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Pulse radiolysis study of the mechanism of electron trapping in alcohol glasses at 77°K
Yctiume 11, number 1
CHEMICAL PHYSICS LEiTERS
PULSE RADIOLYSIS STUDY OF THE ~C~~S~ OF ELECTRON TRAPPING IN ALCOHOL GLASSES AT 77OK
Received 19 July 197 1
Trapped electron spectra in pulse-irradiated t-propanol &SS at 77°K shift toward the blue on a ,use;ctimescale. Solute effects indicate that molecular reorientation around the electron is responsible for the transient spectrai shift rather than a redistribution of electrons among traps.
It is well documented that electrons are trapped in y-tiadiated alcohol glasses at 77°K [ I] . Recently, Richards and Thomas [Z] were able to observe the electron trapping process in ethanol &ass at 77% OR
-amicrosecondtimescale,by usingpulse radialysk. Thek strikingob~~a~io~ wasthat the spectrummaximum shifted from beyond 1300 nm to 594 nm within 4 ~sec. This transient spectral shift indicates that the electron is shifting from a shallow trap to a deeper trap on this timescale. Richards and Thomas suggested two possible mechanisms. in mechanism I the electron is trapped in a shallow trap and then the strong electric field of the electron promotes reorientation of the surrounding molecular dipoles to produce a deeper trap. In mechanism II it is postuiated that there exists a distribution,of preformed shallow and deep zaps in the glass matrix. initially the electrons are predominantly. trapped iri the shallow traps, but on a microsecond timescale the electrons are thermally excited out of &ailow trips a& ultimately redistributed into deeper traps. Richards and Thomas favored mechanism I as did Hamill [3] in a further discussion of their data. But mechanism I1 could not be ruled out. Here we present expkrimental evidence for mechanism I and against mechanism II which is based on solute effects in pulseirradiated 2-propanol glasses. l
1970,7X Guggenheim F&o& at Fe Danish Atomic En&y Establishment R&-i, RoskiIde, Denmark.
Yl&-
Reagent grade 2-propaF.ol (l liter) was refluxed with concentrated H,SO, (1 ml) and 2,4&nitrophenyl hydrazine (2 g) and was slowly distilled. Reagent grade betlzyl chloride and biphenyi were used as solutes. The szunple ceti’consisted
of 3n aluminum cylinder
with spectrosil windows and an access tube cemented
in with Silastic 732 RTV (Dow-Coming). Samples were degassed by argon bubbling and the cells were seaied off. The dewar used was similar to that described earlier f2] IIThe pulseradiolysisset-upat the Danish Atomic Energy Establishment Risii was used; it has been described [4]. Ten MeV electrons were used at typically a nominal pulse width of 0.4 ~sec and an average dose of about 3 krad. A photomultiplier detector with a limit of about 850 nm was used. When pure 2-propanof glass was pulse irradiated at 77% an absorption rising to beyond 850 nm is observed about 0.2 gsec after the pulse. At 700 nm the absorption shows a transient growth after the pulse. This growth is due to a spectrai shift of the absorbing species. The spectrum shows a peak at 800 nm 1OO~sec after the pulse. This’spectrum is attributed to trapped electrons and its transient behavior is sin& lar to’that reported for electrons in ethanol glass [2]. Biphenyi reacts with electrons to produce the biphenyl anion which has an absorption peak near, 400 sun in organic glasses ES). Thus with added biphenyl; both the biphenyl anion at 400 run and the trapped eIectron at 7%,,~ can be monitored.‘Biphenyl concentrations of 1 X lo:?,3 X 10~~ and
Volume 11,. number 1
CHEMICAL. PHYSICS LE-ITERS
1 X 10-l M were used. In this concentration ranRe 40 to 60% of the electrons are scavenged to form biihenyl anions: The trapped electron at 700 nm showed a transient growth as seen in pure 2-propkol. However, the biphenyl anion spectrum remained constant in time. Similar experiments were carried out with 1 X low2 and 1 X 10-l M benzyl chloride. With this solute the electron reacts by dissociative attachment to form the benzyl radical [5] which has a maximum absorption at 3 19 nm in 2-propanol glass. With 1 X 10m2 M benzyl chloride about 40% of the electrons are scavenged. Again, the trapped electron at 700 MI showed a transient growth. However the benzyl radical at 3 19 nm was constant in time. These experiments show that trapped electron redistribution among different traps does not occur after the electron pulse to any significant extent. Since we have enough solute to trap about half of the mobile electrons we expect that trapped electron redistribution would lead to a transient increase in the solute spectrum (i.e. biphenyl anion or benzyl radical). In other
