10715
J. Phys. Chem. 1993,97, 10715-10719
Formation of Excited OH Radicals in High-Energy-Electron-IrradiatedIce at Very Low Temperature Tetsuo Miyazaki,' Shigeru Nagasaka, and Y asunobu Kamiya Department of Applied Chemistry, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan
Katsumi Tanimura Department of Physics, School of Science, Nagoya University, Furo-cho, Chikusa- ku, Nagoya 464-01, Japan Received: June 7 , 1993; In Final Form: August 4, 1993"
-
We study the luminescence from ice excited by irradiation with a 2-MeV electron pulse a t 8 and 12 K. The 330-nm emission that is ascribed to the A22+ X2ntransition in the excited hydroxyl radical is also observed for DzO ice, but the emission intensity is about one-half that from H2O ice. In order to clarify the mechanism of the formation of excited hydroxyl radical, the effect of additives on the emission has been examined. The emission intensity decreases upon the addition of HCl (1 mol %), while it is not affected upon the addition of N a O H (1 mol 7%). The emission is completely suppressed by the addition of CzH50H (20 mol 7%). The emission from ice at 8 K decays in a time of 27-35 ps, which corresponds roughly to the reported decay lifetime of trapped electrons in irradiated ice a t 6-7 K. It is concluded from the effects of additives and the lifetimes of the emission that the excited hydroxyl radicals may be formed by neutralization of trapped holes (H*O+) with trapped electrons.
Introduction The electronic excited states of ice and their reactions have been a subject of extensive studies in the field of radiation chemistry and radiation physics as well as radiation biology. The details of the electronic manifold of ice are still subjects of debate.' The kinetic behavior of excited states of ice has not been elucidated as yet. A number of studies on the luminescence from electronirradiated ice has been carried out previously.2 All of them except for the recent 01103 were limited to a temperature range above 77 K. The most intense emission at 77 K has been observed around 380 nm. The origin of the luminescence is still controversial; several models have been proposed such as phosphorescence from the lowest quartet state of OH radicals? the 3B1 'AItransition of H20?the transition from the excited OH- ions,s and the C'BI A'BI transition of H20.6 Quite recently Miyazaki et al. have observed a new strong emission around 330 nm from electronirradiated ice at temperatures below 20 K.3 The 330-nm emission has been ascribed to the AZZ+ X211transition in the excited OH radicals. It has been shown that the emission increases with increasing accumulated dose of electron pulses, indicating that it is caused by trapped species produced by electron irradiation. However, the mechanism of the formation of the excited O H radicals could not be discussed previously because of scanty information. In order to clarify the mechanism of generating the 330-nm emission at very low temperature, we have studied here the isotope effect and the effect of additives on the emission. The mechanism of the formation of the excited OH radicals in the irradiated ice will be discussed.
-
-
-
Experimental Section H20 used in this study was prepared by triple distillation from alkaline permanganate solutions in order to remove organic impurities. D20 was more than 99.8 mol 7'% in purity. Cylindrical
* Author to whom correspondence should be addressed.
Abstract published in Advance ACS Absrracrs, September 15, 1993.
0022-3654/93/2097- 107 15!§04.00/0
"
300
400
500
Wavelength, nm
Figure 1. Emission spectra of electron-irradiated ice at 8 K. (0)D20. (-)
Hz0.
ice samples (9 cm long and 0.8 cm in diameter) were prepared by freezing degassed water. The ice sample was then placed in the irradiation chamber of a cryostat. The purity of C ~ H S O H was more than 99.4 mol %. Pulse irradiations were made by using 2-MeV electron beams generated with a Febetron 707 accelerator. The pulse duration was 20 ns. The radiation dose delivered by each pulse is about 0.6 kGy. The energy of electrons from the accelerator was nearly constant during the present experiment, where the number of irradiation pulses is less than ca. 50. The temperature of the sample could be controlled to a given value within f0.5 K by the use of an Oxford CF 1204 cryostat utilizing liquid helium. Light emitted from the irradiated ice was passed through a monochromator and detected by a photomultiplier tube using two methods. The first method comprises photocurrent measurement using an ordinary oscilloscope. The second method is a one-shot single-photon-counting system that was described in the previous paper.3
Results When D20 and H20 specimens are irradiated with an electron pulse at temperatures below 20 K, the luminescence is observed in the ultraviolet region. Figure 1 shows the emission spectra from D20 (open circles) and from H2O3 (solid curve) upon 0 1993 American Chemical Society
Miyazaki et al.
