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Optical absorption spectrum of trapped electron in crystalline ice at −150°C. Pulse radiolysis study
Volume
14,
number
2
CHEhIlCAL
PHYSICS LETTERS
15 hfay 1972
OPTICAL ABSORPTION SPECTRUM OF TRAPPED ELECTRON IN CRYSTALLLNE ICE AT -150°C. PULSE RADIOLYSIS STUDY K. KAWABATA, H. HORII Radiation
Center
of Osaka Prefecture.
and S. OKABE
Shbzke-rho.
Sakai,
Received 27 January 1972 Revised manuscript received 28 February
Osaka, Japan
1972
A pulse rsdiolysis study of single crystals of D20 ice at - 150°C has been made. In addition to the visible absorption with the peak at 6300 A due to trapped electrons, e brood IK absorption with B peak-like structure at 10000 X is found at the end of the electron beam pulse of 5 CISCCduration. The absorption of the IK region decays faster than that of the visible region. The IR absorption stems to be due to electrons trapped in shallow traps’Thc striking ?ransformstion from IR to the visible region, such as in ethanol, cmnot bc observed. In a sample of D20 crystals doped with lo-” hi NH4F, about 1% of the absorption in the visibic region shows delayed building up with a half life of about 0.1 msec.
Recently, Richard and Thomas [ I] found for ethanol glass at 77°K that an IR absorption continuous in wavelength from the visible region to 13000 A or more, exists transiently after pulsed electron irradiation and that it transforms in 4 _~sec to the visible spectrum with the peak at 6000 8. This transient IR absorption was ascertained by ESR study at 4”K, by Higashimura et al. [2] , to be due to the trapped electron. The IR absorption is considered to be due to the electron captured on a shallow trap existing in the glassy region where the permanent dipoles are randomly oriented. The striking transformation from IR to visible spectra is considered [l] to be the result of either, electrons “digging-own-hole” mechanism by the orientation of the surrounding dipoles, or by electron transfer from the shallow trap to the preexisting deep trap. However, in glassy ice (frozen solid of aqueous solution of concentrated alkali), the similar broad IR absorption has not been observed [ 1, 31. We have made a pulse radiolysis study below -1OO’C on crystalline ice and have found a broad IR absorption similar to the one in ethanol but with different characteristics. A few pulse radiolysis studies on crystalline ice have been made. It has been observed [4] * that the optical peak position of the hydrated electron continuously shifts to the higher energy side
with decreasing temperature: from 7200 A at 20°C (liquid) to about 6300 A at -135°C (crystalline ice). However, IR absorption has not been reported. In crystalline ice the G-value of the trapped electron at low temperature is very small. We used the sample ofD,O and D,O doped with lo-* M NH4F instead of H,O to obtain enough absorbance. (The Gvalue [5] of the trapped electron at - 196’C in D,O and that in 10-I M-NHjF-doped D,O are about 1.5-2 times and about ten times greater than that in H,O, respectively.) Spectrometric grade D,O of Merck Ltd. was used without further purification. The method of growing a single crystal of D,O ice was almost the same as was reported in the previous paper [S] _ An ice crystal block of optical path length of 20 mm, with cross section of 14 mm X 16 mm, was attached to a copper holder which was suspended in a dewar box designed for pulse radiolysis. Under the holder a copper finger was connected. The temperature of the crystal was adjusted by changing the fength of the finger immersed in liquid nitrogen stored in the bottom of the dcwar, and detected by a copper-constantan * Schubin et al. studied ihc temperature Taub and Eiben studied the range -10 Nilsson et a!. the range - 10 - -50°C.
range - IO - -80°C; - - 165°C and
223
CHEMICAL PHYSICS LETTERS
Volume 14, number 2 Photon
Energy
tevt
15 May 1972
Photon
Energy
3.0
9.
