G. H. Morine and J. E. Willard
original N160 and that this was true regardless of the proportions of N160 and Cl80 used. Finally, although stable metal carbonates are reported to have infrared bands in the 1500-1300-~m-~spectral regionlg no such bands were observed in this work. This may indicate that in excess CO reaction 3 occurs rapidly or more probably it may indicate that our proposed carbonate type intermediate has absorption bands below the silica cutoff at 1350 cm-l. Acknowledgment. Financial support for this research was provided by the National Research Council of Canada and by Imperial Oil Ltd. References and Notes ,100
2200
\
1900
2000
1800
C M-'
Flgure 1. The dashed line indicates the background infrared spectrum of a NilSi02 sample. A compensating Si02 disk was placed in the reference beam in order to partially cancel out strong bands due to silica. The break in the spectrum at 2000 cm-' arises because of a grating change which requires new instrument settings: (A) after adding 10 Torr of NO and evacuating for 30 min; (B) after adding 10 Torr of CO, immediate scan. (C, D, E) after 15, 45, and 180 min, respectively.
quently C 0 2 [1:2:1] might be generated. At this point we do not know if we should have written NiCI80 in place of Cl8O in any of these equations, nor whether isocyanate formation only occurs after NiNO has decomposed into NiN and NiO. The latter process has been reported in the literature 8~16-18 and it is possible to devise a large number of schemes involving NiN, NiO, or NiCO which will produce the same result. The important point is that if eq 3 represents the final step leading to C02 formation, then the two oxygen-16 atoms must have come from the
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)
R. A. Dalla Betta and M. Shelef, J. Mol. Catal., 1, 431 (1976). F. Solymosi, J. Sarkany, and A. Schauer, J. Catal., 46, 297 (1977). M. L. Unland, J. Catal., 31, 459 (1973). M. L. Unland, J. fhys. Chem., 77, 1952 (1973); 79, 610 (1975). A. Hiromichi and H. Tominaga, J. Catal., 43, 131 (1976). H. Niiyama, M. Tanake, H. Iida, and E. Echigoya, Bull. Chem. SOC. Jpn., 49, 2047 (1976). M. F. Brown and R. D. Gonzalez, J . Catal., 44, 477 (1976). G. Blyholder and M. C. Allen, J . fhys. Chem., 69, 3998 (1965). S. V. Batychko, M. I. Rossov, and L. M. Roev, Dokl. Akad. Nauk SSSR, 191, 328 (1970). B. A. Morrow and P. Ramamurthy, J. fhys. Chem., 77, 3052 (1973). B. A. Morrow and I. A. Cody, J . Chem. SOC.,Faraday Trans. 1 , 71, 1021 (1975). M. Primet and N. Sheppard, J . Catal., 41, 258 (1976). R. R. Ford, Adv. Catal., 21, 103 (1970). K. Uematsu and W. Komatsu, J. Catal., 17, 398 (1970). G. El Shobaky, P. C. Gravelle, and S. J. Teichner, J . Catal., 14, 4 (1969). C. R. Brundle, J . Vac. Sci. Techno/., 13, 301 (1976). H. Conrad, G. Ertl, J. Kuppers, and E. E. Latta, Surface Scl., 50, 296 (1975). G. L. Price, 6.A. Sexton, and 8. G. Baker, Surface Scl., 50, 506 (1976). L. H. Lmle, "Infrared Spectra of Adsorbed Species", Academic Press, London, 1966.