10716 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993
6 0
0.11
0
'
'
20
'
"
40
'
60
"
Time, yS
Figure 2. Time dependence of the 330-nm emission from ice at 8 K. (0) Difference between the emission from H20 ice preirradiated with 6 electron pulses and the emission from fresh HzO ice. (0)Difference between the emission from D20 ice preirradiated with 10 electron pulses and the emission from fresh D2O ice.
( . . . I J I . I . I I . .
OO 5 10 Number of Irradiation Pulses Figure 4. Effect of the accumulated dose (Le., number of electron pulses) of electron-pulse irradiation on the 330-nm emission at 12 K. (0)Mean value of four samples of H20 (cf. Figure 3). (A,V) HzO-HCI (1 mol % HC1).
I . I l . l l ( l l l l l j j OO 5 10 Number of Irradiation Pulses
Figure 3. Effect of the accumulated dose (Le., number of electron pulses) of electron-pulse irradiation on the 330-nm emission at 12 K. (b, 0-, 9 , -0) H20. (A,0 ) D20.
irradiation by electron pulses at 8 K. As described later, the intensity of the luminescence from D20 increases with increasing number of electron pulses preirradiated, similar to the case of H20 reported previously. Therefore, the spectrum was measured after sufficient preirradiation of a fresh sample in order to avoid possible errors which come from the pulse-number dependence in intensity. It is clear from the results shown in Figure 1 that the emission spectrum from DzO is the same as that from H20. The peak of the luminescence band is located around 330 nm, which is approximately similar to the wavelength (-310 nm) corresponding to the maximum emission intensity of the hydroxyl radical in solid KBr at 77 K.' Figure 2 shows the decay curves of the 330-nm emission a t 8 K from H20 (denoted by squares) and D20 (denoted by circles). The 330-nm emission is caused by the trapped species produced by preirradiation of a ample.^ In order to clarify the emission that is related to the trapped species, the emission from H20 shown here is the difference between the emission from the sample, irradiated with 6 electron pulses before measurement, and the emission from a fresh sample. The emission from D20 is the difference between the emission from the sample, irradiated with 10 electron pulses before measurement, and the emission from a fresh sample. It is evident from the Figure that the decay time constant is slightly shortened in D2O. In Figure 3, we plot the intensity of the 330-nm emission from D20 and from H20 as a function of the accumulated dose of electron pulses, Le., the number of electron pulses deposited. The intensity is the total emission intensity during a decay of the emission. For H20, results obtained for four specimens are shown by different circular symbols (6, 0-, 0 , -O),while results for two specimens of D20 are shown by triangular symbols (A,V). Although the results for different specimens fluctuate, it is clear that the rate of enhancement of the intensity, namely the growth rate, is more strongly reduced in D20 than that in H20. The
/ I I I . I I . . . I . . . OO 5 10 Number of Irradiation Pulses
Figure 5. Effect of the accumulateddose (Le., number of electron pulses) of electron-pulse irradiation on the 330-nm emission at 12 K. (0)Mean value of four samples of H20 (cf. Figure 3). (A,V) H2O-NaOH (1 mol % NaOH). 6 0
;
0 0
5 10 Number of Irradiation Pulses
Figure6. Effect of the accumulated dose (Le., number of electron pulses) of electron-pulse irradiation on the 330-nm emission at 12 K. (0)Mean value of four samples of H20 (cf. Figure 3). (A,V) H20-CzHsOH (20 mol % C2H50H).
result indicates that an isotope effect is involved in the excitation of the 330-nm emission. In order to obtain more information on the 330-nm emission, we measured the effects of additives on the excitation yield of the 330-nm emission. To evaluate the excitation yield, we measured the intensity of the emission as a function of electron pulse preirradiated. In Figures 4-6 are shown the results for adding HCI, NaOH, and C2HsOH in H20 specimens, respectively. In these figures, the average value of the results for the four samples shown in Figure 3 is plotted as the emission intensity from pure H20. Figure 4 shows the result for H20 doped with HCI a t 1 mol 5% that corresponds to pH = 2.5. Measurements were made for two samples; the results were represented by A and V. It is evident that the growth rate is suppressed significantly by adding HCI. Figure 5 shows the effects of adding NaOH at 1 mol % that correspond to pH = 11.5. Also, two samples were used for measuring the emission intensity from H20-NaOH (1 mol %).
Formation of Excited OH Radicals in Ice
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10717
In contrast to the case of HC1, no effects are induced in the growth rate by adding NaOH. Figure 6 shows the results for a specimen doped with C2HsOH at 20 mol %, obtained for two samples. It is noted that the effect of accumulated dose is completely suppressed by this additive.