1.5 September 19?1
in the near infrared [7]. On warming to 77°K the EPI? line width increases [6] and the absorption band shifts toward the blue [7]. These changes are irreversible in that no further changes occur on retooling to 49,i(. These changes are most plausibly explained i.$&ms of molecular reorientation around the trapped’electron on warming. They can also be expIained in terms of electron redistribution among different preformed traps on warming. However, our results imply that the molecular reorientation model is the correct picture. It appears that a consequence of moIecular reorientation of the neighboring matrix molecuIes around trapped electrons would be to emphasize theimportance of interactions between the electron and the matrix. This is precisely the aspect that has been emphasized in recent theoretical caicuIations of electron salvation and trapping in polar systems [S-l 11. It has been found that inclusion of a short range chargedipole interaction in addition to a long range polarization interaction leads to improved agreement between calculated and experimental energy levels of trapped and solvated electrons [lZ, I3f.
words some of the electrons should be scavenged
by the solute during the redistribution
process and
should lead to an increase in the biphenyl anion and benzyl radical intensity (not a spectral shift) on a wc time&e. On the other hand the observation of a transient growth in the residual trapped electron spectrum at 700 nm indicates that the process leading to the spectral shift of the trapped electron is still occurring. Although these results do not satisfy the requirements of mechanism II, they are readily explained in terms of mechanism 1. In mechanism I the molecular dipoles around the electron reorient to deepen the trap and cause a spectral shift. In this mechanism the electron stays in the same trap but the nature (i.e. depth) of the trap changes. So during the reorientation process no additional electrons are expected to be scavenged by solute molecules. The solute spectrum should not change in intensity while the trapped electron spectrum undergoes its spectral shift. Other recent experiments of a somewhat different type also indicate that molecular reorientation occurs in the process of electron trapping at 77OK in alcohol glasses [6,7]. If ethanol or other alcohol glasses are yirradiated at 4”K, the trapped electrons are characterized by a singlet EJ’R line [6] ,tid an optical band
I thank the John Simon Guggenheim Foundation
for a fellowship which made this work possible, the Danish Atomic Energy Establishment for their facilities and hospitality, and S.O. Nielsen, P. Pagsberg, K. Sehested, J. Eriksen and the acceIerator operators for their assistance. The data analysis was supported by the U.S. Atomic Energy Commission under Contract AT(ll-Q-2086.
References [II L.Kevau, Actions Chirn. Biolog. Radiations 13 (1969) 57.
PI J.T.Richards and J.K.Thomas. I. Chem. Phys. 53 (1970)
218. 131 W.H.Hamill. J. Clam. Phys 53 (1970) 473. (41 H.C!.Christensen,C.Nil;son. P.Pagsberg and S.O.f!ielsen, Rev. Sci. Instr. 40 (1969) 786. [51 W.H.HamilI, in: Radical ions, eds. E.T.Kaiser and L. Kevan (Wiley-Interszience, New York, 1968) ch. 9. (61 T.Higzhimura, M.Noda, T.Warashina and H.Yoshida, J. Chem. Phys. 53 (1970) 1152. [71 H.Hase. ki.Nadaaud THigashimura, I. Chem. Phys 54 (1971) 2975. [81 K.Fueki, D.F.Feng and L.Kevan, I. Fhys. Chem. 74 (1970) 1976.
141
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-**‘nl
CHEMICAL PHYSICS LEiTERS
PULSE RADIOLYSIS STUDY OF THE ~C~~S~ OF ELECTRON TRAPPING IN ALCOHOL GLASSES AT 77OK
Received 19 July 197 1
Trapped electron spectra in pulse-irradiated t-propanol &SS at 77°K shift toward the blue on a ,use;ctimescale. Solute effects indicate that molecular reorientation around the electron is responsible for the transient spectrai shift rather than a redistribution of electrons among traps.