Evidence on the Isothermal and Warm-up Luminescence from ?-Irradiated 3-Methylpentane Glass'i2 G. H. Morine and J. E. Willard" Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706 (Reoeived August 15, 1977) Publication costs assisted by the United States Department of Energy
The isothermal luminescence of y-irradiated 3-methylpentane glass at 77 K has been found to grow in proportion to the concentration of trapped electrons to at least 2 X 1019eV g-l, in contrast to the implication of reports in the literature. The appearance of luminescence peaks at two different temperatures during warm-up is evidence for neutralization of two types of cation-anion (or radical-radical) pairs, one of which is present in enhanced concentration in samples containing 02.The dose dependence of the integrated warm-up luminescence indicates that the cations and/or anions responsible for the luminescence at doses less than 1 X 1019eV g-l are different from those at considerably higher doses. The extent and limits of knowledge available for assignment of the luminescence peaks to specific elementary processes are discussed. Introduction Following exposure of glassy hydrocarbons to ionizing radiation, trapped electrons radicals,3band anions3c are observable at 1 7 7 K. Cations equivalent to the e; and anions must also be present, but no optical or definitive ESR spectra have been found for these in pure hydrocarbons, although cations of dilute additives such as olefins, biphenyl, and organic amines with known optical, ESR, or luminescent spectra are identifiable.3d The types of The Journal of Physical Chemistry, Vol. 81, No. 26, 1977
trapped cations which may be present in a typical y-irradiated hydrocarbon glass such as 3-methylpentane (3MP) include C6H14+,CJ-I13+,C6H16+,C12HB+,and cations of impurities present initially or as the result of radiation-induced decomposition. If the spectroscopy of these species in the glassy state were accurately known, it should be possible to identify them by the spectra of the isothermal, warm-up, and photostimulated luminescence4 emitted as a result of their neutralization by electrons and
2669
Luminescence from ?-Irradiated 3-Methylpentane Glass
anions in the y-irradiated samples. Attempts a t such identification have been inconclusive because of lack of knowledge of the luminescent spectra. In this paper we extend the information on isothermal and warm-up luminescence from y-irradiated 3MP glass, and discuss alternative interpretations. We also show that the growth in isothermal luminescence intensity with dose parallels the growth in e; concentration, in contrast to p r e v i o ~ s ~ ~ t ~ evidence.
Experimental Section Phillips Pure Grade 3-methylpentane was further purified by passage through a 70-cm column of freshly activated silica gel with collection under dry nitrogen, followed by pumping on the liquid at -75 "C for 10 min and freeze-pump-thaw cycles using liquid nitrogen. It was stored over Na-K alloy, LiA1H4,or Linde 1OX molecular sieve freshly activated under vacuum. Following such treatment the optical density at 200 nm was 10.08 for a 1-cm path. (Current practice in our laboratory indicates that equal purity is obtained by degassing and exposure to activated molecular sieve without silica gel treatment.) Samples for the luminescence studies (-3.5 mL) were prepared by distillation under vacuum into 1 cm X 1cm i.d. 5 cm high Suprasil cells. Measurements of the luminescence intensity were made with the y-irradiated sample mounted in a quartz Dewar with flat windows. The light was focused by a quartz lens onto the slit of a Bausch and Lomb high intensity monochrometer, or the desired wavelength range was selected by filters. Detection was by means of an RCA 1P28 photomultiplier tube coupled to an electrometer and chart recorder. The warm-up luminescence was determined during spontaneous warming following removal of the liquid nitrogen from the sample Dewar. The temperature of the sample was monitored by a thermocouple attached to the exterior of the cell. The spectra reported here were determined by scanning the monochrometer by hand, with alternate scan directions to compensate for luminescence growth and decay during scanning. For the