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The photoconversion of solvated electrons into trapped electrons in ethanol glass at 6°K
Volume 27, number 4
CHEMICAL PHYSICS LETTERS
THE PHOTOCONVERSION
OF SOLVATED IN ETHANOL
ELECTRONS
IS August 1974
INTO TRAPPED ELECTRONS
GLASS AT 6°K
Lewis M. PERKEY, FARHATAZIZ
* and Robert R. HENTZ
Department of Chemistry and the Radiation Labomtory **. University of Notre Dame. Notre Dame, Indiana 46556. USA
Received 14 May 1974
The 7 irradiation of an ethanol glass at 77°K produces solvated electrons (e,) which have an absorption spectrum = 520 nm at 6°K. The conversion of es- into trapped electrons which absorb in the infrared (i.e., electrons in unrelaxed traps denoted by et) has been observed in bleaching es- at 6“K with light of A < 436 nm. with hax
1. Introduction Higashimura and co-workers [l] have found that y irradiation of ethanol glass at 4°K produces an optical absorption spectrum with a wavelength at the absorption maximum, Amax, of 1500 nm. They ascribe this spectrum to electrons in shallow unrelaxed traps which are denoted by e,. With increase in temperature to 77°K the spectrum with X,, = 1500 nm converts to a spectrum with A,,, = 540 run which is identical with the spectrum produced by y irradiation of ethanol glass at 77°K. The spectrum with &, = 540 nm is ascribed to electrons in relaxed (and, therefore, deeper) traps which are referred to as solvated electrons and denoted by e;. In a recent paper [2], Namiki et al. have reported a study of the photobleaching of e, and e; in ethanol glass. With 50&m light it was found that (1) bleaching of e; in the neat glass at 4OK does not reproduce the near-infrared et band and (2) bleaching of e; in a glass containing benzyl chloride does not produce benzyl radicals. From these results the authors conclude that (1) conversion from e, to e, CQ?Z~ZO~ occur in ethanol glass at 4OK z&d, therefore, *.On leave from the Pakistan Atomic Energy Commission. ** The Radiation.Laboratory of the University of Notre Dame is operated under contkct with the U.S. Atomic Ek er& Commission.This is AEC Document No. COO-38-951.
_ ‘..
:. ..:
retrapping of e; may be impossible at 77’K as well as 4OK and (2) e; is excited to a bound state by visible light. They then appropriately ask how much conclusions can be reconciled with the reported [3] onset at = 540 mn of a monotonic increase in the e, bleaching quantum yield at 77OK with decrease in wavelength and identification of that onset with the . threshold for formation of mobile electrons. As an extension of our pulse-radiolysis study of e;; in a number of liquid alcohols near room temperature [4], we have begun a -r-radiolysis study of e; in some of the same alcohols as glasses at temperatures in the range 4-77°K. Certain of the initial results for ethanol are reported here because they augment the results and, thereby, resolve the dilemma of Namiki et al. [2].
2. Experimental Absolute ethanol of Commercial Solvents was purified prior to use. A solution containing 5 g of 2.4 d.initrophenylhydr&.ine and 5 ml of 8N H2SO4 per liter of ethanol was refluxed under nitrogen for four hours and then was distiied under nitrogen on a Nester-Faust spinning-band column. The middle frae ion of distilled ethanol was stored in contact with l&e& in a bulb on vacuum line. The only impurity detectable by gas chromatography with a 64 Porn pak Q column was 0.2 wt 96 of water. 531
: -..
