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Structure in the optical absorption spectrum of the solvated electron?
Int..T. I~t~d!n_t.Phys. Chem. 197S, Vol. 7, pp. 227-231. Pergamon Press. Prln~d in Great Britain
STRUCTURE IN THE OPTICAL ABSORPTION SPECTRUM OF THE SOLVATED ELECTRON?* JAMm F. GAVLAS and LEON M. D O W N Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, U.S.A.
(Received 25 March 1974; in revised form 6 June 1974) Abstract~The optical absorption spectrum of the solvated electron in ethanol has been determined again to;establish whether a report of observed structural features by Vannikov and Marevtsev, which al~pears to he discordant with earlier data, can he sustained. Our ,datado not reveal such structural features, and are, furthermore, in sian;fle.ant disagreement with t h e general contour of the absorption band reported by these authors.
INTRODUCTION
THE OPTICALabsorption spectrum of the solvated electron in liquids has generally been reported as a broad, structureless band (l-e). The question of whether this band is composed of several narrower bands representing transitions between quAntized energy states is of theoretical~74) interest. For a purely continuum model, there has been speculation as to whether bands of the nature of a Rydberg series may exist. For a cavity model(',~°), in which short-range interactions are important, there may conceivably be structure related to molecular properties of the near neighbors of the solvated electron. This question of structure may be viewed from two aspects: (a) whether the smooth, structureless band may be deconvoluted mathematically into separate, overlapping bands, as suggested by Delahayt11), to represent bound-bound and bound-continuum transitions, or Co) whether structure may indeed by observable if the wavelength resolution and reproducibility of optical density measurements are sufficient. It is only the latter aspect of this question to which this paper is directed. Observable structure in the optical absorption spectrum of e,- in ethanol has recently been reported by Vannikov and Marevtsev(X*~. Their observations do not seem to be in accord with earlier datata,4) for %- in the alcohols. It is possible that this recent observation of structure is the result of improvements in technique. We have, !therefore, carefully repeated the observations for ethanol, accumulating a rather large mass of data. Our data do not support the claim (a) that structure is obserVable in the form of four separate peaks, over the wavelength region 680-820 nm. EXPER/MENTAL The source of the electron pulse, as in earlier work (xt), was a Varian V-7715A electron linear accelerator. In this investigation, 400 ns pulses of 3-4 MeV electrons, with a beam current of about 325 mA were used. Suchpulses deliver a dose of about 2.4x 10x7 eV g-X. The irradiation cells were made of fused quartz, with optical windows of high purity silica, and were 20-0 mm long in the direction of the analyzing light beam. A double pass was used giving a 40.0 mm optical path through the cell. Each cell was conne~ed to a 50 cms Pyrex storage bulb. The bulbs were equipped with Teflon stopcocks and Teflon-seal joints to allow connection to a vacuum system. * This work was supported by the U.S, Atomic Energy Commission under Contract No. AT(11-1)-1763. 227
228
J . ~ r a F. GAVgASand Lt.ON M, D'o~-ivo~
The scatter of repeated optk:al density measurements would normally depend upon pu reproducibility, whieda in our case is approximately + 3 per cent. This source of error was elff hated by splitting the analyzing light beam into two parts with the use of a partially transmitti mirror, so that the transient absorption could be observed at two wavelengths simultaneous Thus, one wavelength could remain fixed while the other was varied. The fixed wavelen[ 500 nm in this case, was tu~d as a reference standard to normalize any variations in pulse intensi and thus to improve the reproducibility of the optk~tl density measurements. Bausch & Lo~ grating monochromators, Type 33-86-25, fl3.5, were used with three different gratings of t following wavelength range* and reciprocal dispersions: 350-800 tma, 6.4 nm mm -~; 200-700 nl 7.4 nm mm-~; 700-1600 nm, 12.8 nm mm -1. Monochromator entrance and exit slits were alwa set to give a band pass of 3.6 rim. Appropriate Coming glass filters were placed in the optic path before the cell 1o prevent photolysis of the sample, and in front of the monochromators eliminate s e c o n d - o ~ ~ o n