 16816.55

15599.88

0 For the 0 2 band systems, the band positions are vacuum-corrected values,16while for the 0 2 + band systems the workers'l did not state if this is the case. However, the vacuum correction in this wavelength region amounts to only between ca. 4 and 11 cm-I. The ice emission bands in this table have been located adjacent to the Herzberg bands of 0 2 (bold type) having similar wavenumbers. The errors shown for the positions of the measured bands are 50% confidence intervals.

a 30 .-I

0

X

28

In the present work, the shortest wavelength peak in Table V 7 is around 350 nm, which is red shifted by ca. 5 nm from the gas-phase position of the 0 4 Herzberg I transition and by 15 1 /4 nm from the gas-phase position of the 0 3 Herzberg I11

-

0, (Herzberg I )

O, (Herzberg 111)

m=0.999 v)

D

29

30

28

27

0.062 26

25

0 w

-

0

-

17

18

19

20

21

Wavenumbers of Emission Bands From 3 G a s Phase Oxygen Species / 10 cm-' Figure 7. Best-fitting linear least-squares plots (solid lines) of the band positions of the short-lived luminescence from UV irradiated H20 ice versus the band positions of the luminescence from gas-phase 0 2 and 0 2 + . A slope of unity would indicate perfect correlation between the ice bands and the gas-phase bands. series and were red shifted from their gas-phase positions by around 2 nm (for theO-4 transition). This observation suggests that a shift of 5-10 nm or even a little larger should be quite acceptable in an ice matrix as the latter would be expected to perturb the energy levels in the 0 2 molecule by a t least as much as would a rare-gas matrix.

-

transition. Shifts of these magnitudes would seem reasonable in view of the above-reported studies, and therefore on the basis of matrix shifts 02 is an acceptable candidate species. On the other hand, a large matrix blue shift of ca. 120 nm is required to produce the line of best fit for 02+in Figure 7. This shift is greatly in excess of the wavelength shifts of up to a few tens of nanometers which have been reported for absorption and emission bands of diatomic molecules in liquids and solids. This point is reinforced by the observation that the emission bands from OZ+in a solid neon matrix are red shifted by only around 20 nm.18 It is clear that assignment of the ice emissions to the second negative system of 02+would require an improbably large blue shift. The foregoing arguments relating to species assignment lead to the conclusion that the band system observed in the short-lived luminescence from UV-excited H20 and D20 ices is most probably the Herzberg I A3Z: X3Z;system or the Herzberg 111 QA,, X3Z;system of the neutral 02 molecule. The best fits (Figure 7) of the experimental band positions to those of 02were obtained for the Herzberg I O 4-7 transitions and the Herzberg 111 0 3-6 transitions of 02,assignments that would mean that the gas-phase emission has been red shifted by 5 or 15 nm, respectively, in the ice matrix. Assignment to the 0 3-6 transitions of the Herzberg I system which requires a red shift of 22 nm is also a possibility. These options could be acceptable as the magnitudes of the spectral shifts required are comparable with those 0bserved21-~~,~~ for gaseous species in solid or liquid matrices. Although it has not been possible to determine which of the Herzberg I and 111 systems is responsible for the ice emissions, it is noted that the previous 02 emissions observed from solid matrices by other workers have been assignedzkz3to the Herzberg 111 system. However, none of that work involved ice matrices and it is not at all clear that the matrix effects of inert gases will simulate those of a strongly hydrophilic environment. The long-lived (7112 = 1.3 s) component of the luminescence has not been investigated in this study. It does not show detectable fine structure, which makes spectroscopic identification difficult. It does however have similar intensity to the short-lived emissions

-

-

UV-Excited H2O and D20 Ices


TABLE VI: Slopes of the Linear Least-Squares Plots of the Positions of the Ice Emission Bands versus the Positions of Sequences of Four Bands from 0 2 and 02+' species whose emission bands were fitted to the ice emission bands ~

-- -0 2

transitions (v' 0 0 0 0 0

(Herzberg I) v")

2,3,4,5 3,4,5,6 4,5,6,7 5,6,7,8 6,7,8,9

a The best

-- --

02 (Herzberg 111)

slope

transitions (u'

0.977 f 0.060 0.992 f 0.062 1.007 f 0.062 1.022 f 0.063 1.037 f 0.064

0 0 0 0 0

slope

v")

1,2,3,4 2,3,4,5 3,4,5,6 4,5,6,7 5,6,7,8

transitions (v'

0.965 f 0.074 0.982 f 0.075 0.999 f 0.076 1.016 f 0.078 1.035 f 0.079

-

02+

slope

v")

0-9,10,11,12 0 - 10,11,12,13 0 - 11,12,13,14 0 - 12,13,14,15 0 - 13,14,15,16

0.939 f 0.059 0.959 f 0.060 0.980 f 0.062 1.036 f 0.085 1.055 f 0.033

fit for each oxygen species was obtained when the slope was nearest to unity (values in bold type). The errors are 50%confidence intervals.