(a)
20
1.0 I
I o Wavelength
thermocouple buried directly at the top of the ice block. The focused light from a 150 W Xe lamp was
through the sample in a direction per-
pendicular to ‘he electron beam, and was led to a Narumi RM-23 monochromator. The analyzed light was detected by an RCA 7102 photomultiplier. The transient signal was amplified 5Ci times before being fed to an oscilloscope. The time range measurable with this detection system wx from 0.5 psec to 10 msec. The bombarding electron beam was delivered from the ORC Linac, and was 10 MeV in energy, 300 mA in peak current, 5 I.rsec duration. The beam was focused on the crystal to a 15 mm diameter. At -1OO’C the IR absorption could not be observed. The spectrum has the single peak at about 6300 A. At - 15O’C the new transient
absorption
in
the IR region was observed. The wavelength distributions of the visible and the IF, spectra are shown in fig. l.for t = 0,0.4 msec, and 2 msec (t is the elapsed time after the end of the electron beam Fulse). At t = 0, the optical absorption in the IR increases with wavelength from about 7500 A to 10000 X at which a peaklike structure is formed, and the IR spectrum is plainly distinguishable from the peak in the :-isible region (at 6300 a). The entire spectrum reaches the maximum at the end of the electron beam prtlse. The decay of the IR spectrum is faster than the visible spectrum, and seems to be non-uniform in wavelength. Similar spectra in D,O doped with 10el M NH,F are shown in fig. 2. The waveleng*h distribution at t = 224 :
5
6 Wavelength
Fig 1. Transient absorption spectra observed in pulse radioIysisof single crystals ofDl0 ice at -1jO’C. Spectrum at the end of the pulse, (0); spectru$ after 0.4 msec, (a); spectrum after 2 fisec, (x).
transmitted
I
I
4
(2,
7
8
9
IO ll(Xld,
(i,
Fig. 2. Transient absorption spectra observed in pulse radiocrystals of D20 ice doped with 10e2 hf NHJF. Spectrum at the end of the pulse, (0); spectrum after 0.4 msec, (0); spectrum after 2 msec, (x). The dot-dash line indicates Iysis of single
the assumed spectrum at f = - which is inferred from the stable absorption at -196’C IS.]. (The real peak height at - is not equal to that at r = 2 mscc.)
t=
0 is almost the same as that in the pure ice. The relative absorbance becomes about twice that in the pure crystal, and the decay becomes slower over the entire region. Especially, the decay of the absorption in the visible region becomes much slower. Probably, this is caused by limited recombination of the electron due to trapping of holes by fluorine ions. In the IR region the non-uniformity of the decay on wavelength becomes pronounced compared with that in the pure crystal. The IR absorption seems to be due to several types of shal!ow traps of various depths, especially in the doped sample. In the doped sample, about 10% of the absorbance in the visible region builds up slowly with a half life of about 0.1 msec. This building up observed at 6300 a is shown in fig. 3. It rather re-
sembles that reported by Kenny and Walker 161, whc found building up of 50 nsec at 6328 K in pure water and low3 M OH- aqueous solution by using a Febetron machine and a He-Ne laser. From the analogy to the case in ethanol, the IR
absorptions observed in the pure and the doped crystal are considered to be due to the electrons in shallow traps. The striking transformation from the IR region to the visible region, as is observed in ethanol, does not occur in crystalline ice. Mostof the electrons in deep traps seem to be produced independently of : those in shallow traps. However, there remains the possibility that the small building up in the visible :
Volume 14, number 2
< e
CHEMICAL PHYSICS LETTERS
PlJk 1
15 May 1972
References
5
Richards and J.K. Thomas, J. Chem. Phys. 53 (1970) 218. hf. Noda, T. Warclshina and H. Yoshida, PI T. Hipshimurn, J. Chem. Phys. 53 (1970) 1152; H. Hase, T. Warashina, >I. Noda, A. Namiti and T. Higashimura, Proceedings of the 14th Congress on Radiation Chemistry, Sapporo, Japan (197 1) p. 111. [31 H. Hase, hl. Noda and T. Higashimura, J. Chem. Phys 54 (1971) 2975. r41 V.N. Schubin, V.A. Zhigunov, V.I. Zolotarevsky and P.L. Dolin. Nature 212 (1966) 1002; LA. Taub and K. Eiben, J. Chem. Phys. 49 (1968) 2499; G. Nilsson, H.C. Christensen, J. Fenger, P. Pagobcrg and SO. Nielsen, Advan. Chem. Ser. 81 (1965) 7 1. [51 K. Rawrtbata, J. Chem. Phys. 55 (1971) 3672. [61 G.A. Kenney and D.C. Walker, J. Chcm. Phys. 53 (1970)
[II
‘D % ._
‘0 ‘I0 G
Time
Fig. 3. OsciJloscope trace showing the writion of transient absorption as a function of time after the pulse for single crystals of D20 ice doped with lO_’ hi NH4F. region of the doped crystal may be due to the retrapping of the electron thermally ejected frun the shallow trap.
J.T.
1282.
225
14,
number
2
CHEhIlCAL
PHYSICS LETTERS
15 hfay 1972
OPTICAL ABSORPTION SPECTRUM OF TRAPPED ELECTRON IN CRYSTALLLNE ICE AT -150°C. PULSE RADIOLYSIS STUDY K. KAWABATA, H. HORII Radiation
Center
of Osaka Prefecture.
and S. OKABE
Shbzke-rho.