effects. RCA phot0multiplier tubes 7102 and 7200, having S,1 and S-19 spectral responses, respective] were used to monitor the optical absorption, These photomultiplier tubes were coupled, throul an amplifier system with a 70 ns risetime (for 10-90 per cent), to a dual-beam 0scilloscop Polaroid photographs of the rate curves were enlarged by a factor of three before measuremen were made of the traces on them. Absolute ethyl alcohol, U.S.P., was obtained from Commercial Solvents Corporation, Ter: Haute, Indiana, U.S.A. The ethanol was distilled under argon at one atmosphere, first from acidic 2,4-dlnitrophenylhydrazine solution, and then from a sodium ethoxide solution, The purilic ethanol was degassed immediately after c~llection (three freeze-pump-thaw cycles) and store under vacuum in the dark until needed. The spectrum reported here is a composite of repeated results obtained from the irradiatic of four separate samples. Sample, were prepared by vacuum distillation of from 35 to 40 ml ( ethanol into each of the Pyrex storage bulbs. After each electron pulse, the irradiated etham was mixed with that in the storage bulb to give a twenty-fold dilution of any stable radiolys products that m a y have been formed. No sample was ever subjected to more than 60 pulse and over the course of any experiment no change was perceptible in either the yield or the deca kinetics of ea- as monitored at the fixed wavelength of 500 nm. The spectral region 630-770 m was covered once with each of the four samples and twice with two of them. With two of th samples the ~,ntire 500-900 nm region was covered. RESULTS AND DISCUSSION The purpose o f this investigation is to ascertain whether the structure in th, r o o m temperature spectrum o f % - in ethanol shown in the paper o f V a n n i k o v an( Marevtsev (1~), and again in a current report (14) f r o m their laboratory, which a refereq h a s kindly b r o u g h t to o u r attention, is indeed real. T w o characteristics o f ott imeasurement capability are i m p o r t a n t : (1) the scatter in the amplitude o f the optica density measurements at any given wavelength, c o m p a r e d with the magnitude o f :th( difference between structure-determining points in their paPer(~), and (2) the bandpam u s e d in o u r m e a s ~ m e n t s c o m p a r e d with the width o f the reported structural features The max/mum spread in o u r optical density measurements at any wavelength (see Fig. 2) over the region 650-850 n m is + 2 . 2 per cent; the average deviation f r o m the m e a n at any wavelength is a b o u t + 1.3 per cent. Since the structural features reported by V a n n i k o v and Marevtsev show ~,spread o f 3.5 per cent at 680 rim, and only slightl) ,less;at other wavelengths(~),and, indeed, an even greater spread, exceeding 10 per cen| a t another wavelength in their m o r e recent Fape r(t4), o u r measurements Should reveal •these features; The b a n @ a s s we h a v e used is 3'6 nm, rather better t h a n the bandpass . o f 9--15 n m w h i c h we infer f r o m the literature (m) was used by Vannikov and M a r e v t s e v (t~) in their first report, a n d at least comparable to the resolution o f 2.5-6.0 n m which they have indicated m o r e recently (i4). A bandpass o f 3'6 n m should be adequate f o r m e a s u r e m e n t o f the r e p o r t e d slruetural features w h i c k exhibit a width at half-height o f a b o u t 16 rim.
Structure in the optical absorption spectrum of the solvated electron ?
229
Shown in Fig. 1 is the spectrum of % - in ethanol that we have obtained. Each point is an average of from 2 to 7 repeated determinations. The smooth curve passing throush the points we believe best fits the data. The curve reported by Vsnnikov and Marevtscv(~) has been included in Fig. 1 for comparison. Clearly, their curve does not fit our data well. The discrepancy is particularly apparent in the decline of their optical density from 680 nm to a structural minimum at 700 rim, and 0.25
1
I
i
~z 0 2 0
o o " ' 0.I~ .J u.l
/ / / ,i s#
0.1(
I 600
500
I 700
WAVELENGTH
I 800
(nm)
900
FIG. l. Optical absorptio n spectrum of the solvated electron in ethanol at 296 K. The open circles represent our experimental data, each point heing the average of two to seven individual data points. The dashed line represents the data of Vannikov and Marevtsev normalized to our data at 680 rim. I
I
I
0"25 I
,~ o.ao e l
II
._1
°
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o.
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.
.
