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Photobleaching of radiation induced trapped electrons in neutral and alkaline glasses. Hydrogen atom formation
Volume 47, number 2
CHEMICAL PHYSICS L”TTERS
PHOTOBLEACHINGOF RADIATION
BNDUCED T
15 April 1971
WED ELECTR(-jNS
M. KONCSHAUC, .I. MOAN, B. BiiCKMANN and H. H@VIK Norsk Hydra’s Institute for Cancer Research, The Norwegian Radium Hospital, Montebello, Oslo 3, Norway Received 1 December 1976 Revised manuscript received 24 January 1977
Photomobilization of trapped electrons in 7 M NaCl04- and 9 M NaOH-glassgives rise to trapped hydrogen atoms. This is probably due to the reaction e, + Hz0 - H + OH-. Experiments with an electron scavenger indicate that electrons are not precursors to radiolytically produced hydrogen atoms. It seems that the mobile electrons produceci during pkotobleaching are not slowed down to thermal energy before they react to produce hydrogen atoms, since the yield of the latter species is strongly dependent on the wavelength rji the bleaching light.
II. Introduction
tions which may shel; light on the above mentioned topic.
Small yields of trapped hydrogen atoms may be observed in neutral and alkaline matrices X-irradiated at 77 K [l-8]. The H atoms are not stably trapped in pure ice at this temperature, but they may be readily observed at 4.2 K [9,10]. The mechanism of production of H atoms in such systems, has been proposed to be decomposition of excited water molecules [7,11]. The excited water molecules may result from primary excitation or from recombination of electrons and HzO+ ions. However, we also observed I-I atoms in W-irradiated NaOH glass containing potassium ferrocyanide or tryptophan [ 121. We suggested that these H atom3 were formed through energy transfer from excited ferrocyanide or tryptophan molecules to surrounding water molecules. In addition one should consider the reaction: e; tHiO-+HtOH-,
(1)
where e, represents a mobile electron. Even though the dissociation of H20- is beiieved to be endotherm in the gas phase, reaction (1) may proceed in the condensed phase by utilizing the solvation energy of the OH- ion [ 131 and the energy of e;. As part of an extensive study of formation and decay of H atcms [ 141 we have m .de some observa-
2. Experimental Glassy samples pf 9 M NaOH and 7 M NaCiO, were prepared by allowing drops of sample solution to fall into liquid N,. The glassy pellets w&r X-irrsdiated 2nd transferred to quartz ESR-tubes. During this procedure the pellets were shielded from light and kept immersed in liquid N2. The source of UV-irradiation and bleaching was a 200 W high pressure mercury lrmp fitted to a Bausch and Lomb grating monochromator. The irradiated sampies were examined by ESR-methods. Details of the experimental procedure may be found elsewhere [12].
3. Resldts The experimental result:: may be summarized as follows: (A) X-irradiation results in the formation of tr:lpp~~i electrons (e;) and 1-latoms (El,), the ratio [I-I,]/[e, ] being about 0.06 in 7 M NaClO, and 0.01 in 9 M NarLPH. (B) Addition of small amounts of the electron scav. 255
CHEMICAL PHYSICS LETTERS
Volume 47, number 2
enger NOT (=lO-*M) reduces the yield of et significantly while the yield of Ht is constant to within 5%. (Cc) Complete bleaching of the trapped eJectrons with 580 run results in an increase in the concentration of H, by about 70% in 7 M NaCIOd and a decrease of about 40% in 9 M NaOH? (Dj During this bleaching the amount of O- radicals in 9 M NaOH decreases by about 65%. initially the concentration of et and that of O- was equal to within J 10%. (E) Annealing (5 min at420 K for 7 M NaCIO, and 5 min at 143 K for 9 M NaOH) rest&s in a coinpIete removal of H, while a large fraction of e; remains, i.e. [e; ] decreases by less than 10% in 9 M NaOH and by 40-50% in 7 M NaC104. When e; in such samples are bleached with visible light, H, reappear to about 20% or more of their concentration prior to annealing. (F) Notably in 7 M NaC104 there is a prominent wavelength dependence of the amount of H atoms that ap pear during photobleaching of annealed samples: at 366 nm about 20% reappear and at 580 run about 50% reappear_ In both cases it was thoroughIy checked that the bleaching resulted in complete removal of et. (G) UV-irradiation of matrices containing the photoionizable solute tryptophan gives rise to e; and Ht, the ratio [H, J/ [et] being about 0.003 in 9 M NaOH and 0.02 in 7 M NaCIO,. Addition of the electron scavenger NOT causes an equal reduction of e; and Ht. Traces of O- radicals were observed in the NaOH samples. Both the yield of O- and the totaJ radical yieId are significantly enhanced by the presence of NOT.