Volume 27, number 4
CHEMICAL PHYSICS LE-I-I-ERS
A portion of the purified degassed ethanol was distilled under vacuum from the storage bulb into a rectangular Suprasil cell (with a 2-mm width for the optical path) and the cell was sealed. The cell was plunged into liquid nitrogen to produce a clear ethanol glass which then was cracked by bubbling helium through the liquid nitrogen. The cracked glass was r irradiated at 77OK to a dose of 3.8 X lOi eV ml-l, and the absorption spectrum of the irradiated glass at 77’K was recorded on a Cary 14R spectrophotometer. The irradiated glass then was transferred to a variable-temperature helium dewar (Andonian Cryogenics, Inc.) with a sample compartment which tits into that of the spectrophotometer. Temperature of the glass was reduced from 77 to 6”K, and the absorption spectrum was recorded again. Sample temperature was measured by a method essentially the same as that described by Timm and Willard [S]. The spectrophotometer sample compartment was equipped with a holder for interference filters (Optics Technology, Inc. with bandwidth at half-maximum of less than 20 nm) to permit bleaching of a sample in place at selected waveIengths of the light from the IR-2 tungsten lamp of the spectrophotometer. By this means, then, the irradiated glass at 6°K in the spectrophotometer was subjected to a sequence of bleaching periods with measurement of the absorption spectrum after each bleaching period. For each wavelength of bleaching light that was used, the intensity incident on the sample was determined with an Eppley Thermopile (SeriIa No. 635) in a separate experiment under the same conditions. Such a sequence of bleaching periods was terminated by.bleaching the glass at 6OK with the unfiltered light of the tungsten Iamp until the absorption spectrum remained constant. The residual spectrum was subtracted from each of the previously measured spectra of the glass to obtain the spectra attributed to electron species in the glass.
15 August 1974 WAVELENGTH
. nm
2
5
?
2 d d
I
5
lo
I5 WAVENUMBER
a0 , CM-’ x IO-’
25
30
Fig. 1. Absorption spectra of eg produced by 7 irradiation of an ethanol glass at 77°K: broken curve is the spectrum meas ured at 77OK: solid curve is the spectrum measured at 6°K. mu, the e, spectrum was bleached at 6OK without the appearance of another absorption. Such results are consistent with the observations of Namiki et al. [2]. However, with light of wavelengths 366,404, and 436 nm, bleaching of e, at 6’K was accompanied by the appearance of an absorption in the infrared at wavelengths greater than the 830~nm threshold of e,. After an initial bIeaching period of two minutes, the increase in optical density at 1300 nm per unit deof e; was 0.05,0.08, and 0.08 for crease at h,, bleaching wavelengths of 436,404, and 366 run, respectively. Typical results for bleac_hing oFe, at 6OK
with concomitant appeararice gf an infrared absorption are presented in fig. -2 for bleaching at 404 run of the sample for which the initial es spectra at 77 and
3. Results
6°K
Fig. 1 shows spectra obtained for e, at 77 and 6°K after 7 irradiation of en ethanol glass at 77’K. The spectrum at6”K is narrower and has a smaller X,, than the spectrum at 77’K. Similar results have been reported by.others .[I J._-
With @hC of wavelengths 486,52&579,
and. 65Q
are shown in fig. 1. As shown in fig. 2, the infrared absorption induced by 404-nm light was complete-
ly removed, leaving a reduced e, spectrum, by exposure to 656~nm Ii&-for-one minute. In&ease in temperature from 6 to 77I”K alSo removed the infrared absorption induced by 404-run Iight but with a concomitant increase in the ec absorption. : .-
Volume 27. number 4
a
/
loo0
CHEhIICAL PHYSICS LETTERS WAVELENGTH 667
, nm 500
15 August 1974
400
0.8
0.6
0’
6 0.4
Fig. 3. Relative quantum yields for bleaching of e; at 6OK as a function of the bleaching wavelength. 0.2
e; into e; can occur in ethanol glass at temperatures near 4°K and, therefore, it is plausible to infer that photoejection of ey and retrapping at a different site IO
15 WAVENUMBER
25 20 , CM-’ x lO-3
30
Fig. 2. Photobleachingof er at 6’K. Curves 1, 2, 3, and 4 were recorded after successive exposures of the glass irradi-
ated at 77°K (for which ec spectra are presented in fig. 1) for 2,2,.X and 5 min, respectively. to 404-nm tight at 6°K. For clarity, presentation of curves 1 and 2 is restricted to the induced infrared absorption. Curve 5 was recorded after a lmin exposure to 656-nm light following the last 5min exposure to 404-run tight. Relative quantum yields for bleaching of e; at 6 “K were calculated from the measured incident intensity at the bleaching wavelength and the decrease in optical density at X,, of e;. The results are given in fig. 3. Similar results have been reported for photobleaching of e; in ethanol [3] and methanol [6] at 77 OK.