5O0
I
600
I 700 WAVELENGTH
: (rim)
I
800
900
' ~FIo. 2. Optical absOrption spectrum of the solvated electron in.etlm~l at296 K showing each individualdata point obtainS_ :in thepresent inv.. tigatlon..
r
230
JAMES F. G A r b s and LEON M. D O l m e N
in the rise to a peak at 820 rim. In both cases this discrepancy is some twofold larg, than the uncertainty in our measurements, which thus do not sustain their report ( observed structure. Moreover, comparison o f the spectrum reported in the recel work (u) with the earlier data(is) indicates an inconsistency in b o t h the amplitude i some peaks and valleys in the reported structure, (~,~) as well as opposite shifts i the wavelength of two peaks. These authors have commented (14) upon such variatio! and have attributed them to differences in the individual alcohol samples. Perhaps some understanding of the problem in defining such structure may [ obtained from Fig. 2. Each point in Fig. 2 represents an optical density measureme] made after a separate electron pulse. The scatter in these points at any given waw length reflects the uncertainty in the measurement of the transient absorption. The deviations o f our data points from the smooth curve which has been draw through them in Fig. 1 are all less than our uncertainty o f about + 1-3 per cent. , smooth curve without structural features thus seems to be called for by the data. A separate but not totally unrelated question arises regarding the disagreemei o f our data with the spectrum o f Vannikov and Marevtsev at wavelengths short¢ than 600 nm and longer than 820 nm. Contrary to their assertion (1~) that their dat
I
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~0.20 Z ur'tJ . ..J pCL
0
uj 0.1~
...A
!
\
/
he"
/
O.IC 500
I I
600
I
700 WAVELENGTH(nm)
I
800
90(
FIG. 3. Optical absorption spectrum of the solvated electron in ethanol at r o o m ,
temperature obtained by different authors. , Sauer et al.Ca); Gavlas and Dorfman, present investigation; , Jha et a/.(le); Vannikov and Marevtsev (x=).
,
are in agreement with the general contour line o f the spectrum in earlier work (s), there seems to be substantial disagreement at the longer and particularly at the shorter wavelengths. Their disagr~ment is not only with the earlier work (s), but also with our present repetition o f those m©asuremcnts as well as with a recent set o f data o f Jha et al. cls) A l l four sets o f data are shown in Fig. 3.
Structure in the optical absorption spectrum of the solvated electron?
231
Acknowledgement--It is an honour to participate in this memorial issue dedicated to Professor Robert L. Platzman. The intellectual stimulation and the guidance he provided to our work on solvated electrons as well as to other areas of our research, over the past decade, have been of profound importance to us. We would also like to acknowledge helpful discussions of the present work with Ms. Judith Weinstein. Dr. A. Melnyk has been kind enough to translate the Russian papers for us.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
H. BLADESand J. W. Ho~3nss, Can. J. Chem. 1955, 33, 411. J. W. BOAGand E. J. HART, Nature 1963, 197, 45. M. C. SALTER,JR., S. ARSd and L. M. DORVMAN,J'. chem. Phys. 1965, 42, 708. S. AR~ and M. C. SAUER,JR., J. chem. Phys. 1966, 44, 2297. J. P. K ~ z , Radiat. Res. 1964, 22, 1. F. Y. Jou and L. M. DO,tO, N, J. chem. Phys. 1973, $8, 4715. A. S. DAVYDOV,Zh. eksp. teor. Fiz. 1948, 18, 913. R. L. PLATZMAN,Physical and Chemical Aspects of Basic Mechanisms in Radiobiology, Publ. No. 305, National Academy of Sciences, National Research Council, Washington, D. C.,1953. D. A. COPet.AND, N. R. KESTmma n d J. JORTNeR, J. chem. Phys. 1970, $3, 1189. L. K e v ~ , K. Ft~KI, D. F. F~NOand R. E. CHRIS'roI~EnSEN,J. phys. Chem. 1971, 75, 2297. R. Luoo and P. DELAHAY,J. chem. Phys. 1972, b'7, 2122. A. V. VAISNtKOVand V. S. MARBVrsEv,Int. J. Radiat. Phys. Chem. 1973, $, 453. W. D. l~tzx, B. L. GALL and L. M. DORVMAN,J. phys. Chem. 1967, 71, 384. V. S. MARBVTSeV,A. V. V ~ O V and N. A. B ~ , Khim. vys. Enorg. 1974, 8, 52. V. N. SnunxN, V. I. ZmGtnsov, V. I. ZOLOTAnEVSKnand P. I. DOIXN, Dokl. Akad. Nauk SSSR 1967, 174, 416. K. N. J~IA, G. L. BOLTONand G. R. F~EMAN, Can. J. Chem. 1972, 76, 3876.