4. Discussion The fol!owing reactions known from the radiolysis of water (see ref. [IS] and references cited therein) should be considered: eTq +H20*H+OH-, II-,, = 16 M-l s-l,
(lb)
eTis + O- --, 20H-,
k2 =2.2X 10'0M-ls-1, eaq + e& +
(2)
Hz f 20H-,
kg = 5 X log M-‘s-1,
(3)
k4 = 3 X 10”’ M-Is-l,
(4)
e&+HHHH2+OH-,
256
15 April 1977
e&l+H,O-tH+H,O, ks
= 2 X lOlo
k6
= 1.3 X
M-ls-L ,
(3
M-‘s-l,
(6)
eG+HH202*OH+OH-, lOi
H+H-tH2, k7
H + OH-
= 1 X 10’0
M-1
s-l
,
(7)
j e&, kg
= 2.2 X i07 M-l s-l,
(8)
H+O-*OH-,
kg = 3 X lOlo M-Is-l, H30+ + OH-
(9)
--, 2H,O,
k,,= 1.5 X IO”
M-ls-I.
(10)
In the discussion which follows we assume that e;ls in these reactions may be replaced by e,, mobile electrons resulting from photobleaching of et. This may imply that the rate constants differ from those given above. As seen from observation (D) the photobleached electrons mainly disappear according to reaction (2) or probably the reaction [l I] :
e,+O-+O2-.
(2b)
However, other pathways must exist since 35% of the initial concentration of O- remains after complete bleaching of et [observation (D)]. Reaction (3) probably does not take place in the present matrix since bielectrons, (e&,, seem to be stabIe, notably when hole scavengers are present [ 1 I]. When excited either by annealing or optical bleaching, the bielectrons give rise to et. However, according to ref. [ 1 l] about 70% of the electrons are lost during this process, so it cannot be safely concluded that reaction (3) does not take place. Reaction (6) may take place, but we have no means of estimating its importance since the OH radicals give rise to O- radicals. However, if we assume that the yield of H202 is not larger than that in alkaline aqueous solutions at room temperature, we find that at most 25% of the electrons may disappear according to reaction (6) during bleaching. Reaction (4) may take place according to observation (Cc), but since the yield of H, is small, it can only account for the disappearance of a tiny fraction of the photomobilized electrons. Some of the photomobilized electrons seem to undergo reaction (1). This is indicated by observations (E) and (G). Reaction (1) may be of more importance than the yieId of H, seems to show, since a relatively
Volume 41, number 2
CHEMICAL PHYSICS LETTERS
large fraction of the produced H atoms may disappear before trapping. Thus the yield of primary atomic hydrogen in the radiolysis of water is about 0.6 (ref. [ 151 p. 73), i.e. a factor 3-4 times higher than the yield of H, in 7 M NaClO, and at least a factor 20 times higher than the yield of Ht in 9 M NaOH. Therefore, if it is assumed that the primary yield of atomic hydrogen is not much different in frozen and liquid solutions, a large fraction of the H atoms must disappear prior to trapping. The scavenger experiment (B) indicates that reaction (4) does not explain this. However, the present data do not enable us to decide which reaction is of most importance, (7), (8) or (9). It is somewhat surprising that reaction (1) seems to take place, the low rate constant k,, taken into consideration and also the fact that the reverse process (8) takes place when the matrix is annealed at 110 K [16]. Reaction (1) has to compete with reaction (2) and reaction (6). Thus one can make a rough estimate of its rate constant. At the X-ray doses used in this work (~135 krad) the concentration of O- radicals is about 3 X 10m4 M. Using the rate constant of reaction (2) and assuming that a fraction of the order of 0.01 of the electrons undergoes reaction (1) during bleaching one can estimate k, to 1.3 X lo3 M-l s-l_ The kinetic energy