Results essentially identical to those reported here for 6OK have been obtained for 4°K in our most recent work using liquid helium and the techniques of Higashimura and co-workers 11,2]. *
4. Discussion
can occur at 77OK. Transfer of electrons into shallow
unrelaxed traps in the bleaching of e; with light of A G 436 run has been demonstrated by (1) observation of the concomitant appearance of an infrared absorption, (2) removal of the induced infrared absorption with 656-MI light, leaving a somewhat reduced e, absorption, and (3) removal of the induced infrared absorption with enhancement of the e, absorption, by warming to 77°K. Further, the observed photoconversion of e; into et is compatible with the observed wavelength dependence of the relative quantum yield for bleaching of e; at 6°K (cf. fig. 3) and with the previously proposed interpretation [3] of such a wavelength dependence. Thus, the results in fig. 3 indicate _ a threshold near 530 nm (2.3 eV) for formation of the mobile electron precursors of et_ Consistent with such an interpretation are the reported [2] formation of benzyl radicals from benzyl chloride in bleaching of e; in ethanol glass at 77°K with wavelengths in the. range 250-380 nm and the reported [7] wavelength dependence of photoconductivity at 77OK of a methanol glass r-irradiated at 77°K. The failure to detect a product of e; photoejection with 486-rtm light in this work or SOO-run light in the work of Namiki et ah [2] is plausibly attributed to a.combination at these wavelengths of a small quantum yield for mobile electrons, e,, &rnda.small probability of their localizaticin in : 533
The results of this work show.that, contrary. to the conclusion of Nan&i et al:[2], photoconversion of .r E .’ : : _ :. .,. : -... y _,-. ‘, .. . :. _ : .: -.: :. :1 . ... :..., ‘;‘-,. .I ‘._. I .. . -:. _- 1,-.,:_ ., . _--’ ._)___.., ....;_ : _- :-_;;:_......‘.‘;:y : ..:::-_.._ ,.: -;::._ .; ‘_, ._ .: _:_. ..., _:,.:-.., ..‘..(_..’ _.1’ I.-. .~;‘,.‘_> ....’.~_:.~,-‘,_: ‘.:’ z_:_:, fy_ I _;; ‘.‘.y; .’.i :_. :m ,:Y I_ : ,.‘-. . :.. ..‘-. . 1.. I.:, _‘ -_.‘..._ ;...,.---_:.. .I.-..,-. I _.. _..,..t~T-:l~:_I.:.;.:; .-.;._;,_~ ,:;;j,:, ,-.A: __., -<;.,_:,._:_ L_: ..L.. 1._. _,,;:..-:I ..<_‘.:;: :~~~~--~;:__.::
Volume 27, number 4
CHEMICAL PHYSICS LETTERS
shallow unrelaxed traps or on benzyl chloride at the concentrations present [2]. In the spectra of fig. 2, obtained by bleaching e; at 6°K with 404-run light, it is noteworthy that the infrared portion wholly attributable to e; (i.e., for wavenumbers less than 12 X lo3 cm-l) differs significantly from the spectrum with X,, = 1500 nm that is obtained by r radiolysis of ethanol glass at 4’K [1,2]. In the photobleaching experiments, the initial. distribution of trapped-electron species is biased toward greater population of the deeper unrelaxed traps [2] and the distribution shifts with increased bleaching toward that obtained by y radiolysis. Such a difference between the photob!eaching and y-radiolysis results may be related to the difference in energies of the mobile electrons produced end the presence in photobleaching of both a cation and solvation hole in the vicinity of em _Photoejection of e; gives a relatively low-energy mobile electron (= 0.7 eV with 404-nm light] and leaves a solvation hole that probably relaxes slowly and partially, perhaps negligibly, at 4°K. The mobile electron can recombine with the solvation hole, neutralize a nearby correlated cation, or be trapped close to the solvation hole or *he cation. Proximity of the solv&on hole or the cation may cause tunnelling from the shallower traps or, in some manner, a deepening of the trap and, thereby, result in a relatively greater population of deeper unrelaxed traps. Continued bleaching, then, promotes electrons from their initial traps [2] to traps more remote from the vicinity