-

and therefore it might also come from an O2emission. If it did, it might possibly be a transition from the c'Z; aIAg series which was observed by Goodman and Brus21 and by Okada et al.22 in inert-gas matrices. However, as the observed lifetime of this ice emission is far longer than that observed for this transition (ca.8 ps at 32 KIl4) some slow precursor process (such as electronion recombination) would need to be invoked if this explanation were to be viable. Such proposals for the long-lived emission are purely speculative at this stage. Mechanism of Formation of Excited Molecular Oxygen in UVExcited Ice. Energetics of the Photodissociation of H20 in Ice. Any mechanism that explains the formation of O2in UV-irradiated ice must first consider the primary process of water dissociation. The threshold energy of 5.13 eV (242.0 nm) for the dissociation of H20 is well established85-g7 in the gas phase, but while H20 dissociation is ~ell-known36.44,88,~9 in water and in ice (Appendix A), the energy threshold for this process in the condensed phases is not available. In water vapor photolyzed with light at wavelengths of 145 nm and longer, H and O H are the onlys9 primary products of photolysis, while photolysis with wavelengths around 120 nm will produce the minor89 (ca. 25%) primary products which are H2and 0. However, in liquid water H2 and 0 are not formed as primary photoproducts. In their study of the photolysis of pure liquid water and aqueous solutions of alcohols and 0 2 ,Sokolov and Stein88 found that all of the molecular hydrogen produced therein was from the recombination of H atoms and not from the photodissociation of the water molecule to produce H2 and 0. In the absence of direct experimental measurements, the available thermochemical data and the reactions presented below, enable one to estimate the threshold dissociation energy for the H 2 0molecule in ice. This value is the enthalpy change for the process

H2O(s)+ hv

-

(1)

H(s, + OH(,)

at 77 K which will be designated by AHD(H20,s,77K)and is given by the sum

literature values in Appendix B and give AHD(H,0,s,77K)= 474 f 15 kJ mol-' This important result indicates that in order to dissociate the water molecule in ice into H and OH at 77 K, exciting light with X I 252 f 8 nm should be required. The wavelength of the exciting light used in this study was 260 nm (bandpass = 10 nm), and therefore, within the limits of the accuracy of the above calculation, it is reasonable to expect that H and OH would indeed be formed in these experiments. The calculation outlined in Appendix B has ignored the assistance which would be given to the dissociation of a water molecule in ice, if the products OH and H were trapped in the lattice. In a number of published studies of the decay kinetics of H90391 and OH29992 in ice, trapping energies have been obtained by determining the activation energies of the mobilities of these species in the ice lattice. On this basis, the trapping energies of OH29 and H91 are given as 23.8 f 3.0 kJ mol-' and ca. 8.3 kJ mol-1, respectively. If it is assumed that these trap energies can be added directly to the dissociation energies calculated above, then A&(H20,s,77K) = 442 f 18 kJ mol-', and thus less energetic UV photons with X 5 271 f 11 nm should be able to effect dissociation. Observations Which the Luminescence Mechanism Must Explain. Any mechanism which is postulated to account for the production of the series of ice luminescence bands excited by 260 nm light must be consistent with two fundamental observations, viz. (1) The luminescence must arise from the deexcitation of chemically produced 02*, as it is shown (see Appendix C) that direct optical excitation of oxygen molecules in the ice lattice is not the source of the luminescence. (2) The mechanism must account for the first-order dependence of luminescence intensity on the intensity of the exciting light observed by Litjens.2 Reactions Which Produce Electronically Excited 02.Reactions 11 and 12 are the only potential 02-forming reactions in

IO ..