of the photomobilized electrons may explain why k, is larger than klb_ During bleaching reaction (4) probably takes place in 7 M NaC104 as it does in 9 M NaOH, but still the Ht concentration increases in the former case while it decreases in the latter case. The reason for this is probably that H atoms are formed more efficiently during photobleaching in NaClO,- than in NaOH-glass. At least three mechanisms may explain this. H,O+ or analogous ions are formed in the matrices during Xirradiation. These ions are hardly stable in NaOH-glass [cf. reaction (lo)], but may be so in NaCIOq-glass. Thus, in the latter case some of the trapped H atoms may result from reaction (5) as proposed by Ershov and Pikaev [ 1 l] _ It is also possible that reaction (1) should be regarded as an equilibrium: e- + H,O + H + OH-. This is in accordance with the annealing experiments in ref. [16] (showing that H, gives rise to et) and would readily explain that H atoms are formed more e Wciently in NaC104- than in NaOH-glass. One could also propose that different trapping efficiencies explain the differences in H, yield in the two matrices. However, three observations seem to be in contrast
15 Aprif I977
with the latter explanation. Firstly the H atoms are more stable during annealing in NaOH- than in NaC104glass. Secondly, the ESR linewidth of Ht is larger in NaOH- than in NaC104-glass [ 141. These two observations indicate that the configuration of the trap is more stable and that the H atoms interact stronger and possibly in a more varied way with their surroundings in NaOH-glass than in NaClOq-glass. The third observation is probably more conclusive but still not evidence: the H-atom scavenger ally1 alcohol reduces the yield of H, more efficiently in NaClO,- than in NaOH-glass
E141. However, ally1 alcohol may exist in its anionic form in 9 M NaOH and the anionic form may react differently with H atoms. It is also possible that ally1 alcohol reacts with a precursor of the H atoms, and that the lifetime of such precursor
is longer in NaClC14-
than in NaOH-glass.
One astonishing conclusion may tentatively be drawn from these observations. It seems clear from observations (E) and (G) that mobile electrons give rise to H,_ Observation (B) is consistent with the assumption that X-irradiation does not produce H, with mobile eIectrons as precursors, but rather through the commonly accepted mechanism, dissociation of excited water molecules [ 11,7]. These two observations together indicate that during X-irradiation trapped electrons may be formed without having mobile electrons as precursors. The idea that e; are formed without having e; as precursors substantiates a new model for radioIysis of water [17] and a model for radioIysis of condensed media proposed by Steen [18] to explain several shortcomings of the present radiolysis models. In the latter model it is suggested that the trapped electrons are formed directly from CTTS states. Also in this case there is one reservation that shouId be considered before the above mentioned observations can be regarded as proof of the correctness of the model. The electrons may be mobile a much shorter time during X-irradiation than during photobleaching, since in the latter case each electron may be mobilized and retrapped several times before it undergoes reaction. Photoconduction experiments with and without scavengers and accurate and quantitative photobIeaching experiments may help us to reach a safe conchrsion. Observation (G) indicates that W-irradiation of matrices containing tryptophan causes dissociation of H20. The quantum energy of the W-light is only 257