Discussions Mechanism of Formation of Excited Hydroxyl Radicals. As shown in Figure 1, the emission spectrum from DzO is essentially the same as that from H20. Since the electronic states for the OH and OD radicals are similar,8 the present result is consistent with our assignment that the 330-nm emission is due to the transition from A2Z+to X211of excited O H radicals. As isotope effects on the 330-nm emission, we note the following two features. The first is the suppression of the growth rate of the intensity upon an accumulated dose of electron pulses, shown in Figure 3. The increase of the emission intensity from D2O upon the preirradiation is much smaller than that from H2O; the emission intensity from D20 is about one-half that from H20. The second feature of the isotope effects is shortening of the decay time. As portrayed in Figure 2, the decay curve of the 330-nm emission is described roughly by first-order kinetics except for the initial part at less than 10 1.1s. The time constants for the emission from H20and D20 are 35 and 27 fis, respectively. These two features, together with results of the effects of additives, give important implications in elucidating the mechanism of the formation of excited OH radicals upon electron irradiation. The intensity of the 330-nm emission increases with increasing accumulated dose of electron pulses. This characteristic indicates clearly that the excited OH radicals are formed through a reaction involving some trapped species, denoted here as P, produced by preirradiation. Therefore, the reactions leading to the excited OH radicals are described as follows:
--
H,O
P X
(1) (2)
P+X-OH* OH* -OH
(3)
+ hv
(4)
The trapped species, P, formed by preirradiation and other entities, X, generated by ionizing radiation produce an excited hydroxyl radical, OH*, that emits 330-nm light. Then the primary problem is what are P and X. The properties of P,which have been clarified in the previous study,3 are as follows: (a) it is stable for more than 1 h after irradiation a t 6 K and (b) it decays out thermally above 77 K. Below we examine possiblecandidates for the trapped species in order, based on these properties and the results obtained in the present study. 1. OH radicals: If OH radicals are responsible for the formation of the excited OH radicals, the following mechanism may be possible:
-
OH + H 2 0
4K 77 K
H20
--
H20* + OH
OH
(5)
H20*
(6)
H 2 0+ OH*
(7)
The excitation transfer from excited ice to an OH radical will produce an excited OH radical. The previous study3 of the annealing effect on the preirradiated ice indicates that the trapped species, P, decay completely at 77 K. O H radicals produced in
+ H30+
(8)
--
prcirradiation
H20 H20
H
(9)
H20++ e-
(10)
+ H2
(12)
OH*
Trapped H atoms produced by the preirradiation capture mobile holes (H2O+) to produce H30+ ions, which neutralize with electrons to produce excited hydroxyl radicals. When HC1 is added to H20, the reaction (12) should be enhanced by the presence of a large amount of H30+ions. The C1- ions do not affect reaction 12. The results in Figure 4, however, show that the 330-nm emission is suppressed upon the addition of HCl. Therefore, reactions 9-1 2 are not responsible for the formation of excited OH radicals. The neutralization of H3O+ and e- in ice will produce only H atoms by the following reaction:ls
H 3 0 ++ e-
-
H + H20
-
OH-
(13) 5. OH- ions: OH- ions may be formed in irradiated ice by an electron-capture reaction of O H radicals, and excited hydroxyl radicals may be formed from the OH- ions by the following reactions: prcirradiation
H20
preirradiation
H20
0-
More than 90% of the initial yields of 0-ions are measured again a t 4 K after the annealing at 77 K.13 Therefore, the trapped species P that decay out completely at 77 K are not ascribed to the 0- ions. 4. H atoms: H atoms produced in ice are stable at 4 K, while they decay completely a t 77 K.14 A tentative mechanism for the formation of excited hydroxyl radicals (OH*) by H atoms was proposed previously.3
H30++ e--
preirradiation
H20
the irradiated ice do not decay at 77 K but around 100 K.9 Thus, OH radicals are not responsible for the trapped species P. 2. Trapped electrons: Since trapped electrons produced in ice decay fast in a time less than several hundreds of microseconds at 6 K,loJ1the trapped electrons are not the trapped species P, which are stable for more than 1 h at 6 K. 3. 0-ions: When H20 (or D20) ice is irradiated with y-rays (or X-rays) a t 4 K, the ESR spectrum at g = 2.08 is observed and assigned to 0- ions.12 The spectrum disappears completely upon annealing the irradiated iceat 77 K, but the spectrum appears again at 4 K after the annealing.13 The reversible change of the spectrum was interpreted in terms of the following equilibrium reactions:
OH- + H20+
-
OH*
+ H20
(14)
(15)
OH- ions, produced by preirradiation, react with mobile holes (H20+) and produce OH* radicals. When NaOH a t 1 mol % is added, the concentration of OH- ions is ca. 500 times larger than that of OH- ions produced by preirradiation of 10 electron pulses, where the G-value of OH- ions is assumed as 2. Then, a large yield of OH* radicals, i.e., the strong 330-nm emission, will be expected in the H20-NaOH (1 mol % NaOH) mixture without preirradiation. It is noted that Na+ ions do not react
Miyazaki et al.