15 August 1974
of solvation hole and cation with a resultant shift of the distribution toward that produced by -y radiolysis. In .‘yradiolysis, the greater energy of most of the mobile electrons produced results in a larger separation distance from the correlated cation and, therefore, a smaller effect of the cation on et. Such an interpretation also is consistent with the observed small increase in optical density at i300 nm per unit decrease at &ax of e;, corresponding to 0.08 in bleaching at 404 run. In accord with the interpretation, initial experiments on the bleaching of e; at 4°K with 253.7~run Light (mobile-electron energy of = 2.6 eV) have given a spectrum of ec more closely resembling that produced by T radiolysis and a larger increase in optical density at 1300 run per unit decrease at &ax of e,.
References.
[l] H. Hase, T. Warashina. M. Noda, A. Namiki and T. Higashiiura, J. Chem. Phys. 57 (1972) 1039. [2] A. Namiki, M. Noda and T. Higashimura, Chem. whys.
Letters 23 (1973) 402. [3] A. Bemas, D. Grand and C. Chachaty, Chem. Commun. (1970) 1667. [4] R.R. Hentz and G.A. Kenney-Wallace, J. Phys. Chem. 78 (1974) 514. [S] D. Tim
and J.E. Willard, Rev. Sci. Instr. 40 (1969)
848.
[6] A. Habersbergerovi, Lj. Josimovidand J. Teplt, Trans. Faraday Sot. 66 (1970) 656. [7] V.A. Smimov and M.V. Alfunov, High Energy Chem. 6 (1972) 156.
CHEMICAL PHYSICS LETTERS
THE PHOTOCONVERSION
OF SOLVATED IN ETHANOL
ELECTRONS
IS August 1974
INTO TRAPPED ELECTRONS
GLASS AT 6°K
Lewis M. PERKEY, FARHATAZIZ
* and Robert R. HENTZ
Department of Chemistry and the Radiation Labomtory **. University of Notre Dame. Notre Dame, Indiana 46556. USA
Received 14 May 1974
The 7 irradiation of an ethanol glass at 77°K produces solvated electrons (e,) which have an absorption spectrum = 520 nm at 6°K. The conversion of es- into trapped electrons which absorb in the infrared (i.e., electrons in unrelaxed traps denoted by et) has been observed in bleaching es- at 6“K with light of A < 436 nm. with hax
1. Introduction Higashimura and co-workers [l] have found that y irradiation of ethanol glass at 4°K produces an optical absorption spectrum with a wavelength at the absorption maximum, Amax, of 1500 nm. They ascribe this spectrum to electrons in shallow unrelaxed traps which are denoted by e,. With increase in temperature to 77°K the spectrum with X,, = 1500 nm converts to a spectrum with A,,, = 540 run which is identical with the spectrum produced by y irradiation of ethanol glass at 77°K. The spectrum with &, = 540 nm is ascribed to electrons in relaxed (and, therefore, deeper) traps which are referred to as solvated electrons and denoted by e;. In a recent paper [2], Namiki et al. have reported a study of the photobleaching of e, and e; in ethanol glass. With 50&m light it was found that (1) bleaching of e; in the neat glass at 4OK does not reproduce the near-infrared et band and (2) bleaching of e; in a glass containing benzyl chloride does not produce benzyl radicals. From these results the authors conclude that (1) conversion from e, to e, CQ?Z~ZO~ occur in ethanol glass at 4OK z&d, therefore, *.On leave from the Pakistan Atomic Energy Commission. ** The Radiation.Laboratory of the University of Notre Dame is operated under contkct with the U.S. Atomic Ek er& Commission.This is AEC Document No. COO-38-951.
