J. A. GHORMLEY AND C . J. HOCHANADEL
40
imposed on this is a second spectrum which appears to have four lines, also with splittings of approximately 24 G.13 The number of lines and splittings as well as the rapid decay relative to the 3MP radical suggests the methyl radical, although C-C bond rupture is unexpected in such a system. The metal-photosensitized formation of radicals may be presumed to occur as a result of the transfer of energy from the neutralization process (M+ e- -t M*) to matrix molecules. Energy Threshold for Electron Production. The relative yields of the electron singlet as a function of metal irradiated and of wavelength (Figure 6) depend on the wavelength dependence of (a) the extinction coefficient of the metal, (b) the intensity of the AH4 lamp, and (e) the photoionization quantum yields. Despitte this combination of variables which affect the absolute yields, the relative yields of Figure 6 are adequate to show that the photoionization thresholds of all of the metals in the 3RIP matrix are lower than the gas-phase ionization potentials and higher (with the possible exception of AJg) than the photoelectric work function for removal of electrons from the solid (Table 11). It may he reasoned that the energy for ionization of isolat,ed metal atomsin 3MP must equal the gas-phase ionization potential minus the electronic polarization energies of the separated and cation in the medium plus thie repulsive energy for the electron in
+
Table I1 : Approximate Photoionization Thresholds in 3MP-Metal Matrices Compared to Gas-Phase Ionization Potentials and Photoelectric Work function^"^^ Thresh- c--Ionization old Mi Mz
Li Na K Mg Cd
5.1
3.4 2.8 3.1 5.9
5.4 5.14 4.34 7.65 8.99
5.15 4.9 4.0
... ...
potentialsc-------. Ms
Work
M4
function
2.3-2.4 2.27 2.24 3.6-3.8 4
...
..
3.9 3.4
4.2 3.6
... ...
...
...
P. J. Foster, R. E. Leckenby, and E. J. a In units of eV. Robbins, Proc. Phys. Soc., London (At.Mol. Phys.), 2,478 (1969). c For the species with one to four metal atoms.
the medium. If the metal atoms in the experiments of Figure 6 are monatomic or in aggregates of only a few atoms, the polarization energies must exceed the repulsive energy, to be consistent with the data of Table 11. It seems possible, however, that many or all of the metal atoms exist as larger agglomerates. (12) D. J. Henderson and J. E. Willard, J . Amer. Chem. Soc., 91, 3014 (1969)(13) Work of L. Glasgow in our laboratory shows that photolysis of carefully purified 3MP glass a t 77OK with 185- or 254-nm radiation produces 3MP radicals and a species which gives some added structure to the esr signal. This is presumed to result from absorption of radiation by difficultly removed oxygen complexes. Similar spectra have been observed when deposits of pure 3Mp were irradiated on the cold finger used in the present work, but their intensity for identical irradiations was less than one-tenth that observed for the 3MP-Na matrices.
Production of H, OH, and H202in the Flash Photolysis of Ice1 by J. A. Ghormley and C. J. Hochanadel" Chemistry Division, Oak R&e National Laboratory, Oak Ridge, Tennessee 67830 (Received August 94, 1970) Publication costs assisted by the Oak Ridge National Laboratory
NQabsorption by trapped electrons was observed in the flash photolysis of ice at - 10" by light above 1600 k although electrons had previously been observed on flashing water at 24'. Absorption in the ultraviolet, produced during the flash, was very similar to that observed in the flash photolysis of water and is attributed to 3HT and OH radicals.
We had prevjoiisly observed2 the formation of hydrated electrons (also H, OH, and H,Oz) in the flash photolysis of pure water a t 24" with photons having energy 26.5 eV (1920 A). The overall reaction
be stabilized in preexisting traps (configuration of suitably oriented water dipoles). It was of interest, therefore, to examine the possible formation of trapped electrons by the flash photolysis of pure ice.
2€[20,, -+ea,OH,, H3OSq+ (1) is endothermic by about 5.8 eV, and for the hydration energy to be available in time requires that the electron
(1) Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corp. (2) J. W. Boyle, J. A. Ghormley, C. J. Hochanadel, and J. F. Riley, Rudiat. Res., 31, 582 (1967); J . Phys. Chem., 73, 2886 (1969).
+
The Journal
of
+
Physical Chemistry, Vol. 76,No. 1 , 1971
PRODUCTION OF H, OH, AND HzOz IN TIIE FLASH PHOTOLYSIS OF ICE An extensive study of the pulse radiolysis of pure ice a t various temperatures wm made recently by Taub and Eiben.a l n ice irradiated a t temperatures below - 56" they observed optical absorption bands with peaks near 6700,2800, and2300 8 which they attributed to solvated electrons, OH radicals, and HOz radicals, respectively. ilt -14" only the 6700- and 2300-A peaks were observed. Sliubin, et u Z . , ~ arid Nilsson, et uLj5also studied the trapped electron produced by the pulse radiolysis of ice. They all found a broad absorption barid with a maximum (-6800 A at - 10") in the same spectra! range as in liquid water (-7200 8 at 25')) indicating about the same trapping energy in both phases. The b a d is narrower in ice than in water, and the extinclion coefficient at, the maximum may be about double that in water.8 Taub and Eiben3 and also Nilsson, et ~ l . found , ~ the yield of trapped electrons to be temperature dependent, decreasing rapidly from - 5 to --40" and then slowly at lower tempcratures. G vnlues at - 5 , -40, -130, and -196" nwe approximately 0.3 0.07, 0.003, and 0.0006, compwed wiih -3 for liquid water at room temperature. l u the y radiolysis of ice a t -196", Ghormley and Stewart6 observed a large peak at 2800 with a small shoulder at 2800 A. On warming, the two bands annesled out simultaneously in two temperature ranges. Taub and Eiben3 observed that neither the buildups at 196" nor the annealings of the tmo bands after pulse radiolysis at - 56" were simultaneous.