AHD(H,0,s,77K)

0

=

+ H20,

i=3

where the AHi values are for the processes H2°(s,77K)

----

0

H2°(s,273K)

AH3

(3)

H2°(s,273K)

H2°(1,273K)

AH4

(4)

H2°(1,273K)

H2°(1,298K)

AH5

(5)

H2°(1,298K)

H2°(g,298K)

H2°(g,298K)

H2°(g,77K)

H2°(g,77K)

OH(g,77K)

H(g,77K)

OH(s,77K)

+ OH(,77K)

(6) AH7

(7)

AH8

(8)

AH9

(9)

H(g,77K) H(s,77K) AH', (10) The enthalpy changes for processes 3-10 are calculated from

-

+0

0, + H 2 0

-

0,

AH,,

= -355.3 k J mol-' (1 1)

AH,, = -495 kJ mol-'

(12)

irradiated ice which release anywhere near sufficient enthalpy to produce the 0 2 molecule in the excited, A3Z:(u' = 0), state for example, which is 419 kJ mol-' (or 35 008.02 cm-1)71 above the ground, X3Z;(v'' = 0), state. The enthalpy changes shown above are for the temperature of 100 K and have been estimated from gas-phase values for various enthalpies of formati0n.93-~~Enthalpy values were not available for these species in an ice matrix and hence the AH values in eqs 11 and 12 should be regarded as approximate. In view of this approximation, eq 11 has not been ruled out on energetic grounds even though the estimated exothermicity of 355.3 kJ mol-' is somewhat less than the spectroscopically required value of 41 9 kJ mol-I. The alternative

Matich et al.

10548 The Journal of Physical Chemistry, Vol. 97, No. 41. 1993

reaction pathways involving the separate reactions 1 1 and 12 will be termed the “peroxide pathway” and the “0 atom pathway”, respectively, the names being derived from the species trapped in the ice lattice in each case. The accepted mechanism of formation of the H202 needed to supply eq 1 1 is via the reaction between two OH radicals, although reaction between 0 and H20 is also a pos~ibility.2~,36,~9,51 Interestingly, there appears to be no direct evidence for H202 formation in either radiolyzed or photolyzed ice because H202 is only detected, and its yield measured, after the ice has been melted.36~~9It is therefore at least possible that the H202 could be produced only by the reactions between trapped radicals which are liberated during the warming or melting process. At 77 K, OH is immobile50~51~92~96 in ice and would not be expected to react to form H202 until the temperature of the ice exceeded ca. 100 K, at which stage OH becomes mobile. Therefore, it is possible that at temperatures above 100 K, some H202 could be formed from reacting OH radicals in the lattice. However, there is indirect evidence for the formation of H202 in ice even at 77 K. The reaction

OH

+ H,02

-

P

H20

+ HO,

+0 +M

-

+ 0,

-

or 0+0

02* +M

0,

+ 02*

P‘

radiationless transition

(13)

is an important reaction for the destruction of H202 in the atmosphere97 and is generally accepted29~49~51~96 as the reaction which forms the w e l l - k n o ~ n HOz 2 ~ ~radical ~ ~ ~in~ice ~ ~at ~77~ K. This reaction implies the presence of HzOz in ice at this temperature, and K e ~ a has n ~suggested ~ that if this is indeed the case, then OH is not necessarily immediately trapped at its point of formation in the ice but could be mobile for short periods of time afterward. If H202 does form in ice, then under continuous irradiation its concentration would significantly exceed that of its mobile reaction partner, 0, thus providing a mechanism (eq 1 1) that gives the observed pseudo-first-order dependence of emission intensity on irradiance. An expression for the dose dependence predicted by the peroxide pathway and the 0 atom pathway is derived in the following section. At first sight, it might be suggested that a doping study could be undertaken to determine the involvement of H202 in the mechanism of formation of the luminescence. However, Litjens has pointed out that even in rapidly frozen dilute solutions, H 2 0 2 is not distributed homogeneously throughout the ice but separates out and forms microcrystal^.^* Therefore, such procedures do not provide a useful or reproducible way of homogeneously inserting dopant molecules into the crystal lattice. Consequently, doping ice with H202 would not unequivocally determine whether H202 which has been formed in the lattice is an intermediate in the mechanism for the production of the luminescence. Furthermore, such a dopant study would not differentiate between the mechanisms described in eqs 11 and 12 because in both cases one of the reactants is the atom, 0,which can be a photoproduct of H202. The most likely source of excited 0 2 is the 0 atom pathway as it uses eq 12 which is the most exothermic of the reactions which form 0 2 and thus provides more energy than the 419 kJ mol-’ required to form the electronically excited state. In a gasphase study of the Herzberg I emission, Broida and Gaydon15 proposed that this species is formed via a three body collision such as