_ ‘..
:. ..:
retrapping of e; may be impossible at 77’K as well as 4OK and (2) e; is excited to a bound state by visible light. They then appropriately ask how much conclusions can be reconciled with the reported [3] onset at = 540 mn of a monotonic increase in the e, bleaching quantum yield at 77OK with decrease in wavelength and identification of that onset with the . threshold for formation of mobile electrons. As an extension of our pulse-radiolysis study of e;; in a number of liquid alcohols near room temperature [4], we have begun a -r-radiolysis study of e; in some of the same alcohols as glasses at temperatures in the range 4-77°K. Certain of the initial results for ethanol are reported here because they augment the results and, thereby, resolve the dilemma of Namiki et al. [2].
2. Experimental Absolute ethanol of Commercial Solvents was purified prior to use. A solution containing 5 g of 2.4 d.initrophenylhydr&.ine and 5 ml of 8N H2SO4 per liter of ethanol was refluxed under nitrogen for four hours and then was distiied under nitrogen on a Nester-Faust spinning-band column. The middle frae ion of distilled ethanol was stored in contact with l&e& in a bulb on vacuum line. The only impurity detectable by gas chromatography with a 64 Porn pak Q column was 0.2 wt 96 of water. 531
: -..
Volume 27, number 4
CHEMICAL PHYSICS LE-I-I-ERS
A portion of the purified degassed ethanol was distilled under vacuum from the storage bulb into a rectangular Suprasil cell (with a 2-mm width for the optical path) and the cell was sealed. The cell was plunged into liquid nitrogen to produce a clear ethanol glass which then was cracked by bubbling helium through the liquid nitrogen. The cracked glass was r irradiated at 77OK to a dose of 3.8 X lOi eV ml-l, and the absorption spectrum of the irradiated glass at 77’K was recorded on a Cary 14R spectrophotometer. The irradiated glass then was transferred to a variable-temperature helium dewar (Andonian Cryogenics, Inc.) with a sample compartment which tits into that of the spectrophotometer. Temperature of the glass was reduced from 77 to 6”K, and the absorption spectrum was recorded again. Sample temperature was measured by a method essentially the same as that described by Timm and Willard [S]. The spectrophotometer sample compartment was equipped with a holder for interference filters (Optics Technology, Inc. with bandwidth at half-maximum of less than 20 nm) to permit bleaching of a sample in place at selected waveIengths of the light from the IR-2 tungsten lamp of the spectrophotometer. By this means, then, the irradiated glass at 6°K in the spectrophotometer was subjected to a sequence of bleaching periods with measurement of the absorption spectrum after each bleaching period. For each wavelength of bleaching light that was used, the intensity incident on the sample was determined with an Eppley Thermopile (SeriIa No. 635) in a separate experiment under the same conditions. Such a sequence of bleaching periods was terminated by.bleaching the glass at 6OK with the unfiltered light of the tungsten Iamp until the absorption spectrum remained constant. The residual spectrum was subtracted from each of the previously measured spectra of the glass to obtain the spectra attributed to electron species in the glass.
15 August 1974 WAVELENGTH
. nm
2
5
?