Volume 47, number 2
CHEMICAL PHYSICS LElTERS
about 4 eV, but by triplet absorption (a biphotonic process) it is possible to reach an energy well above the energy necessary to break an OH bond in water, which is about 5.1 eV 1191. Due to the broad ar.d asymmetric line of O- it is hard to determine the yield from the ESR-spectra. The scavenger effect of NO, on the yield of O- indicates that recombination of electrons and cations may produce excited states of tryptophan that transfer energy to H20_ In accordance with this is the observation that the total radical yield is enhanced by the presence of NOT [observation (C)J. Similar observations have previously been made with ethylene glycol water glass as the trapping matrix [20]. The wavelength dependence in observation (F) indicates that the electrons are not slowed down to thermal energy before they undergo reaction. If this were the case one wou!d expect to find no wavelength dependence. The wavelength dependence cannot be explained by retrapping in traps of different depths, since in both cases the bleaching resulted in complete removal of et, i.e. electrons in all kinds of traps absorb light at both wavelengths. Reaction (1) may be more favoured the lower the energy of the mobile electrons. In addition it is possible that H atoms produced with high energy diffuse further than those produced with low energy and therefore have higher probability to undergo reaction prior to trapping. This is in accordance with the fmdings of Kaalhus [21] who showed that the immediate precursor of H, in H2S04glass is not e& but a mobile H atom. In conclusion we have shown that photomobiIized electrons in 7 M NaCI04- and 9 M NaOH-glass give rise to trapped hydrogen atoms, while electrons are not precursors to radiolyticalIy produced trapped hydrogen atoms. Attention is drawn to the reaction e& + H20 + H + OH-. Possibly this reaction should be regarded as an equilibrium. Furthermore, it seems that mobile electrons produced during photobleaching are
258
15 April 1977
not sIowed down to thermal energy before they react to produce hydrogen atoms. References [ 1] L. Kevan, P.N. Moorthy and J.J. Weiss, Nature 199 (1963) 689. [2] L. Kevan, P.M. Moorthy and J.J. Weiss. J. Am. Chem. Sot. 86 (1964) 771. [ 3 ] V.N. Belevski and LT. Bugaenko, UI. Fit Khim. 42 (1968) 105. [4] L. Xevan and C. Fine, J. Am. Chem. Sot. 88 (1966) 869. [S] B.C. Ersbov and A.K. Pikaev, Khirn. Vys. Energ. 1 (1967) 29. [6] J. I&oh, B.C. Green and J-W-T. Spinks. Can. 3. Chem. 40 (1962) 413. [7] T. Heruiksen, Radiation Res. 23 (1964) 63. [8] B-G_ Ershov, AX. Pikaev. P.Ya GIazunov and V.I. Spitsyn, DokL Akad. Nauk SSSR 149 (1963) 163. [9] H.N. Rexroad and W. Gordy, Phys. Rev. 125 (1962) 242. [lo] H.S. Judeikis, J-M. Fluornoy and S. Siegei, J. Chem. Phys. 37 (1962) 2272. [ll 1 B.G. Ershov and A-K. Pikaev, Radiation Res. Rev. 2
(1969) 1. 112) 3. Moan and 0. Kaalhus, J. Chem. Phys. 61 (1974) 3556. [13] J-L._Magee and M. Burton, J. Am. Chem. Sot. 73 (1951) 523. [ 14 J B. Biickman, Cand. Real. Thesis, Trondheim University,
in preparation. [15] I-G_ Draganid and 2-D. DraganiE. The radiation chemistry of water (Academic Press, New York, 1971) p. 40. [16] M.C.R. Symons and D.N. Zimmerman, J. Phys. Chem. 80 (1976) 395. [ 17 ] M. Kongshaug, in preparation. [IS] I-LB. Steen, in: Electron-solvent and anion-solvent interactions, eds. L. Kevan and B. Webster (Elsevier, Amsterdam, 1976) p. 175. [19] M.S. Matheson and J. Rabani, J. Phys. Chem. 69 (1965) 1324_ 120) J. Moan and B. Hfivik, Chem. Pbys. Letters 43 (1976) 477. [21] 0. Kaalhus. Nature 239 (1972) 111.