0

the mode of formation of 02* in solid rare-gas matrices and it could also be the mechanism of formation of 0 2 * in photolyzed ice. To use the 0 atom pathway to explain the observed first-order dependence of the emission intensity on the irradiance of the exciting light, a pseudo-first-order mechanism can be invoked. This involves 0 atom accumulation in the ice during irradiation, the rate-determining step being the diffusion of mobile, photoproduced 0 atoms (Om) to immobile accumulated 0 atoms (OT). The disparity between the concentrations of 0, and OT would then account for pseudo-first-order kinetics. Derivation of the Dependence of the Luminescence Intensity on Irradiance. Any satisfactory mechanism for the emission of luminescence must predict the observed2 first-order dependence of luminescence intensity on irradiance. Both of the mechanisms under consideration (the peroxide pathway and the 0 atom pathway) do this as shown by the following derivation. These two pathways to the formation of 0 2 in its excited A3Z: state are laid out as a single generalised scheme, uiz.

(14)

(15)

and noted that M = H2 or H20, enhanced the intensity of the emission. 0 atom recombination has also been suggested99 as

where XT (which is either H202 or OT)is a trapped species, the concentration of which has accumulated in the ice lattice to a level substantially in excess of that of the mobile species and which does not change significantly with a transient change in dose. In this mechanism, OTis formed by the photodissociation of photolytically produced OH, and two OH radicals react to produce H202. Although atomic oxygen is not a known photolysis or radiolysis product of ice, the reactant species, OH, is,29~3731J00J0~ and its photodissociation is energetically feasible. H202 is a known photolysis36and product of ice. k ~ isl the rate constant for the photodissociation of H2O molecules to form H and OH1 radicals and k ~ is2 that for the photodissociation of the trapped OH radicals (OHT) to form H and 0 atoms. k , is the rate constant for the reaction between a mobile oxygen atom (0,) and XT. The pseudo-first-order rate constant for this reaction is k’,,, = km[XT]. After 7 h of accumulation, the concentrations of XT and OHT, which also accumulate, may be assumed to be essentially independent of the irradiances used to study the dose dependence and are thus treated as “bulk” species. kr is the rate constant for the fluorescence emission step in this scheme and kl is the rate constant for the reaction between OHT and any mobile scavenger species, A, to form products, P. k2 describes the reaction between 0, and any “bulk” scavenger species, B, to give products P’. k~ is the rate constant for the quenching of OZ* by the “bulk” species Q. Because continuous irradiation is employed, a steady-state treatment can be used. The luminescence intensity is given by

4 = kr[O,*I and as

UV-Excited H20 and D2O Ices

The Journal of Physical Chemistry, Vol. 97,No. 41, 1993

where kt2 = k2[B]. Combining eqs 17, 19, and 21 gives

I, =

[OHTI (k,+ k b ) ( k ; + k;) ‘fkmkD2

From the Beer-Lambert law, the rate of formation of 0, is

d[o,]/d?

@DIoCY~[OHT]

(23) where a linear approximation to the Beer-Lambert law is used because the concentration of OHT in ice is small, and thus the ice sample is optically thin. In this expression, IOis the intensity of the light incident on the ice sample; a = 2.303t, where t is the decadic extinction coefficient for OH; x is the optical pathlength; and QD is the quantum efficiency for the dissociation of O H radicals. It follows that

kD2

(24)

@DZOCXX

whence from eqs 22 and 24

+

1, = kfk,@D~oax[OHT]/(k\ k’,)(k,

+ kb)

(25)

which can be rewritten as

I, = KZo where K is a constant given by

Thus, the intensity of the luminescence from 02*produced by both the 0 atom pathway and the peroxide pathway should be directly proportional to the irradiance. This is indeed what is observed.2 A somewhat different approach to theabove would be to suggest that the first step in eq 16 produces H OH pairs which are caged in the ice lattice and can then react, without the need of a second partner, to give H2 + 0, which can then form 0 2 * by either of the two pathways shown in its reaction with XT, viz.

+

H,O

+ hv

- h

(H OH]

H,

+ 0,

XT

02*

(27)

km

where the brackets indicate that the products are caged together in the ice sufficiently long for them to react rather than for them to diffuse away from one another. If the same kinetic analysis is applied to this alternative sequence, the dependence in eq 25 is replaced by

This result predicts the same observed dependence of the luminescence intensity, I,, on the irradiance. The main difference between the two approaches is that the first shows that the luminescence will not reach a steady value until [OHTI has reached a steady state value in the ice lattice. The second approach does not require the occurrence of such a delay.