isothermal luminescence, the spectrum was scanned in 5-nm steps and the band shape determined. For the warm-up luminescence, only the band maxima was determined. Results Isothermal Luminescence. The spectrum of the isothermal luminescence a t 77 K from 3MP following y irradiation at 77 K has been reported4 to have a small band with ,A, at 230 nmk and a much more intense broad band values of 425,4a9b1f415,4cand 380 nm4ehave for which A,, been given. Our value for the latter band is 420 nm. The samples were quenched in liquid nitrogen immediately before a 5-min 1X 1019eV g-l y irradiation and the spectral measurements were made during -10 min following irradiation. For determination of the dose dependence of the isothermal luminescence, the luminescence was monitored a t 400 nm at 77 K for 1h starting 6 min after irradiation of the 3MP a t 77 K (Figure 1). The samples were quenched to 77 K 25 min before the end of their respective irradiations, which varied from -3 to 13 min. The results (Figure 1) indicate an approximately linear increase in luminescence with dose to 2 X 1019eV g-l. To obtain the true relative luminescent intensities, correction has been made for the increase in self-absorption with dose, assuming the following: the average distance traveled by the photons in escaping the 1 cm X 1 cm cell is 0.5 cm; 54% of the e; decay during 1h over which the luminescence
Figure 1. Luminescence of y-irradiated 3MP at 77 K as a function of y dose. Each point indicates the luminescence integrated over 1 h starting 6 min after the end of y irradiation at a dose rate of 1.8 X 10" eV g-' min-'. The luminescence was viewed through a monochromator set to give a band pass at half-height of about 20 nm centered at 400 nm: (0)relative intensities uncorrected for self-absorption; (0) relative intensities corrected for self-absorption (see text); (0)normalized data of Funabashi et ai.4d 87K
91K
--L ._
+ Temperature
Figure 2. Warm-up luminescence from 3MP samples y irradiated at 77 K. (A) Degassed sample bleached 10 min with 100-W tungsten lamp at 5 cm following 1 X 10'' eV g-' y dose. (B) Sample saturated with air before cooling to 77 K; dose, 5 X 10'' eV g-'; no photobleaching. The recording of A was done with higher amplification than B. For each plot the cell luminescence was essentially constant in the temperature range directly under the peaks, giving a background intensity at the level of the high temperature end of the plot.
was integrated; G(e;) = 0.7; t(eJ at 1600 nm = 3 X lo4; p(3MP, 77 K) = 0.87 g ~ m - ~~(;4 0 nm)/t(1600 0 nm) = 0.08. The average absorption may be estimated from the average of the OD values at 400 nm at the start and end of 1h over which the luminescence was integrated. Warm-up Luminescence. Samples of pure degassed 3MP which have been y irradiated (5 X 1018-1 X 1020eV g-') at 77 K followed by bleaching the electrons with near-IR light show a broad structureless peak with maximum intensity at 86-87 K when warmed. The temperature and structure of the peak was independent of whether the luminescence was monitored at 360 or 400 nm with a monochromator or by the total light which passed a filter which eliminated wavelengths below 350 nm. The integrated warm-up luminescence intensity was approximately the same as the isothermal luminescence intensity integrated for 1h starting 6 rnin after irradiation. Exposure of y-irradiated samples to light in the 300700-nm range before warming reduces and, with prolonged exposure, removes the 86 K peak but exposes a much smaller peak with a maximum at -91 K (Figure 2A), which is only partially bleached on exhaustive illumination. The A,, of the spectrum of the 86 K luminescence is a t The Journal of Physical Chemistry, Vol. 81, No. 26, 1977
2670
-
JUJ
9:.
I
I
/
i
Flgure 3. Integrated warm-up luminescence and electrical conductivity of y-irradiated (77 K) 3MP as a function of dose to 2.5 X 10lgeV g-':
(0) luminescence, this work, measured at 400 nm; ( 0 )luminescence, ref 4d, measured at 425 nm; (0)electrical conductivity, 85 K peak, ref 19.
$l/ ' ' 0
I
1 i
a A xA ;
DOSE .v9-I
b
10-19
Figure 4. Integrated warm-up luminescence of y-irradiated (77 K) 3MP measured at 400 nm as a function of dose to 1.0 X 10''
eV g-'.