CHEMICAL PHYSICS L”TTERS
PHOTOBLEACHINGOF RADIATION
BNDUCED T
15 April 1971
WED ELECTR(-jNS
M. KONCSHAUC, .I. MOAN, B. BiiCKMANN and H. H@VIK Norsk Hydra’s Institute for Cancer Research, The Norwegian Radium Hospital, Montebello, Oslo 3, Norway Received 1 December 1976 Revised manuscript received 24 January 1977
Photomobilization of trapped electrons in 7 M NaCl04- and 9 M NaOH-glassgives rise to trapped hydrogen atoms. This is probably due to the reaction e, + Hz0 - H + OH-. Experiments with an electron scavenger indicate that electrons are not precursors to radiolytically produced hydrogen atoms. It seems that the mobile electrons produceci during pkotobleaching are not slowed down to thermal energy before they react to produce hydrogen atoms, since the yield of the latter species is strongly dependent on the wavelength rji the bleaching light.
II. Introduction
tions which may shel; light on the above mentioned topic.
Small yields of trapped hydrogen atoms may be observed in neutral and alkaline matrices X-irradiated at 77 K [l-8]. The H atoms are not stably trapped in pure ice at this temperature, but they may be readily observed at 4.2 K [9,10]. The mechanism of production of H atoms in such systems, has been proposed to be decomposition of excited water molecules [7,11]. The excited water molecules may result from primary excitation or from recombination of electrons and HzO+ ions. However, we also observed I-I atoms in W-irradiated NaOH glass containing potassium ferrocyanide or tryptophan [ 121. We suggested that these H atom3 were formed through energy transfer from excited ferrocyanide or tryptophan molecules to surrounding water molecules. In addition one should consider the reaction: e; tHiO-+HtOH-,
(1)
where e, represents a mobile electron. Even though the dissociation of H20- is beiieved to be endotherm in the gas phase, reaction (1) may proceed in the condensed phase by utilizing the solvation energy of the OH- ion [ 131 and the energy of e;. As part of an extensive study of formation and decay of H atcms [ 141 we have m .de some observa-
2. Experimental Glassy samples pf 9 M NaOH and 7 M NaCiO, were prepared by allowing drops of sample solution to fall into liquid N,. The glassy pellets w&r X-irrsdiated 2nd transferred to quartz ESR-tubes. During this procedure the pellets were shielded from light and kept immersed in liquid N2. The source of UV-irradiation and bleaching was a 200 W high pressure mercury lrmp fitted to a Bausch and Lomb grating monochromator. The irradiated sampies were examined by ESR-methods. Details of the experimental procedure may be found elsewhere [12].
3. Resldts The experimental result:: may be summarized as follows: (A) X-irradiation results in the formation of tr:lpp~~i electrons (e;) and 1-latoms (El,), the ratio [I-I,]/[e, ] being about 0.06 in 7 M NaClO, and 0.01 in 9 M NarLPH. (B) Addition of small amounts of the electron scav. 255
CHEMICAL PHYSICS LETTERS
Volume 47, number 2
enger NOT (=lO-*M) reduces the yield of et significantly while the yield of Ht is constant to within 5%. (Cc) Complete bleaching of the trapped eJectrons with 580 run results in an increase in the concentration of H, by about 70% in 7 M NaCIOd and a decrease of about 40% in 9 M NaOH? (Dj During this bleaching the amount of O- radicals in 9 M NaOH decreases by about 65%. initially the concentration of et and that of O- was equal to within J 10%. (E) Annealing (5 min at420 K for 7 M NaCIO, and 5 min at 143 K for 9 M NaOH) rest&s in a coinpIete removal of H, while a large fraction of e; remains, i.e. [e; ] decreases by less than 10% in 9 M NaOH and by 40-50% in 7 M NaC104. When e; in such samples are bleached with visible light, H, reappear to about 20% or more of their concentration prior to annealing. (F) Notably in 7 M NaC104 there is a prominent wavelength dependence of the amount of H atoms that ap pear during photobleaching of annealed samples: at 366 nm about 20% reappear and at 580 run about 50% reappear_ In both cases it was thoroughIy checked that the bleaching resulted in complete removal of et. (G) UV-irradiation of matrices containing the photoionizable solute tryptophan gives rise to e; and Ht, the ratio [H, J/ [et] being about 0.003 in 9 M NaOH and 0.02 in 7 M NaCIO,. Addition of the electron scavenger NOT causes an equal reduction of e; and Ht. Traces of O- radicals were observed in the NaOH samples. Both the yield of O- and the totaJ radical yieId are significantly enhanced by the presence of NOT.