It is well documented that electrons are trapped in y-tiadiated alcohol glasses at 77°K [ I] . Recently, Richards and Thomas [Z] were able to observe the electron trapping process in ethanol &ass at 77% OR
-amicrosecondtimescale,by usingpulse radialysk. Thek strikingob~~a~io~ wasthat the spectrummaximum shifted from beyond 1300 nm to 594 nm within 4 ~sec. This transient spectral shift indicates that the electron is shifting from a shallow trap to a deeper trap on this timescale. Richards and Thomas suggested two possible mechanisms. in mechanism I the electron is trapped in a shallow trap and then the strong electric field of the electron promotes reorientation of the surrounding molecular dipoles to produce a deeper trap. In mechanism II it is postuiated that there exists a distribution,of preformed shallow and deep zaps in the glass matrix. initially the electrons are predominantly. trapped iri the shallow traps, but on a microsecond timescale the electrons are thermally excited out of &ailow trips a& ultimately redistributed into deeper traps. Richards and Thomas favored mechanism I as did Hamill [3] in a further discussion of their data. But mechanism I1 could not be ruled out. Here we present expkrimental evidence for mechanism I and against mechanism II which is based on solute effects in pulseirradiated 2-propanol glasses. l
1970,7X Guggenheim F&o& at Fe Danish Atomic En&y Establishment R&-i, RoskiIde, Denmark.
Yl&-
Reagent grade 2-propaF.ol (l liter) was refluxed with concentrated H,SO, (1 ml) and 2,4&nitrophenyl hydrazine (2 g) and was slowly distilled. Reagent grade betlzyl chloride and biphenyi were used as solutes. The szunple ceti’consisted
of 3n aluminum cylinder
with spectrosil windows and an access tube cemented
in with Silastic 732 RTV (Dow-Coming). Samples were degassed by argon bubbling and the cells were seaied off. The dewar used was similar to that described earlier f2] IIThe pulseradiolysisset-upat the Danish Atomic Energy Establishment Risii was used; it has been described [4]. Ten MeV electrons were used at typically a nominal pulse width of 0.4 ~sec and an average dose of about 3 krad. A photomultiplier detector with a limit of about 850 nm was used. When pure 2-propanof glass was pulse irradiated at 77% an absorption rising to beyond 850 nm is observed about 0.2 gsec after the pulse. At 700 nm the absorption shows a transient growth after the pulse. This growth is due to a spectrai shift of the absorbing species. The spectrum shows a peak at 800 nm 1OO~sec after the pulse. This’spectrum is attributed to trapped electrons and its transient behavior is sin& lar to’that reported for electrons in ethanol glass [2]. Biphenyi reacts with electrons to produce the biphenyl anion which has an absorption peak near, 400 sun in organic glasses ES). Thus with added biphenyl; both the biphenyl anion at 400 run and the trapped eIectron at 7%,,~ can be monitored.‘Biphenyl concentrations of 1 X lo:?,3 X 10~~ and
Volume 11,. number 1
CHEMICAL. PHYSICS LE-ITERS
1 X 10-l M were used. In this concentration ranRe 40 to 60% of the electrons are scavenged to form biihenyl anions: The trapped electron at 700 nm showed a transient growth as seen in pure 2-propkol. However, the biphenyl anion spectrum remained constant in time. Similar experiments were carried out with 1 X low2 and 1 X 10-l M benzyl chloride. With this solute the electron reacts by dissociative attachment to form the benzyl radical [5] which has a maximum absorption at 3 19 nm in 2-propanol glass. With 1 X 10m2 M benzyl chloride about 40% of the electrons are scavenged. Again, the trapped electron at 700 MI showed a transient growth. However the benzyl radical at 3 19 nm was constant in time. These experiments show that trapped electron redistribution among different traps does not occur after the electron pulse to any significant extent. Since we have enough solute to trap about half of the mobile electrons we expect that trapped electron redistribution would lead to a transient increase in the solute spectrum (i.e. biphenyl anion or benzyl radical). In other
1.5 September 19?1
in the near infrared [7]. On warming to 77°K the EPI? line width increases [6] and the absorption band shifts toward the blue [7]. These changes are irreversible in that no further changes occur on retooling to 49,i(. These changes are most plausibly explained i.$&ms of molecular reorientation around the trapped’electron on warming. They can also be expIained in terms of electron redistribution among different preformed traps on warming. However, our results imply that the molecular reorientation model is the correct picture. It appears that a consequence of moIecular reorientation of the neighboring matrix molecuIes around trapped electrons would be to emphasize theimportance of interactions between the electron and the matrix. This is precisely the aspect that has been emphasized in recent theoretical caicuIations of electron salvation and trapping in polar systems [S-l 11. It has been found that inclusion of a short range chargedipole interaction in addition to a long range polarization interaction leads to improved agreement between calculated and experimental energy levels of trapped and solvated electrons [lZ, I3f.