2 d d
I
5
lo
I5 WAVENUMBER
a0 , CM-’ x IO-’
25
30
Fig. 1. Absorption spectra of eg produced by 7 irradiation of an ethanol glass at 77°K: broken curve is the spectrum meas ured at 77OK: solid curve is the spectrum measured at 6°K. mu, the e, spectrum was bleached at 6OK without the appearance of another absorption. Such results are consistent with the observations of Namiki et al. [2]. However, with light of wavelengths 366,404, and 436 nm, bleaching of e, at 6’K was accompanied by the appearance of an absorption in the infrared at wavelengths greater than the 830~nm threshold of e,. After an initial bIeaching period of two minutes, the increase in optical density at 1300 nm per unit deof e; was 0.05,0.08, and 0.08 for crease at h,, bleaching wavelengths of 436,404, and 366 run, respectively. Typical results for bleac_hing oFe, at 6OK
with concomitant appeararice gf an infrared absorption are presented in fig. -2 for bleaching at 404 run of the sample for which the initial es spectra at 77 and
3. Results
6°K
Fig. 1 shows spectra obtained for e, at 77 and 6°K after 7 irradiation of en ethanol glass at 77’K. The spectrum at6”K is narrower and has a smaller X,, than the spectrum at 77’K. Similar results have been reported by.others .[I J._-
With @hC of wavelengths 486,52&579,
and. 65Q
are shown in fig. 1. As shown in fig. 2, the infrared absorption induced by 404-nm light was complete-
ly removed, leaving a reduced e, spectrum, by exposure to 656~nm Ii&-for-one minute. In&ease in temperature from 6 to 77I”K alSo removed the infrared absorption induced by 404-run Iight but with a concomitant increase in the ec absorption. : .-
Volume 27. number 4
a
/
loo0
CHEhIICAL PHYSICS LETTERS WAVELENGTH 667
, nm 500
15 August 1974
400
0.8
0.6
0’
6 0.4
Fig. 3. Relative quantum yields for bleaching of e; at 6OK as a function of the bleaching wavelength. 0.2
e; into e; can occur in ethanol glass at temperatures near 4°K and, therefore, it is plausible to infer that photoejection of ey and retrapping at a different site IO
15 WAVENUMBER
25 20 , CM-’ x lO-3
30
Fig. 2. Photobleachingof er at 6’K. Curves 1, 2, 3, and 4 were recorded after successive exposures of the glass irradi-
ated at 77°K (for which ec spectra are presented in fig. 1) for 2,2,.X and 5 min, respectively. to 404-nm tight at 6°K. For clarity, presentation of curves 1 and 2 is restricted to the induced infrared absorption. Curve 5 was recorded after a lmin exposure to 656-nm light following the last 5min exposure to 404-run tight. Relative quantum yields for bleaching of e; at 6 “K were calculated from the measured incident intensity at the bleaching wavelength and the decrease in optical density at X,, of e;. The results are given in fig. 3. Similar results have been reported for photobleaching of e; in ethanol [3] and methanol [6] at 77 OK.
Results essentially identical to those reported here for 6OK have been obtained for 4°K in our most recent work using liquid helium and the techniques of Higashimura and co-workers 11,2]. *
4. Discussion
can occur at 77OK. Transfer of electrons into shallow
unrelaxed traps in the bleaching of e; with light of A G 436 run has been demonstrated by (1) observation of the concomitant appearance of an infrared absorption, (2) removal of the induced infrared absorption with 656-MI light, leaving a somewhat reduced e, absorption, and (3) removal of the induced infrared absorption with enhancement of the e, absorption, by warming to 77°K. Further, the observed photoconversion of e; into et is compatible with the observed wavelength dependence of the relative quantum yield for bleaching of e; at 6°K (cf. fig. 3) and with the previously proposed interpretation [3] of such a wavelength dependence. Thus, the results in fig. 3 indicate _ a threshold near 530 nm (2.3 eV) for formation of the mobile electron precursors of et_ Consistent with such an interpretation are the reported [2] formation of benzyl radicals from benzyl chloride in bleaching of e; in ethanol glass at 77°K with wavelengths in the. range 250-380 nm and the reported [7] wavelength dependence of photoconductivity at 77OK of a methanol glass r-irradiated at 77°K. The failure to detect a product of e; photoejection with 486-rtm light in this work or SOO-run light in the work of Namiki et ah [2] is plausibly attributed to a.combination at these wavelengths of a small quantum yield for mobile electrons, e,, &rnda.small probability of their localizaticin in : 533