Conclusions The short-lived ice luminescence bands have been assigned to either the Herzberg I or 111 series of 02,and this assignment is partly based upon the reasonable agreement between the positions of the ice luminescence bands and the bands in both Herzberg systems. The earlier tentative assignment by Litjens et al.1,2of this previously unresolved luminescence to the A2Z+ (v’ = 0) X2n (v” = 1) transition of OH has been ruled out because the expected isotopic wavelength shift in DzO ice was not observed. Furthermore, the band spacings of the ice emissions are much smaller than those in the OH band system. Emissions arising from other oxygen band systems were discounted on either

-

10549

spectroscopic or energetic grounds, as were emissions from all other species which might be likely photoproducts of ice. However, it is not yet known whether all the short-lived ice luminescence observed between 320 and 420 nm in the present work is attributable to a Herzberg series of oxygen, or whether these bands are superimposed upon an underlying continuum emission from some other transitions. For the latter to be the case, the underlying luminescence would have to also have a half life shorter than ca. 10 &02 and a first-order dependence on the irradiance of the exciting light. It has been shown that negligible 02*can be formed by the direct absorption of UV light by 0 2 located in the ice lattice, whence 02*must be formed chemically. Two energetically feasible chemical pathways for the formation of 02* have been proposed. Either two 0 atoms react together to give 02*, or this species is formed by the reaction of an 0 atom with a molecule of H202. It is shown that the former pathway is most likely to provide enough energy to produce excited oxygen molecules. Furthermore, an expression has been derived which shows that the intensity of the luminescence from 0 2 * produced by either pathway should be directly proportional to the irradiance of the exciting light, as was observed by Litjens et The long-lived ( q p = 1.3 s) component of the luminescence has not been investigated in this study. It does not show detectable fine structure, which makes spectroscopic identification difficult. It does however have similar intensity to the short-lived emissions, and therefore it might also come from an 0 2 emission.

Acknowledgments. A.J.M. gratefully acknowledges a Commonwealth Postgraduate Research Award from the Australian Government. D.L. gratefully acknowledges the tenure of a University of Western Australia Postdoctoral Research Fellowship, and M.G.B. the tenureof a Postdoctoral Research Fellowship from the Australian Institute of Nuclear Science and Engineering. Appendix A Literature Assignments to the Absorption Peaks in Photolyzed and Radiolyzed Ice. The identification of the species and the reactions responsible for UV excited’ice luminescences is aided by information about the species which are known to be produced by ice photolysis. In their 263 K study of ice which was flash photolyzed by light at wavelengths longer than 160 nm, Ghormley and HochanadeP found a broad absorption peak a t CLI. 245 nm, at a time 8 ms after the exciting flash. This peak was attributed to the O H radical because it is similar to the 230-nm absorption peak in irradiated water, which Boyle et al.” assigned to hydrogenbonded OH radicals. Ghormley and Hochanadel performed a computer simulation of the kinetics of formation of HO2 via the reaction

OH + H,O,

-

HO,

+ H,O

(29) in liquid water. The results suggested that H02 was much less likely to be the absorbing species than OH, as the simulation predicted that the HOz concentration would be very small, 8 ms after the exciting flash. Boyle et al.3 assignment of the 230-nm absorption peak in irradiated liquid water to OH was supported by the doping experiments of Thomas et a1.,Io3who observed that the height of the 230-nm peak in electron irradiated solutions increased when either N 2 0 or H202 was added to the water. These substances enhance the O H yield via the reactions

+

e-aq N,O e-aq

+ H,O

+ H,O,

-

N,

+ OH + OH-

OH + OH-

(30) (31)

The O H assignment was supported by the observation that the

Matich et al.


230-nmabsorption decayed rapidly when the water was irradiated under 10-20 atm of hydrogen. The latter is a well-known scavenger of O H and removes it according to

OH

+ H,+H + H20

(32)

Other workerslwJ03 have observed similar behavior in studies of doped, electron-irradiated water, and their results are in accordance with the assignment to O H by Thomas et aI.lo3 However, it is not clear in such studies how the absorption by O H has been differentiated from the H02 absorption around 230 nm. This species is detected in irradiated water both by its optical absorption and by its ESR signal,28 and it is quite possible that the absorption peak attributed to OH by Ghormley and Hochanadel36 in ice and by other workers in liquid waterl03-105 is in fact H02. Furthermore, although doping irradiated water with H202, N20, and H2 may have marked effects on the yield of OH, the yield of HO2 will be similarly affected because it is a reaction product of OH293,49,50,51.96 via