4. Discussion The fol!owing reactions known from the radiolysis of water (see ref. [IS] and references cited therein) should be considered: eTq +H20*H+OH-, II-,, = 16 M-l s-l,
(lb)
eTis + O- --, 20H-,
k2 =2.2X 10'0M-ls-1, eaq + e& +
(2)
Hz f 20H-,
kg = 5 X log M-‘s-1,
(3)
k4 = 3 X 10”’ M-Is-l,
(4)
e&+HHHH2+OH-,
256
15 April 1977
e&l+H,O-tH+H,O, ks
= 2 X lOlo
k6
= 1.3 X
M-ls-L ,
(3
M-‘s-l,
(6)
eG+HH202*OH+OH-, lOi
H+H-tH2, k7
H + OH-
= 1 X 10’0
M-1
s-l
,
(7)
j e&, kg
= 2.2 X i07 M-l s-l,
(8)
H+O-*OH-,
kg = 3 X lOlo M-Is-l, H30+ + OH-
(9)
--, 2H,O,
k,,= 1.5 X IO”
M-ls-I.
(10)
In the discussion which follows we assume that e;ls in these reactions may be replaced by e,, mobile electrons resulting from photobleaching of et. This may imply that the rate constants differ from those given above. As seen from observation (D) the photobleached electrons mainly disappear according to reaction (2) or probably the reaction [l I] :
e,+O-+O2-.
(2b)
However, other pathways must exist since 35% of the initial concentration of O- remains after complete bleaching of et [observation (D)]. Reaction (3) probably does not take place in the present matrix since bielectrons, (e&,, seem to be stabIe, notably when hole scavengers are present [ 1 I]. When excited either by annealing or optical bleaching, the bielectrons give rise to et. However, according to ref. [ 1 l] about 70% of the electrons are lost during this process, so it cannot be safely concluded that reaction (3) does not take place. Reaction (6) may take place, but we have no means of estimating its importance since the OH radicals give rise to O- radicals. However, if we assume that the yield of H202 is not larger than that in alkaline aqueous solutions at room temperature, we find that at most 25% of the electrons may disappear according to reaction (6) during bleaching. Reaction (4) may take place according to observation (Cc), but since the yield of H, is small, it can only account for the disappearance of a tiny fraction of the photomobilized electrons. Some of the photomobilized electrons seem to undergo reaction (1). This is indicated by observations (E) and (G). Reaction (1) may be of more importance than the yieId of H, seems to show, since a relatively
Volume 41, number 2
CHEMICAL PHYSICS LETTERS
large fraction of the produced H atoms may disappear before trapping. Thus the yield of primary atomic hydrogen in the radiolysis of water is about 0.6 (ref. [ 151 p. 73), i.e. a factor 3-4 times higher than the yield of H, in 7 M NaClO, and at least a factor 20 times higher than the yield of Ht in 9 M NaOH. Therefore, if it is assumed that the primary yield of atomic hydrogen is not much different in frozen and liquid solutions, a large fraction of the H atoms must disappear prior to trapping. The scavenger experiment (B) indicates that reaction (4) does not explain this. However, the present data do not enable us to decide which reaction is of most importance, (7), (8) or (9). It is somewhat surprising that reaction (1) seems to take place, the low rate constant k,, taken into consideration and also the fact that the reverse process (8) takes place when the matrix is annealed at 110 K [16]. Reaction (1) has to compete with reaction (2) and reaction (6). Thus one can make a rough estimate of its rate constant. At the X-ray doses used in this work (~135 krad) the concentration of O- radicals is about 3 X 10m4 M. Using the rate constant of reaction (2) and assuming that a fraction of the order of 0.01 of the electrons undergoes reaction (1) during bleaching one can estimate k, to 1.3 X lo3 M-l s-l_ The kinetic energy of the