500 f 20 nm, and that of the 91 K peak 380 f 20 nm. For samples saturated with air before cooling to 77 K, the integrated warm-up luminescence is greatly increased, and the ratio of the 91 K peak to the 86 K peak is also greatly increased (Figure 2B). (It is worthy of note that the relative intensity of the 91 K peak is enhanced by the decreasing absorption of the luminescence by the sample as the reaction intermediates decay.) The freshly irradiated air-saturated samples are grey or black. This opacity, the 86 K luminescence and part of the 91 K luminescence, can be removed by exposure to light of a medium pressure Hg arc through a Pyrex filter. The growth in warm-up luminescence of the degassed samples of 3MP with increasing y dose was linear at doses above -2 X 1019 eV g-', but not at lower doses (Figures 3 and 4). For the experiments of Figures 3 and 4 the etwere bleached with 950-nm light prior to warm-up. The warm-up luminescence was monitored at 400 nm at warming rates of 2-4 K min-l and was integrated by weighing the peak cut from the recorder paper. Correction was made for the luminescence from the cell walls. Literature values for warm-up luminescence and warm-up electrical conductivity of 3MP following y irradiation a t 77 K are included in Figure 3. Discussion Correlation of Dose Dependence of Luminescence and e< Concentration. The concentration of e; in y-irradiated 3MP a t 77 K measured by ESR or infrared absorption increases linearly with dose at low doses, passes through The Journal of Physical Chemistry, Vol. 81, No. 26, 1977
G. H. Morine and J. E. Willard
a maximum at 1 X lozoeV g-', and then decreasesa5r6By contrast the isothermal luminescence intensity integrated over 15 min following irradiation has been reported to rise to a maximum at 2-4 X l0ls eV g-14d9fand thereafter remain constant to at least 1.4 X 1019eV g-1,4e If these observations are both correct, the apparent lack of correlation between e< concentration and luminescence intensity implies that the photon yield per electron which combines with a cation decreases dramatically with increasing dose above 2 X l O l s eV g-' indicating a changing mechanism of neutralization with increasing dose. We have, therefore, determined the growth in integrated isothermal luminescence with dose under conditions similar to those under which the growth in [e;] has been studied. The data plotted in Figure 1 show linearity to at least 1 X l O I 9 eV g-I and (after correction for the absorption of luminescence by the growing concentration of reaction products2 and some electron decay during irraeV g-', without evidence diation) near linearity to 2 X of the plateau onset at 2-4 X 1OI8 eV g-' previously observed. It appears that the plateau observed in the earlier work may have resulted from the competition between the production and decay of e; having reached a steady state at the relatively low dose rate used. The initial half-life for e; decay in 3MP at 77 K varies from -6 min in unannealed samples in ESR tubes and 25 min for unannealed samples in 1 cm X 1 cm cells to 60 min in well-annealed sample^.^ The dose rates used in the experiments where the plateaus were ~ b t a i n e dappear ~ ~ > ~to have been -2 X lo1' eV g-' min-l, requiring 20 min to reach 4 X 1OI8 eV g-'. The determinations of growth in e; concentration with dose were made in one case5 at a 15-fold higher dose rate, requiring a much shorter irradiation time, and in another6 at 71 K where et- decay is negligible. Above -2 X loL9eV g-l the rate of growth of e; concentration with dose decrease^.^ This is attributable to prompt capture of a portion of the e- by the growing concentration of trapped radicals in competition with stable trapping. In this dose region the molar heat of photobleaching of those e- which are trapped decreases with increasing dose,8 consistent with the conclusion that the fraction reacting with radicals rather than cations is increasing. It may be assumed that, in this dose region, the isothermal luminescence per mole of e; decaying, measured a t 400 nm, would decrease in parallel with the decrease in heat of reaction since the energy of attachment of electrons to radicals is too low (0.5-1 eW9 to produce 400-nm photons. W a r m - u p Luminescence. The data of Figure 2 show that radiation-produced ions and/or radicals which are quite stable in 3MP at 77 K migrate sufficiently rapidly at higher temperatures so that at a warming rate of -3 K min-' they all react at -95 K. A t least two discrete populations are indicated by the peaks at 86 and 91 K. The complete elimination of the 86 K peak by exposure of the