OH

OH + OH -,H202

(33)

+ H,O,

(34)

-,HO,

+ H,O

Therefore, it is not clear whether the absorption at 230 nm in irradiated water is indeed due to OH, as often assigned, or is due to HOz. The absorption spectrum of radiolyzed ice29,37,61~1oo~*07~108 at 77 K shows maxima at 230,280 and 650 nm (assigned to HOz, OH, and e-vis,respectively) but does not show the peak Ghormley and Hochanadel36 reported at 245 nm in flash-photolyzed ice. They proposed that the OH radicals, which absorb around 280 nm at 77 K, are trapped interstitially with little or no hydrogen bonding and that when the ice is heated above 120 K, these disappear completely. Ghormley and Hochanadel further proposed that in ice photolyzed at temperatures above 120 K, the OH radicals formed are trapped in substitutional sites where they are hydrogen bonded and will have an absorption spectrum similar to that of O H in liquid water. If this hypothesis is correct, then it might be expected that an absorption peak at ca. 245 nm would also be observed in ice samples irradiated with ionizing radiation at temperatures just below the melting point of ice. Unfortunately, verification of this assignment to two types of trapped OH in ice is not available, and it remains completely speculative. No absorption peak in ice at 245 nm has been reported by any other workers who have studied the absorption spectrum of this species in ice.29J7 Thus, in their pulsed electron irradiation study of crystalline ice, Taub and Eiben29 studied the absorption spectra of the species produced at a number of temperatures. They observed an absorption peak at ca. 230 nm in ice irradiated at temperatures ranging from 77 to 263 K. On the basis of the peak position, the decay kinetics and the ESR spectrum, they assigned the peak to absorption by HO2. The only other ice absorption observed by Taub and Eiben and 0thers3~was the 280-nm peak which they ascribed to the O H radical. In view of the frequently verified absence of an absorption at 245 nm in ice irradiated with high-energy ionizing radiation, the assignment of Ghormley and Hochanadel's36 optically produced absorption at 245 nm to interstitial OH needs to be regarded with caution. More work is clearly needed before this absorption feature can be assigned unambiguously to any particular species. Regardless of whether or not the 245-nm absorption peakarises from the O H radical, there is little doubt that this species, along with H, is indeed formed in UV irradiated ice. In their flash photolysis study, Ghormley and HochanadeP detected micromolar amounts of H202 in water formed by melting the irradiated ice. The main pathway of H202 formation is by reaction between two photolytically produced OH radicals which are mobile in ice at 263 K. These workers also observed an absorption which had

a half-life of about 1 ms and which did not display a maximum. This absorption continuum, which commenced below 300 nm and increased toward shorter wavelengths, was assigned to the H radical which is known to have a similar absorption profile in aqueous solutions.lwJOs In their ESR study of the products of the far-UV photolysis (121.6 nm) of ice at 20 and 77 K, Kuwata et al." did not detect the characteristic signal of the OH radical but reported a signal which they attributed to the HO3 radical. These workers suggested that the OH signal was absent because OH was consumed by reaction with H202 and with 0 2 . However, because OH and H202 are considered49-sI to be immobile in ice at 77 K, it is unlikely that these reactions would have a significant effect on the concentration of OH. Furthermore, Kuwata et al. did not detect the ESR absorption of H or HOz, both of which have been frequently m e a ~ u r e d ~ ~ ~ ~ , ~in2 electron , 9 6 , ~ and ~ ~ ~y-irradiated 0~ ice samples. The inability to detect species which are known to be formed by both radiolysis and photolysis of ice suggests that in the study by Kuwata et al., the concentrations of the species produced were merely below the detection limits of their ESR spectrometer. In summary, it is clear from a number of studies29~37~61~~06-~08 of the radiolysis of ice that the UV and visible absorption peaks of three radiolytically produced species have definitely been observed. These species are the O H radical which has an absorption peak at 280 nm,29.37the HOz radical which has an absorption peak at 230 nm,29and e-"iS which has an absorption peak around 650 nm.29,61J06-308 These well-established assignments have been made upon the basis of the peak positions, the decay kinetics, and the ESR spectra of these species. However, identification of the species present in ice irradiated by UV light and the assignment of these species to particular absorption features is much less certain than in the case of radiolysis. The two absorption features observed in UV irradiated ice are a peak a t 245 nm and an absorbance continuum which increases as one proceeds from below 300 nm to shorter wavelengths. These two features have been assigned to absorption by OH and H, respectively.36 Neither of these absorptions have been observed in studies of radiolyzed ice,29J7and therefore these two assignments must be regarded as very speculative. However, quite apart from spectral considerations, the existence of H202 in the melts of UV irradiated ice samples indicates that OH and, by implication, H are indeed photoproducts in ice.