photomobilized electrons may explain why k, is larger than klb_ During bleaching reaction (4) probably takes place in 7 M NaC104 as it does in 9 M NaOH, but still the Ht concentration increases in the former case while it decreases in the latter case. The reason for this is probably that H atoms are formed more efficiently during photobleaching in NaClO,- than in NaOH-glass. At least three mechanisms may explain this. H,O+ or analogous ions are formed in the matrices during Xirradiation. These ions are hardly stable in NaOH-glass [cf. reaction (lo)], but may be so in NaCIOq-glass. Thus, in the latter case some of the trapped H atoms may result from reaction (5) as proposed by Ershov and Pikaev [ 1 l] _ It is also possible that reaction (1) should be regarded as an equilibrium: e- + H,O + H + OH-. This is in accordance with the annealing experiments in ref. [16] (showing that H, gives rise to et) and would readily explain that H atoms are formed more e Wciently in NaC104- than in NaOH-glass. One could also propose that different trapping efficiencies explain the differences in H, yield in the two matrices. However, three observations seem to be in contrast
15 Aprif I977
with the latter explanation. Firstly the H atoms are more stable during annealing in NaOH- than in NaC104glass. Secondly, the ESR linewidth of Ht is larger in NaOH- than in NaC104-glass [ 141. These two observations indicate that the configuration of the trap is more stable and that the H atoms interact stronger and possibly in a more varied way with their surroundings in NaOH-glass than in NaClOq-glass. The third observation is probably more conclusive but still not evidence: the H-atom scavenger ally1 alcohol reduces the yield of H, more efficiently in NaClO,- than in NaOH-glass
E141. However, ally1 alcohol may exist in its anionic form in 9 M NaOH and the anionic form may react differently with H atoms. It is also possible that ally1 alcohol reacts with a precursor of the H atoms, and that the lifetime of such precursor
is longer in NaClC14-
than in NaOH-glass.
One astonishing conclusion may tentatively be drawn from these observations. It seems clear from observations (E) and (G) that mobile electrons give rise to H,_ Observation (B) is consistent with the assumption that X-irradiation does not produce H, with mobile eIectrons as precursors, but rather through the commonly accepted mechanism, dissociation of excited water molecules [ 11,7]. These two observations together indicate that during X-irradiation trapped electrons may be formed without having mobile electrons as precursors. The idea that e; are formed without having e; as precursors substantiates a new model for radioIysis of water [17] and a model for radioIysis of condensed media proposed by Steen [18] to explain several shortcomings of the present radiolysis models. In the latter model it is suggested that the trapped electrons are formed directly from CTTS states. Also in this case there is one reservation that shouId be considered before the above mentioned observations can be regarded as proof of the correctness of the model. The electrons may be mobile a much shorter time during X-irradiation than during photobleaching, since in the latter case each electron may be mobilized and retrapped several times before it undergoes reaction. Photoconduction experiments with and without scavengers and accurate and quantitative photobIeaching experiments may help us to reach a safe conchrsion. Observation (G) indicates that W-irradiation of matrices containing tryptophan causes dissociation of H20. The quantum energy of the W-light is only 257