STRUCTURE IN THE OPTICAL ABSORPTION SPECTRUM OF THE SOLVATED ELECTRON?* JAMm F. GAVLAS and LEON M. D O W N Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, U.S.A.
(Received 25 March 1974; in revised form 6 June 1974) Abstract~The optical absorption spectrum of the solvated electron in ethanol has been determined again to;establish whether a report of observed structural features by Vannikov and Marevtsev, which al~pears to he discordant with earlier data, can he sustained. Our ,datado not reveal such structural features, and are, furthermore, in sian;fle.ant disagreement with t h e general contour of the absorption band reported by these authors.
INTRODUCTION
THE OPTICALabsorption spectrum of the solvated electron in liquids has generally been reported as a broad, structureless band (l-e). The question of whether this band is composed of several narrower bands representing transitions between quAntized energy states is of theoretical~74) interest. For a purely continuum model, there has been speculation as to whether bands of the nature of a Rydberg series may exist. For a cavity model(',~°), in which short-range interactions are important, there may conceivably be structure related to molecular properties of the near neighbors of the solvated electron. This question of structure may be viewed from two aspects: (a) whether the smooth, structureless band may be deconvoluted mathematically into separate, overlapping bands, as suggested by Delahayt11), to represent bound-bound and bound-continuum transitions, or Co) whether structure may indeed by observable if the wavelength resolution and reproducibility of optical density measurements are sufficient. It is only the latter aspect of this question to which this paper is directed. Observable structure in the optical absorption spectrum of e,- in ethanol has recently been reported by Vannikov and Marevtsev(X*~. Their observations do not seem to be in accord with earlier datata,4) for %- in the alcohols. It is possible that this recent observation of structure is the result of improvements in technique. We have, !therefore, carefully repeated the observations for ethanol, accumulating a rather large mass of data. Our data do not support the claim (a) that structure is obserVable in the form of four separate peaks, over the wavelength region 680-820 nm. EXPER/MENTAL The source of the electron pulse, as in earlier work (xt), was a Varian V-7715A electron linear accelerator. In this investigation, 400 ns pulses of 3-4 MeV electrons, with a beam current of about 325 mA were used. Suchpulses deliver a dose of about 2.4x 10x7 eV g-X. The irradiation cells were made of fused quartz, with optical windows of high purity silica, and were 20-0 mm long in the direction of the analyzing light beam. A double pass was used giving a 40.0 mm optical path through the cell. Each cell was conne~ed to a 50 cms Pyrex storage bulb. The bulbs were equipped with Teflon stopcocks and Teflon-seal joints to allow connection to a vacuum system. * This work was supported by the U.S, Atomic Energy Commission under Contract No. AT(11-1)-1763. 227
228
J . ~ r a F. GAVgASand Lt.ON M, D'o~-ivo~
The scatter of repeated optk:al density measurements would normally depend upon pu reproducibility, whieda in our case is approximately + 3 per cent. This source of error was elff hated by splitting the analyzing light beam into two parts with the use of a partially transmitti mirror, so that the transient absorption could be observed at two wavelengths simultaneous Thus, one wavelength could remain fixed while the other was varied. The fixed wavelen[ 500 nm in this case, was tu~d as a reference standard to normalize any variations in pulse intensi and thus to improve the reproducibility of the optk~tl density measurements. Bausch & Lo~ grating monochromators, Type 33-86-25, fl3.5, were used with three different gratings of t following wavelength range* and reciprocal dispersions: 350-800 tma, 6.4 nm mm -~; 200-700 nl 7.4 nm mm-~; 700-1600 nm, 12.8 nm mm -1. Monochromator entrance and exit slits were alwa set to give a band pass of 3.6 rim. Appropriate Coming glass filters were placed in the optic path before the cell 1o prevent photolysis of the sample, and in front of the monochromators eliminate s e c o n d - o ~ ~ o n effects. RCA phot0multiplier tubes 7102 and 7200, having S,1 and S-19 