Appendix B Calculation of the Enthalpy of Dissociation of HzO in Ice at 77 K. The enthalpy of dissociation of H20 in ice at 77 K, A H D ( H ~ ~can , ~ be , ~ calculated ~ K ) , from the enthalpy changes for the processes 3-10, shown in the section on the energetics of the photodissociation of H20 in ice, as follows. For process 3 (35) where the mean values of C,between 77 and 100 K, 100 and 195 K, 195 and 255 K, and 255 and 273 K are 10.99,21.44, 34.84, and 36.97 J K-1 mol-I, respectively.66 From the literature,% AH4 = +6.008 kJ mol-' and AH5 is given by

where

Cp(H20,I) =

75.58 J K-I m0l-1,'~~ whence AHs = +1.89 kJ mol-'

From theliterature,I@'AHs = +44.016 kJmo1-I, whileajustifiable linear interpolation of literature valuesg3gives AH,= -7.376 kJ mol-'.

UV-Excited H20 and D20 Ices


The enthalpy of thedissociationofwater vapor at 77 K (process 8) can be obtained from

where AHf(H(,)) = 216.47 kJ mol-', AHf(OH,,,) = 38.47 kJ mol-', and AHf(H20g)) = -240.05 kJ mol-'. These values were obtained at 77 K by fitting second-order polynomials to the literature data93+4' which is presented a t temperatures of 0, 100, and 200 K. The values obtained at 100 K by this fitting procedure differed from the listed values a t 100 K by less than 0.2%. Substitution into eq 37 then gives AH8 = +495.0 kJ mol-'. Calculation of the values for AH9 and AHLO is less reliable because lattice energy values for O H and H radicals in H20 ice are not available. However, it is not unreasonable toassume that the lattice energy of O H is approximately equal to that of the H2O molecule in the ice lattice at 77 K. WhalleylIocalculated the value for the lattice energy of ice at 0 K to be -56.0 f 0.7 kJ mol-'. Using the same method as Whalley and the thermodynamic data given in the references cited therein, an approximate value of AH9 = -58 f 1 kJ mol-' at 77 K is obtained. As a check on this estimate of A H 9 , an alternative calculation was made. This calculation used the enthalpy of solvation of O H at 298 K, and then corrected this value by including terms for the enthalpy of cooling in liquid water from 298 to 273 K, the enthalpy of fusion and the enthalpy of cooling in ice from 273 to 77 K. The heat capacities for OH radicals in liquid or solid water are not known, and similarly, AHfusionis also unavailable. However, in the present case it was assumed that the C, values and the PHfusion for O H are comparable with those for water, and therefore the values for water have been used in eq 39. Thus, for the step OH(g,77K)

-D

OH(s,77K)

the enthalpy change is

where

AH(g+aq,298K)

= Mf(OH(aq))

- Mf(OH(g))

--

-44.5 f 1.5 kJ mol-' (41) The value of AHf(OH(aq)) = -5.5 f 1.5 kJ mol-' used for substitution in eq 41 is the mean of the two values (-7 and -4 kJ mol-') given in a review by Buxton et ~ 1 . ~Reference ~ ' 112 gives AHf(oH(,))= +38.99 kJ mol-'. Thevalue of AH(aq,298-.273~), which is the enthalpy change when a mole of liquid water cools from 298 to 273 K, was calculated to be -1.89 kJ mol-', as it is the negative of AH5 which has already been determined using eq 36. The latent heat of fusionIWis AH(fus) = -6.008 kJ mol-', and the enthalpy change (eq 35) for cooling ice from 273 to 77 K is m ( ~ 7 3 - 7 7 K ) = -5.045 kJ mol-'. This procedure led to A H 9 = -50 f 5 kJ mol-I, where a 10% error is allowed in view of thevarious approximations made above. This value agrees reasonably well with the value of -58 kJ mol-1 calculated from the lattice energy of ice a t 0 K, and the two values were averaged to give A H 9 = -54 kJ mol-'. The remaining quantity to be calculated is A H l o , the lattice energy of H. The H radical is very much smaller than OH (and HzO), and it is expected that its lattice energy would be substantially less than that of OH (and HtO). An estimation of AH10 can be obtained via the method used to calculate AHg. In

this calculation one can AHf(H(aq))= 21 3.5 f 0.5 kJ mol-', M f ( H ( g ) ) = 21 8.0 kJ and once again assume that the Cis and the heat of fusion are comparable with thoseof water. Given the assumptions made in this calculation, the error in AHlocould reasonably be expected to be as high as 50%, whence