words some of the electrons should be scavenged
by the solute during the redistribution
process and
should lead to an increase in the biphenyl anion and benzyl radical intensity (not a spectral shift) on a wc time&e. On the other hand the observation of a transient growth in the residual trapped electron spectrum at 700 nm indicates that the process leading to the spectral shift of the trapped electron is still occurring. Although these results do not satisfy the requirements of mechanism II, they are readily explained in terms of mechanism 1. In mechanism I the molecular dipoles around the electron reorient to deepen the trap and cause a spectral shift. In this mechanism the electron stays in the same trap but the nature (i.e. depth) of the trap changes. So during the reorientation process no additional electrons are expected to be scavenged by solute molecules. The solute spectrum should not change in intensity while the trapped electron spectrum undergoes its spectral shift. Other recent experiments of a somewhat different type also indicate that molecular reorientation occurs in the process of electron trapping at 77OK in alcohol glasses [6,7]. If ethanol or other alcohol glasses are yirradiated at 4”K, the trapped electrons are characterized by a singlet EJ’R line [6] ,tid an optical band
I thank the John Simon Guggenheim Foundation
for a fellowship which made this work possible, the Danish Atomic Energy Establishment for their facilities and hospitality, and S.O. Nielsen, P. Pagsberg, K. Sehested, J. Eriksen and the acceIerator operators for their assistance. The data analysis was supported by the U.S. Atomic Energy Commission under Contract AT(ll-Q-2086.
References [II L.Kevau, Actions Chirn. Biolog. Radiations 13 (1969) 57.
PI J.T.Richards and J.K.Thomas. I. Chem. Phys. 53 (1970)
218. 131 W.H.Hamill. J. Clam. Phys 53 (1970) 473. (41 H.C!.Christensen,C.Nil;son. P.Pagsberg and S.O.f!ielsen, Rev. Sci. Instr. 40 (1969) 786. [51 W.H.HamilI, in: Radical ions, eds. E.T.Kaiser and L. Kevan (Wiley-Interszience, New York, 1968) ch. 9. (61 T.Higzhimura, M.Noda, T.Warashina and H.Yoshida, J. Chem. Phys. 53 (1970) 1152. [71 H.Hase. ki.Nadaaud THigashimura, I. Chem. Phys 54 (1971) 2975. [81 K.Fueki, D.F.Feng and L.Kevan, I. Fhys. Chem. 74 (1970) 1976.
141
,_:
.;
: ..
‘,
-
.:
.*-
.,
>&and, N:R.Keitni
0
,.’
.‘.
‘.
.i, .,
_
..
: :
.” .*
[ 111 ICFtieki,-I);-F.Feng +d tK&n,
aid J.Joriner, JXhem.
10(1971)504; :,_ ‘. . .I
..:
:.
%
C&an. F’h& ‘bttexs
:
.:5;-
!lZj I.Eisete.and L.K&n,‘J. Chcm,Ph#. 53 (@‘&b) 1837. [ 131 ~.Ha&et and L.K&&, 3: Am: Chem. Say.. e3 (1971)
-**‘nl