sample to UV radiation is evidence that it results from reaction of carbanions with cations, the carbanions being removed3cby photodetachment when illuminated. The fact that the 91 K peak is only partially removed by pre-warm-up illumination suggests that it may be the result, in part at least, of radical-radical combination. The difference in photoeffects, the relative enhancement of the 91 K peak by the presence of oxygen, and the difference in A,, for the two peaks indicate that they are caused by different species. However, it should be noted that even if all the luminescence were caused by the same species, two peaks might be resolved in the warm-up spectrum, the first resulting from short-range intraspur encounters and
2671
Luminescence from y-Irradiated 3-Methylpentane Glass
the second resulting from random encounters required for reaction when the spurs are depleted. The contrast between the rates of intraspur reaction and random-encounter reaction is illustrated by these two modes of radical decay in y-irradiated 3MP glass.3b Such spatial control may contribute to the peak separation shown in Figure 2. Two peaks in the warm-up luminescence of y-irradiated a t -83 K and one a t -93 K, which 3MP, one with A,, are qualitatively similar to those of Figure 2, have been reporteda to have different rates of growth with dose. The first increases rapidly with dose below 2 X 10" eV g-l and relatively slowly thereafter. The second peak is negligible below 2 X l0l6 eV g-l but grows rapidly thereafter up to 1.4 x 1019eV g-l, the highest dose tested. A similar feature seems to be reflected in the plot of integrated warm-up luminescence a t 400 nm as a function of dose shown in Figure 4 where a rapid initial rise followed by a leveling off is followed by a renewed but slower rise. This suggests that the nature of the cation-anion population formed during the initial stages of irradiation is such as to yield more photons per neutralization event than are produced by the combining species a t higher doses. The nature of the predominant cationic or anionic species may change with dose because (1) a charge scavenging impurity is depleted; (2) the concentration of a charge scavenging stable product grows; (3) the solvent radicals, which increase in concentration linearly with dose,3b capture progressively more of the electrons or positive charge or both. The latter effect would be expected to become significant only a t concentrations high enough for spur overlap to be significant, There is evidence6for the onset of such overlap of electron spurs at C1.2 X 1019eV g-l in 3MP at 77 K. Sawai has reported that luminescence during warming 3MP to 83 K following y irradiation at 77 K extends from -390 to 500 nm with a maximum a t -425 nm. On heating to 90 K, an emission from -470 to 600 nm with a maximum at 530 nm appeared. The integrated 530-nm luminescence grew linearly with dose to 1 7 X lo1' eV g-', with no plateau, This dose is in the range which might appear linear on the initial rising portion of Figures 3 and 4. Species Which May Be Responsible for the Luminescence. The only reactions in y-irradiated organic glasses which are sufficiently exothermic to produce excited species which can luminesce in the visible-UV region are neutralization (of cations by either electrons or anions) and radical-radical reactions. For such luminescence to occur, the product molecule must have excited states available in the appropriate energy region. Both the spectrum and the probability of light emission from these excited states may be different in the glassy phase than in the gas or liquid. The glass provides a dielectric medium which (1) reduces the total energy of neutralization; (2) constrains free rotation, with the result that the excited molecules may luminesce from a geometry characteristic of the cation rather than the lowest excited state of the neutral molecule; (3) may reduce the decomposition of excited molecules, and hence increase the probability of luminescence from the undecomposed form. The influence which these solid state effects may have on the luminescence spectra, and the current lack of knowledge of the identity of the predominant cation in y-irradiated hydrocarbons, lead to considerable uncertainty in the assignment of the luminescence peaks to specific processes. Also, it is not known whether the luminescence is the product of neutralization of the predominant charged species or only a minor component of the cations and/or anions. There is