AH,o = -17.4 f 9 k J mol-' Summing processes 3-10, according to eq 2 one obtains = 474 f 15 kJ mol-' AHD(H20,s,77K)

Appendix C Calculation Showing That the Herzberg Emission Does Not Arise from Optical Excitation of 0 2 in the Ice Lattice. Direct optical excitation of 0 2 formed in the ice lattice can be rejected as a significant process by considering the absorption cross section of the oxygen molecule a t the excitation wavelength of 260 nm. Unfortunately, it is difficult to find such data at wavelengths above 250 nm. However, Ditchburn and Young1I3have measured the 02 cross section, q, from 200 to 250 nm and find that it decreases steadily with increasing wavelength. Therefore, their value of q = 2 X cm2/molecule at 250 nm serves as a useful upper limit for insertion in the equation ZA

= I,[ 1 - exp(-qnl)]

(42)

for calculating the intensity, I,, of the absorbed light. In this equation, ZOis the intensity of the incident light; q is the absorption cross section; n is the number of absorbing species per cm3; and I is the path length in centimeters. In the present study, the total optical power incident on the 3 cm X 3 cm X 0.5 cm ice sample a t 260 nm was ca. 2 X 10" photons s-l. Although the thickness of the ice sample was ca. 0.5 cm, its optical path length was ca. 1 cm because after the light passed through the ice sample, it was reflected back again by the shiny copper block. An upper limit to the concentration of 0 2 produced photolytically in the ice can be estimated by assuming that all the exciting light is absorbed and that the quantum yield of 02 formation in the ice is unity. On this basis, after say 7 h of irradiation, the 0 2 concentration in the ice would not exceed 1.6 X 10" mol L-I. Equation 42 then shows that only ca. 170 photons s-1 of the exciting light would be absorbed by the oxygen dissolved in the ice sample. Assuming 100% conversion of this absorbed light to luminescence, then 170 photons s-l would be the upper limit for the total short-lived emission from the ice sample due to direct excitation by UV light. However, from the measured spectrum of the ice luminescence and the efficiency of the light detection system, the total radiant power of the short-lived luminescence (over 4 a sr) is ca. 1.8 X 108 photons s-l. This value is a factor of lo6 times greater than the upper limit calculated on the basis of direct optical excitation of 0 2 . It can be concluded that the luminescence from ice is not emission from 0 2 molecules excited directly by UV light. Experiment Reinforcing the View That the Herzberg I or I11 Emissions Do Not Arise from Optical Excitation of 0 2 in the Ice Lattice. If direct excitation of 0 2 was directly responsible for the ice emission bands, it would be expected that doping the ice sample with 0 2 would enhance the luminescence intensity considerably. To test this point, such an experiment was carried out. Instead of using the freeze-thaw degassing technique described in the Experimental Section, ice samples were prepared from water presaturated by bubbling with 02, and three replicate spectral scans were carried out on each of two separate ice samples. The mean spectrum obtained was then compared with the similar spectrum obtained from the three scans of two 02-free samples. No significant enhancement of the luminescence was observed when 0 2 was present.

10552 The Journal of Physical Chemistry, Vol. 97, No. 41. 1993

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UV-Excited

HzO and D20 Ices

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The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10553 (114) Rossetti, R.; Brus, L. E.J . Chem. Phys. 1979, 71, 3963. ( 115) During the course of these experiments, it was discovered that t r a m of amorphousice at ca. 2-pm thicknessformed upon the 4-mm-thickcrystalline ice substrate due to condensation of t r a m of H20 vapor present in the background gases of the vacuum system. Becauseamorphous ice has a greater luminescence quantum yield than other ice polymorphs,102 it is possible that thespectral measurements on our crystalline D20 ice might have been affected byisotopiccontaminationoftheD20surface. However, this point waschecked by carrying out a series of spectral measurements on freshly deposited amorphous H20 and D20 ices which showed that no isotopic shift occurred even in this extreme case. Therefore, the presence of any thin layer of amorphous ice does not affect the important conclusion that the short-lived emission from H2O ice is not wavelength shifted by substitution of D for H.

Oxygen luminescence from UV-excited water (H2O and D2O) ices

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