10718 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993
with holes (H20+) or, probably, with electrons since Na+ ions scarcely react with hydrated electrons in water.16 Figure 5 shows that the emission from the H20-NaOH (1 mol % NaOH) mixture without preirradiation is very weak. Therefore, OH- ions do not correspond to the trapped species P. 6. Trapped holes (H20+t): So far, we examined several radiolysis products as candidates for P, which is responsible for the formation of excited OH radicals, but none of them were the trapped species; experimental results of the present study were not consistent with the models based on them. We now propose the model that P is a trapped hole (H20+,), and a mechanism of the formation of excited OH radicals will bediscussed. In general, H20+ ions produced by irradiation react rapidly with H2O to form H 3 0 +and OH. A part of the H2O+ ions, however, may be trapped in defects produced by irradiation of ice at very low temperature. H20
preinadiatbn
H20+
+ e-
The trapped hole (H20+,) recombines with a trapped electron to form an excited H20 molecule (H20*). The discussion on the lifetime of trapped electrons to follow suggests that a counterpart of the trapped hole in the recombination reaction is not a mobile free electron with a very short lifetime but a trapped electron. When the excited state of H20 is above 9.1 eV, the excited OH(A2Z+) radical is produced by the dissociation of the excited H20 (reaction 2l).I7 Below we examine the mechanism in detail, basedon the present experimental results. When C2HsOH is added to ice, C Z H ~ O H can react with both electrons and holes. When a HZO-CZHSOH (20 mol % C2Hs) mixture is irradiated a t 77 K by y-rays, the formation of trapped electrons is suppressed, whereas H atoms are produced complementarily by reaction 22.18 The proton
C,H,OH
+ e-
-
C2H,0- + H
(22)
affinitieslg of CzHsOH and H2O are so large that the following proton and H-atom transfers (reactions 23 and 24) are exothermic reactions. Thus, reactions 23 and 24 may be possible from an
H20++ C,H,OH
HzO++ C,H,OH
-
OH + C,H,OH,+
H30++ C2H50
TABLE I: Lifetimes of 330-nm Emission, Trapped Electrons, and Excited Hydroxyl Radicals species
lifetime, ps
330-nm emission in H20 at 8 K 330-nm emission in D2O at 8 K ei,- in D2O at 6 and 7 K e,.b- in D2O at 6 K radiative lifetime of OH*(%+) radiative lifetime of OD*(*Z+)
27 27;' 476 66 0.7;c0.8d 0.7;e O.gd
35
Quoted from ref 10. Quoted from ref 1 1. Quoted from ref 21. Quoted from ref 8.
by reactions 20 and 21, the lifetime of the emission corresponds to three processes: a time for reaction 20, a lifetime of the excited water molecule (H20*), and a lifetime of the excited hydroxyl radical (OH*). In order to produce OH*(A2Z+) radicals, the excited state of HzO should be above 9.1 eV,17 which is higher than the energy (ca. 8.5 eV) of the first-excited state ('BI) of H20 in ice.lq20Then, the lifetime of the highly excited state that is responsible for the production of the OH*(A2Z+)radical may be much shorter than 1 ns. Thus, the lifetime of the 330-nmemission cannot beascribed to the lifetime of the excited molecule (H2O*). The radiative lifetimes of OH*(A2Z+)and O D * ( A W ) radicals weremeasured previously as 0.7-0.8 ~ s , ~which J are much shorter than the lifetimes (27 and 35 ps) of the 330-nm emission (cf. Table I). When crystalline ice is irradiated a t very low temperatures by high-energy electrons, two kinds of trapped electrons are produced: one (&is-) has a visible absorption band with A, = 630 nm, and the other (q;) has a broad infrared band with A, > 2500nm.10J1 Thedecay ratesof the twokindsoftrappedelectrons are very different a t 6-7 K. As is shown in Table I, the lifetime (6 ps) of &is- a t 6 K is shorter than the lifetime (27 ps) of the 330-nm emission in D2O at 8 K. It is very interesting that the lifetimes (27 ps in D20, 35 ps in H20) of the emission are approximately similar to the lifetimes (27 and 45 ps) of ei,-. Thus, the lifetimes of the 330-nm emission correspond to the rate for reaction 20, where a counterpart of the trapped hole in the recombination reaction is q,-.