evidencelothat only a small fraction of the neutralization events (e.g., loM3)result in the emission of photons, but also evidence that for liquid hydrocarbonsll the low emission yield is due to a low fluorescence yield from the first excited singlet state, rather than to a low G value for the production of this state. Despite the uncertainities noted above, it is useful to consider what may be concluded from present knowledge about the elementary processes responsible for the luminescence, There is definitive evidence from ESR spectra of y-irradiated 3MP for the presence of secondary 3methylpentyl radicals12 and physically trapped electrons3a (produced with G values of 3.03b and 0.7,13respectively). Anions, presumed to be predominantly C6H13-,are indicated by their photodetachment which produces observab'e electrons3cand by their optical ab~orption.~ The possible cations include C6H14+,C6H13+(formed in the primary ionization event or by capture of migrating positive charge by C6H13radicals), C6Hl6;*(formed by proton transfer), C12H26+ (formed by C6H14 + C6H14 C12H26+ H2),and C6H12+ (formed by capture of positive charge by radiation-produced olefin molecules, or by C6H14+* C&12+ H2 where the asterisk indicates an excited state of the cation). It may be concluded with conviction, based on much evidence, that the isothermal luminescence from y-irradiated 3MP at 77 K results from neutralization of cations by e; which tunnel and/or diffuse from their traps. As noted above, the 86 K peak of the warm-up spectrum may be assigned with some certainty to the neutralization of cations by anions and it appears that the 91 K peak may involve contribution from both anion-cation and radical-radical combination. These species must be heavier and more slowly diffusing than those responsible for the 86 K peak. The ions may be oxy ions (e.g., C6H1302formed by reaction of 02-with C6H13or C6H13- with 0 2 ) and the radicals may be peroxy radicals (e.g., C6H1302. formed by reaction of C6H13- 0 2 ) , since the peak is greatly enhanced in the presence of 02. In carefully degassed samples the peak is small and is detectable only if a portion of the 86 K peak has been removed by UV exposure at 77 K prior to warm-up. This residual 91 K peak must result from (1)oxy ions or radicals formed from trace O2 remaining even after careful degassing; (2) C12H26+; (3) reactions of cations with anions from different spurs after the spur concentrations have been depleted. Some assignments of the luminescence peaks from y-irradiated 3MP glass to specific processes have been suggested in the literature. The 230-nm band observed by Merkel and Hamil14chas been attributed by them to fluorescence from the first excited singlet of C6H14on the basis of the ,A, of 231 nm reported14for the fluorescence from liquid 2-methylpentane when activated by 165-nm radiation. This assignment is plausible but it appears impossible with present knowledge to eliminate alternative possibilities: e.g., the fluorescence of C6H12* formed by (1)C&14+.+ e- C6H14*--+ C6H12* + H2, or (2) C6H13+ + e- C6H12* + H, or (3) C6H12 + a mobile positive hole C&12+, C6H12+ + e- CGH12*. It has been suggestedh that the latter process, involving reaction with a stable radiation product, is ruled out by the observation that the luminescence intensity reaches a saturation level with increasing dose4dand by the finding that when a sample is re-irradiated following melting and refreezing the luminescence is not greater than that from the initial irradiation.4cJ0aAs described above, it now appears that the reported saturation of luminescence with dose was simply a result of proportionality of the luminescence intensity
+
+
+
-+
+
--
-
+
The Journal of Physical Chemistry, Vol. 8 1, No. 26, 1977
2672
D. R. Rosseinsky and I. A. Dorrity
to the e< concentration in experiments where the latter reached a plateau at low doses. Likewise, the re-irradiation tests do not prove that radiation-produced olefins are unimportant since the olefin concentration in the melted and refrozen samples would be much lower than the local olefin concentration in the radiation-produced spurs. Thus the reasons for discarding excited olefins as the source of the observed luminescence do not now appear compelling. It has been reportedlo" that the presence of mole fraction of olefins or diolefins in alkane glasses greatly increases the luminescence observed after y irradiation and, in other work, that mole fraction of 2-methyl1-pentene in 3MP glass increased the lumine~cence.~~ The absorption spectra (So SI) of various olefins show red limits near 200 nm1.l5 The 230-nm luminescence of yirradiated 3MP is in the correct energy region to represent olefin fluorescence and has a lifetime