spectral responses, respective] were used to monitor the optical absorption, These photomultiplier tubes were coupled, throul an amplifier system with a 70 ns risetime (for 10-90 per cent), to a dual-beam 0scilloscop Polaroid photographs of the rate curves were enlarged by a factor of three before measuremen were made of the traces on them. Absolute ethyl alcohol, U.S.P., was obtained from Commercial Solvents Corporation, Ter: Haute, Indiana, U.S.A. The ethanol was distilled under argon at one atmosphere, first from acidic 2,4-dlnitrophenylhydrazine solution, and then from a sodium ethoxide solution, The purilic ethanol was degassed immediately after c~llection (three freeze-pump-thaw cycles) and store under vacuum in the dark until needed. The spectrum reported here is a composite of repeated results obtained from the irradiatic of four separate samples. Sample, were prepared by vacuum distillation of from 35 to 40 ml ( ethanol into each of the Pyrex storage bulbs. After each electron pulse, the irradiated etham was mixed with that in the storage bulb to give a twenty-fold dilution of any stable radiolys products that m a y have been formed. No sample was ever subjected to more than 60 pulse and over the course of any experiment no change was perceptible in either the yield or the deca kinetics of ea- as monitored at the fixed wavelength of 500 nm. The spectral region 630-770 m was covered once with each of the four samples and twice with two of them. With two of th samples the ~,ntire 500-900 nm region was covered. RESULTS AND DISCUSSION The purpose o f this investigation is to ascertain whether the structure in th, r o o m temperature spectrum o f % - in ethanol shown in the paper o f V a n n i k o v an( Marevtsev (1~), and again in a current report (14) f r o m their laboratory, which a refereq h a s kindly b r o u g h t to o u r attention, is indeed real. T w o characteristics o f ott imeasurement capability are i m p o r t a n t : (1) the scatter in the amplitude o f the optica density measurements at any given wavelength, c o m p a r e d with the magnitude o f :th( difference between structure-determining points in their paPer(~), and (2) the bandpam u s e d in o u r m e a s ~ m e n t s c o m p a r e d with the width o f the reported structural features The max/mum spread in o u r optical density measurements at any wavelength (see Fig. 2) over the region 650-850 n m is + 2 . 2 per cent; the average deviation f r o m the m e a n at any wavelength is a b o u t + 1.3 per cent. Since the structural features reported by V a n n i k o v and Marevtsev show ~,spread o f 3.5 per cent at 680 rim, and only slightl) ,less;at other wavelengths(~),and, indeed, an even greater spread, exceeding 10 per cen| a t another wavelength in their m o r e recent Fape r(t4), o u r measurements Should reveal •these features; The b a n @ a s s we h a v e used is 3'6 nm, rather better t h a n the bandpass . o f 9--15 n m w h i c h we infer f r o m the literature (m) was used by Vannikov and M a r e v t s e v (t~) in their first report, a n d at least comparable to the resolution o f 2.5-6.0 n m which they have indicated m o r e recently (i4). A bandpass o f 3'6 n m should be adequate f o r m e a s u r e m e n t o f the r e p o r t e d slruetural features w h i c k exhibit a width at half-height o f a b o u t 16 rim.
Structure in the optical absorption spectrum of the solvated electron ?
229
Shown in Fig. 1 is the spectrum of % - in ethanol that we have obtained. Each point is an average of from 2 to 7 repeated determinations. The smooth curve passing throush the points we believe best fits the data. The curve reported by Vsnnikov and Marevtscv(~) has been included in Fig. 1 for comparison. Clearly, their curve does not fit our data well. The discrepancy is particularly apparent in the decline of their optical density from 680 nm to a structural minimum at 700 rim, and 0.25
1
I
i
~z 0 2 0
o o " ' 0.I~ .J u.l
/ / / ,i s#
0.1(
I 600
500
I 700
WAVELENGTH
I 800
(nm)
900
FIG. l. Optical absorptio n spectrum of the solvated electron in ethanol at 296 K. The open circles represent our experimental data, each point heing the average of two to seven individual data points. The dashed line represents the data of Vannikov and Marevtsev normalized to our data at 680 rim. I
I
I
0"25 I
,~ o.ao e l
II
._1
°
(.,}
0
o.
W,
e e
•
" ~ 0:i5,
.
.