Volume 47, number 2
CHEMICAL PHYSICS LElTERS
about 4 eV, but by triplet absorption (a biphotonic process) it is possible to reach an energy well above the energy necessary to break an OH bond in water, which is about 5.1 eV 1191. Due to the broad ar.d asymmetric line of O- it is hard to determine the yield from the ESR-spectra. The scavenger effect of NO, on the yield of O- indicates that recombination of electrons and cations may produce excited states of tryptophan that transfer energy to H20_ In accordance with this is the observation that the total radical yield is enhanced by the presence of NOT [observation (C)J. Similar observations have previously been made with ethylene glycol water glass as the trapping matrix [20]. The wavelength dependence in observation (F) indicates that the electrons are not slowed down to thermal energy before they undergo reaction. If this were the case one wou!d expect to find no wavelength dependence. The wavelength dependence cannot be explained by retrapping in traps of different depths, since in both cases the bleaching resulted in complete removal of et, i.e. electrons in all kinds of traps absorb light at both wavelengths. Reaction (1) may be more favoured the lower the energy of the mobile electrons. In addition it is possible that H atoms produced with high energy diffuse further than those produced with low energy and therefore have higher probability to undergo reaction prior to trapping. This is in accordance with the fmdings of Kaalhus [21] who showed that the immediate precursor of H, in H2S04glass is not e& but a mobile H atom. In conclusion we have shown that photomobiIized electrons in 7 M NaCI04- and 9 M NaOH-glass give rise to trapped hydrogen atoms, while electrons are not precursors to radiolyticalIy produced trapped hydrogen atoms. Attention is drawn to the reaction e& + H20 + H + OH-. Possibly this reaction should be regarded as an equilibrium. Furthermore, it seems that mobile electrons produced during photobleaching are
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not sIowed down to thermal energy before they react to produce hydrogen atoms. References [ 1] L. Kevan, P.N. Moorthy and J.J. Weiss, Nature 199 (1963) 689. [2] L. Kevan, P.M. Moorthy and J.J. Weiss. J. Am. Chem. Sot. 86 (1964) 771. [ 3 ] V.N. Belevski and LT. Bugaenko, UI. Fit Khim. 42 (1968) 105. [4] L. Xevan and C. Fine, J. Am. Chem. Sot. 88 (1966) 869. [S] B.C. Ersbov and A.K. Pikaev, Khirn. Vys. Energ. 1 (1967) 29. [6] J. I&oh, B.C. Green and J-W-T. Spinks. Can. 3. Chem. 40 (1962) 413. [7] T. Heruiksen, Radiation Res. 23 (1964) 63. [8] B-G_ Ershov, AX. Pikaev. P.Ya GIazunov and V.I. Spitsyn, DokL Akad. Nauk SSSR 149 (1963) 163. [9] H.N. Rexroad and W. Gordy, Phys. Rev. 125 (1962) 242. [lo] H.S. Judeikis, J-M. Fluornoy and S. Siegei, J. Chem. Phys. 37 (1962) 2272. [ll 1 B.G. Ershov and A-K. Pikaev, Radiation Res. Rev. 2
(1969) 1. 112) 3. Moan and 0. Kaalhus, J. Chem. Phys. 61 (1974) 3556. [13] J-L._Magee and M. Burton, J. Am. Chem. Sot. 73 (1951) 523. [ 14 J B. Biickman, Cand. Real. Thesis, Trondheim University,
in preparation. [15] I-G_ Draganid and 2-D. DraganiE. The radiation chemistry of water (Academic Press, New York, 1971) p. 40. [16] M.C.R. Symons and D.N. Zimmerman, J. Phys. Chem. 80 (1976) 395. [ 17 ] M. Kongshaug, in preparation. [IS] I-LB. Steen, in: Electron-solvent and anion-solvent interactions, eds. L. Kevan and B. Webster (Elsevier, Amsterdam, 1976) p. 175. [19] M.S. Matheson and J. Rabani, J. Phys. Chem. 69 (1965) 1324_ 120) J. Moan and B. Hfivik, Chem. Pbys. Letters 43 (1976) 477. [21] 0. Kaalhus. Nature 239 (1972) 111.