The results of this work show.that, contrary. to the conclusion of Nan&i et al:[2], photoconversion of .r E .’ : : _ :. .,. : -... y _,-. ‘, .. . :. _ : .: -.: :. :1 . ... :..., ‘;‘-,. .I ‘._. I .. . -:. _- 1,-.,:_ ., . _--’ ._)___.., ....;_ : _- :-_;;:_......‘.‘;:y : ..:::-_.._ ,.: -;::._ .; ‘_, ._ .: _:_. ..., _:,.:-.., ..‘..(_..’ _.1’ I.-. .~;‘,.‘_> ....’.~_:.~,-‘,_: ‘.:’ z_:_:, fy_ I _;; ‘.‘.y; .’.i :_. :m ,:Y I_ : ,.‘-. . :.. ..‘-. . 1.. I.:, _‘ -_.‘..._ ;...,.---_:.. .I.-..,-. I _.. _..,..t~T-:l~:_I.:.;.:; .-.;._;,_~ ,:;;j,:, ,-.A: __., -<;.,_:,._:_ L_: ..L.. 1._. _,,;:..-:I ..<_‘.:;: :~~~~--~;:__.::
Volume 27, number 4
CHEMICAL PHYSICS LETTERS
shallow unrelaxed traps or on benzyl chloride at the concentrations present [2]. In the spectra of fig. 2, obtained by bleaching e; at 6°K with 404-run light, it is noteworthy that the infrared portion wholly attributable to e; (i.e., for wavenumbers less than 12 X lo3 cm-l) differs significantly from the spectrum with X,, = 1500 nm that is obtained by r radiolysis of ethanol glass at 4’K [1,2]. In the photobleaching experiments, the initial. distribution of trapped-electron species is biased toward greater population of the deeper unrelaxed traps [2] and the distribution shifts with increased bleaching toward that obtained by y radiolysis. Such a difference between the photob!eaching and y-radiolysis results may be related to the difference in energies of the mobile electrons produced end the presence in photobleaching of both a cation and solvation hole in the vicinity of em _Photoejection of e; gives a relatively low-energy mobile electron (= 0.7 eV with 404-nm light] and leaves a solvation hole that probably relaxes slowly and partially, perhaps negligibly, at 4°K. The mobile electron can recombine with the solvation hole, neutralize a nearby correlated cation, or be trapped close to the solvation hole or *he cation. Proximity of the solv&on hole or the cation may cause tunnelling from the shallower traps or, in some manner, a deepening of the trap and, thereby, result in a relatively greater population of deeper unrelaxed traps. Continued bleaching, then, promotes electrons from their initial traps [2] to traps more remote from the vicinity
15 August 1974
of solvation hole and cation with a resultant shift of the distribution toward that produced by -y radiolysis. In .‘yradiolysis, the greater energy of most of the mobile electrons produced results in a larger separation distance from the correlated cation and, therefore, a smaller effect of the cation on et. Such an interpretation also is consistent with the observed small increase in optical density at i300 nm per unit decrease at &ax of e;, corresponding to 0.08 in bleaching at 404 run. In accord with the interpretation, initial experiments on the bleaching of e; at 4°K with 253.7~run Light (mobile-electron energy of = 2.6 eV) have given a spectrum of ec more closely resembling that produced by T radiolysis and a larger increase in optical density at 1300 run per unit decrease at &ax of e,.
References.
[l] H. Hase, T. Warashina. M. Noda, A. Namiki and T. Higashiiura, J. Chem. Phys. 57 (1972) 1039. [2] A. Namiki, M. Noda and T. Higashimura, Chem. whys.
Letters 23 (1973) 402. [3] A. Bemas, D. Grand and C. Chachaty, Chem. Commun. (1970) 1667. [4] R.R. Hentz and G.A. Kenney-Wallace, J. Phys. Chem. 78 (1974) 514. [S] D. Tim
and J.E. Willard, Rev. Sci. Instr. 40 (1969)
848.
[6] A. Habersbergerovi, Lj. Josimovidand J. Teplt, Trans. Faraday Sot. 66 (1970) 656. [7] V.A. Smimov and M.V. Alfunov, High Energy Chem. 6 (1972) 156.