5O0
I
600
I 700 WAVELENGTH
: (rim)
I
800
900
' ~FIo. 2. Optical absOrption spectrum of the solvated electron in.etlm~l at296 K showing each individualdata point obtainS_ :in thepresent inv.. tigatlon..
r
230
JAMES F. G A r b s and LEON M. D O l m e N
in the rise to a peak at 820 rim. In both cases this discrepancy is some twofold larg, than the uncertainty in our measurements, which thus do not sustain their report ( observed structure. Moreover, comparison o f the spectrum reported in the recel work (u) with the earlier data(is) indicates an inconsistency in b o t h the amplitude i some peaks and valleys in the reported structure, (~,~) as well as opposite shifts i the wavelength of two peaks. These authors have commented (14) upon such variatio! and have attributed them to differences in the individual alcohol samples. Perhaps some understanding of the problem in defining such structure may [ obtained from Fig. 2. Each point in Fig. 2 represents an optical density measureme] made after a separate electron pulse. The scatter in these points at any given waw length reflects the uncertainty in the measurement of the transient absorption. The deviations o f our data points from the smooth curve which has been draw through them in Fig. 1 are all less than our uncertainty o f about + 1-3 per cent. , smooth curve without structural features thus seems to be called for by the data. A separate but not totally unrelated question arises regarding the disagreemei o f our data with the spectrum o f Vannikov and Marevtsev at wavelengths short¢ than 600 nm and longer than 820 nm. Contrary to their assertion (1~) that their dat
I
0"25I
1
I
>-
~0.20 Z ur'tJ . ..J pCL
0
uj 0.1~
...A
!
\
/
he"
/
O.IC 500
I I
600
I
700 WAVELENGTH(nm)
I
800
90(
FIG. 3. Optical absorption spectrum of the solvated electron in ethanol at r o o m ,
temperature obtained by different authors. , Sauer et al.Ca); Gavlas and Dorfman, present investigation; , Jha et a/.(le); Vannikov and Marevtsev (x=).
,
are in agreement with the general contour line o f the spectrum in earlier work (s), there seems to be substantial disagreement at the longer and particularly at the shorter wavelengths. Their disagr~ment is not only with the earlier work (s), but also with our present repetition o f those m©asuremcnts as well as with a recent set o f data o f Jha et al. cls) A l l four sets o f data are shown in Fig. 3.
Structure in the optical absorption spectrum of the solvated electron?
231
Acknowledgement--It is an honour to participate in this memorial issue dedicated to Professor Robert L. Platzman. The intellectual stimulation and the guidance he provided to our work on solvated electrons as well as to other areas of our research, over the past decade, have been of profound importance to us. We would also like to acknowledge helpful discussions of the present work with Ms. Judith Weinstein. Dr. A. Melnyk has been kind enough to translate the Russian papers for us.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
H. BLADESand J. W. Ho~3nss, Can. J. Chem. 1955, 33, 411. J. W. BOAGand E. J. HART, Nature 1963, 197, 45. M. C. SALTER,JR., S. ARSd and L. M. DORVMAN,J'. chem. Phys. 1965, 42, 708. S. AR~ and M. C. SAUER,JR., J. chem. Phys. 1966, 44, 2297. J. P. K ~ z , Radiat. Res. 1964, 22, 1. F. Y. Jou and L. M. DO,tO, N, J. chem. Phys. 1973, $8, 4715. A. S. DAVYDOV,Zh. eksp. teor. Fiz. 1948, 18, 913. R. L. PLATZMAN,Physical and Chemical Aspects of Basic Mechanisms in Radiobiology, Publ. No. 305, National Academy of Sciences, National Research Council, Washington, D. C.,1953. D. A. COPet.AND, N. R. KESTmma n d J. JORTNeR, J. chem. Phys. 1970, $3, 1189. L. K e v ~ , K. Ft~KI, D. F. F~NOand R. E. CHRIS'roI~EnSEN,J. phys. Chem. 1971, 75, 2297. R. Luoo and P. DELAHAY,J. chem. Phys. 1972, b'7, 2122. A. V. VAISNtKOVand V. S. MARBVrsEv,Int. J. Radiat. Phys. Chem. 1973, $, 453. W. D. l~tzx, B. L. GALL and L. M. DORVMAN,J. phys. Chem. 1967, 71, 384. V. S. MARBVTSeV,A. V. V ~ O V and N. A. B ~ , Khim. vys. Enorg. 1974, 8, 52. V. N. SnunxN, V. I. ZmGtnsov, V. I. ZOLOTAnEVSKnand P. I. DOIXN, Dokl. Akad. Nauk SSSR 1967, 174, 416. K. N. J~IA, G. L. BOLTONand G. R. F~EMAN, Can. J. Chem. 1972, 76, 3876.