+
H20+, ei,-
-
H20*
It is concluded that trapped holes (HzO+J produced by preirradiation react with electrons (q,-)in the radiolysis of ice a t very low temperature to produce the highly excited H2O molecules, and then the highly excited molecules dissociate into excited OH* radicals that emit light at 330 nm. The trapped holes may disappear by reactions upon the annealing of the irradiated ice a t 77 K.
(23) (24)
energetic point of view. If reactions 22,23, and/or 24 will take place in competition with reaction 20, ethanol suppresses the formation of excited H20. As shown in Figure 6 , the 330-nm emission, Le., the formation of excited hydroxyl radicals, is suppressed completely upon the addition of ethanol. When HCl is added, H30+from HCI reacts with electrons (reaction 13) and interrupts reaction 20. Thus, the emission intensity decreased upon the addition of HC1 (cf. Figure 4). As was noted in the previous part, NaOH may not react with H20+ ions or electrons, resulting in no effect on the emission (cf. Figure 5). Thus the effects of additives on the formation of the 330-nm emission obtained in the present study are all consistent with the model proposed above. NOW,we will discuss the lifetime of the 330-nm emission. The lifetimes from H20 and D2O at 8 K are 35 and 27 ps, respectively (cf. Figure 2 and Table I). If excited OH radicals are formed
Acknowledgment. This work was supported in part by a Grantin-Aid for Scientific Research from the Japanese Ministry of Education, Science and Culture. References and Notes (1) Michaud, M.; Cloutier, P.; Sanche, L. Phys. Reu. A 1991,44,5624. Related papers on electronic states of ice are cited therein. (2) Quickenden, T. T.; Trotman, S.M.;Sangster, D. F. J. Chem. Phys. 1982, 77, 3790. Related papers on emissions from irradiated ice are cited therein. (3) Miyazaki, T.; Kamiya, Y.; Fueki, K.; Yasui, M.; J . Phys. Chem. 1992, 96,9558. (4) Merkel, P. B.; Hamill, W. H. J . Chem. Phys. 1971, 54, 1695. ( 5 ) Buxton, G. V.; Gillis, H. A.; Klassen, N. V. Can. J . Chem. 1977,55, 2385. (6) Vernon, C . F.; Matich, A. J.; Quickenden, T. I.; Sangster, D. F. J. Phys. Chem. 1991, 95,7313. (7) Maria, H. J.; McGlynn, S.P. J. Chem. Phys. 1970, 52, 3402. (8) Elmergreen, B. G.; Smith, W. H. Astrophys. J. 1972, 178, 557. (9) Siegel, S.; Baum, L. H.; Skolnik, S.; Flournoy, J. M. J. Chem. Phys. 1960, 32, 1249. (IO) Wu, Z.; Gillis, H. A.; Klassen, N. V.; Teather, G. G.; J . Chem. Phys. 1983, 78, 2449.
Formation of Excited OH Radicals in Ice (1 1) Kawabata, K.; Buxton, G. V.;Salmon, G. A.; Chem. Phys. Lett. 1979, 64, 487. (12) Symons, M. C. R.J. Chem. Soc., Chem. Commun. 1980,675. (13) Johnson, J. E.; Moulton, G. C. J. Chem. Phys. 1978,69, 3108. (14) Flournoy, J. M.; Baum, L. H.; Siegel, S. J. Chem. Phys. 1%2,36, 2229. (1 5) Muto, H.; Matsuura, K.; Nunome, K. J. Phys. Chem. 1992,96,5211. (16) Baxendale, J. H.; Fielden, E.M.;Capellos, C.; Francis, J. M.;Davies, J. V.;Ebert, M.; Gilbert, C. W.; Keene, J. P.;Land, E. J.; Swallow, A. J. Nature 1964, 201, 468.
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10719 (17) Beenakker, C. I. M.; de Heer, F.J.; Krop, H. B.; Mohlmann, G. R. Chem. Phys. 1974, 6,445. (18) (a) Hase, H.; Kevan, L. J. Phys. Chem. 1970,74,3355. (b) Ito, Y.; Higashimura,T., The 23rd Symposiumon RadiationChemistry,Kyoto, Japan, Oct. 1980. ( 19) Lias, S. G.; Ausloos, P. Ion-Molecule Reacrions; American Chemical Society: Washington, DC, 1975; Chapter 5. (20) Shibaguchi, T.; Onuki, H.; Onaka, R. J. Phys. Soc. Jpn. 1977, 42, 152. (21) German, K. R. J . Chem. Phys. 1975, 63, 5252.