-
Experimental Section Beautifully clear cylindrical samples of ice were prepared by slowlv lowering (1 cm/hr) sealed ampoules of degassed water xito a bath thermostated at -1.5". The samples were then annealed at this temperature at lea& overnight befort. using. The water was purified by several distrllations as before2and degassed by purging with helium, followed by several freeze-pumpthaw cycles. After removing the ice from the tube, about 1-2 cm was removed from each end, and the ends were ffattmed with a smooth aluminum plate giving an optically clear sample about 23 cm long X 2.2 cm in diameter. The sample was skspended between two 25 cm long flash lamps imide a magnesium oxide coated reflecr or. The temperature df the sample ww held a t -10 f 2" by a stream of coid nitrogen blowing throngh the refle~tor.~The flash was usually a 24-kV discharge (288 J) in Xe at 15Torr, with a duration of -3 ctsec. The measuring equipment was described p r e v i o ~ s l y . ~Thc ~ ~ absorption spectra produced by the flash were talcen spectrophotometrically point by point. Measurements UTE made using the mercury lines emitted by a Hanovia 200-W Hg-Xe lamp (901B-1), or the Xe continuiim from a Hanovia 300-W Xe lamp (91463)-1), For some measurements the analytical lamp was pulwd by discharging a capacitor through the
41
1800
1900
WAVL LENGTH
2000
2IOO
(A)
Figure 1. Absorption spectrum of ice at several temperatures compared with t h e spectrum of liquid water. The absorption spectrum of oxygen-saturated water
(1.24 X
hf) is shown also.
glowing lamp, thereby increasing the light output by a factor of -50 for the xenon lamp or for the Hg-Xe lamp at wavelengths off the Hg lines. It is known that the absorption spectra of liquid waterlo and ice11s12are shifted to shorter wavelengths relative to water vapor.13 The peiLks occur at 1500 and 1670 A in the liquid and vapor, respectively. The long-wavelength cutoff of ice relative to liquid water was of interest to us and had not been measured accurately before. We measured the absorption spectrum of pure ice at several temperatures using a Model 15 Gary spectrophotometer purgcd with nitrogen. The ice samples (10 cm long X 2.2 cm in diameter) were cooled below the debwd temperature and then, with therniocouple attached, were placed in an insulating block and the spectra run while the samp1t.s warmed slowly. The spectra are shown in Figure 1, along with the absorption spectra of helium-purged and oxygen-mturated water. The water spectra wrre measured in 10(3) I. A . ' h u b and I(.Eiben, J . Chem. Phya., 49, 2499 (1968). (4) V. N . Shubin, V. A . Zhiynov, V. I. Zolot,arevsky, and 1'. I, D o h , Nalure (London), 212, 1002 (1966). (5) G. Nilsson, If. C. Christensen, J. Fenger, 1'. i'agskrg, and S , 0. Nielson, Advan. Chem. S e r . , No. 81, 71 (1968). (6) J. A. Ghorrnley and .4.C. Stewart, .I. A m c r . Chem. Soc., 78, 2934 (1956). (7) Our first saniples of ice were prepared !,y slowly lowering a n ampoule of water into a bath a t - 5 O . We t,hen tried t u make mensurements while the sample was melting. However, on warming to Oo the samples developed a crazing throughout which great,ly reduced
the light transmission. (8) C . J. Hochsnadel, J. A . Ghormley, and J. W . Bo,-le, J . Chem. Phgs., 48, 2416 (1968). (9) C. J. IIochunadei, J. A. Ghorrnley, J. W. Boylc, and J. 1:. liiloy, Rea. Sci. Instrum., 39, 1114 (1968). (10) L. R. Painter, R. D. Birkhoff, and E. T. hm!rawii, J . Chem. I'hys., 51, 243 (1369). (11) K. Dreasler and 0.Schnepp, ibid., 33, 270 (1960). (12) It. Onalca and T. Tdkahashi, J. Phys. Soc. Jup., 24, 548 (1968). (13) K. Watanabe and M. Zeiikoif, J. Opt. &c. Amer., 43, 753 (1953).
The Journal of PhgsicdL Chemistr,y, Vol. 76, No. 1, 1971
J. A. GHORMLEY AND C. J. WOCHANADEL
42
>-
I
1
I,
I
WAVE LENGTH ( A )
Figure 2. Absorption spectrum produced by the flash photolysis of pure crystalline ice at - 10". The upper curve was measured -40 psec after the start of the flash and the lower curve -8 msec afterward. Values for two different samples are plotted together, normalized to the same 0.d. at 2650 A to compensate for a 20% difference in measured 0.d. Two other samples, measured in the range 2300-3550 A, gave the same spectrum. Samples were 23 cm long x -2.2 cm in diameter. The flash was 288 J at 24 kV.
cm cells with Suprasil windows. The blank (mostly reflection from the cell windows) was taken to be half the reading for the empty (Nz-filled) cell. All spectra were run against NZ in the reference beam. The absorption by ice was ysumed to be the same as that by liquid water at 2100 A. The observed shift of the spectrum of liquid water to shorter wavelengths as the water i s cooled is in agreement with measurements of Weeks, et a1.I4 On changing from liquid to ice the spectrum shifts -70 A. The flash lampsgwere constructed of pure fused silica (Suprasil) which transmits -95% of the light above 1600 b. The ice sample, because of the shift in spectrum, absorbs much less of the flash energy than does liquid water.
Results and Discussion Within the limits of our sensitivity, we could detect no absorption (