Sakai,
Received 27 January 1972 Revised manuscript received 28 February
Osaka, Japan
1972
A pulse rsdiolysis study of single crystals of D20 ice at - 150°C has been made. In addition to the visible absorption with the peak at 6300 A due to trapped electrons, e brood IK absorption with B peak-like structure at 10000 X is found at the end of the electron beam pulse of 5 CISCCduration. The absorption of the IK region decays faster than that of the visible region. The IR absorption stems to be due to electrons trapped in shallow traps’Thc striking ?ransformstion from IR to the visible region, such as in ethanol, cmnot bc observed. In a sample of D20 crystals doped with lo-” hi NH4F, about 1% of the absorption in the visibic region shows delayed building up with a half life of about 0.1 msec.
Recently, Richard and Thomas [ I] found for ethanol glass at 77°K that an IR absorption continuous in wavelength from the visible region to 13000 A or more, exists transiently after pulsed electron irradiation and that it transforms in 4 _~sec to the visible spectrum with the peak at 6000 8. This transient IR absorption was ascertained by ESR study at 4”K, by Higashimura et al. [2] , to be due to the trapped electron. The IR absorption is considered to be due to the electron captured on a shallow trap existing in the glassy region where the permanent dipoles are randomly oriented. The striking transformation from IR to visible spectra is considered [l] to be the result of either, electrons “digging-own-hole” mechanism by the orientation of the surrounding dipoles, or by electron transfer from the shallow trap to the preexisting deep trap. However, in glassy ice (frozen solid of aqueous solution of concentrated alkali), the similar broad IR absorption has not been observed [ 1, 31. We have made a pulse radiolysis study below -1OO’C on crystalline ice and have found a broad IR absorption similar to the one in ethanol but with different characteristics. A few pulse radiolysis studies on crystalline ice have been made. It has been observed [4] * that the optical peak position of the hydrated electron continuously shifts to the higher energy side
with decreasing temperature: from 7200 A at 20°C (liquid) to about 6300 A at -135°C (crystalline ice). However, IR absorption has not been reported. In crystalline ice the G-value of the trapped electron at low temperature is very small. We used the sample ofD,O and D,O doped with lo-* M NH4F instead of H,O to obtain enough absorbance. (The Gvalue [5] of the trapped electron at - 196’C in D,O and that in 10-I M-NHjF-doped D,O are about 1.5-2 times and about ten times greater than that in H,O, respectively.) Spectrometric grade D,O of Merck Ltd. was used without further purification. The method of growing a single crystal of D,O ice was almost the same as was reported in the previous paper [S] _ An ice crystal block of optical path length of 20 mm, with cross section of 14 mm X 16 mm, was attached to a copper holder which was suspended in a dewar box designed for pulse radiolysis. Under the holder a copper finger was connected. The temperature of the crystal was adjusted by changing the fength of the finger immersed in liquid nitrogen stored in the bottom of the dcwar, and detected by a copper-constantan * Schubin et al. studied ihc temperature Taub and Eiben studied the range -10 Nilsson et a!. the range - 10 - -50°C.
range - IO - -80°C; - - 165°C and
223
CHEMICAL PHYSICS LETTERS
Volume 14, number 2 Photon
Energy
tevt
15 May 1972
Photon
Energy
3.0
9.
(a)
20
1.0 I
I o Wavelength
thermocouple buried directly at the top of the ice block. The focused light from a 150 W Xe lamp was
through the sample in a direction per-
pendicular to ‘he electron beam, and was led to a Narumi RM-23 monochromator. The analyzed light was detected by an RCA 7102 photomultiplier. The transient signal was amplified 5Ci times before being fed to an oscilloscope. The time range measurable with this detection system wx from 0.5 psec to 10 msec. The bombarding electron beam was delivered from the ORC Linac, and was 10 MeV in energy, 300 mA in peak current, 5 I.rsec duration. The beam was focused on the crystal to a 15 mm diameter. At -1OO’C the IR absorption could not be observed. The spectrum has the single peak at about 6300 A. At - 15O’C the new transient
absorption
in
the IR region was observed. The wavelength distributions of the visible and the IF, spectra are shown in fig. l.for t = 0,0.4 msec, and 2 msec (t is the elapsed time after the end of the electron beam Fulse). At t = 0, the optical absorption in the IR increases with wavelength from about 7500 A to 10000 X at which a peaklike structure is formed, and the IR spectrum is plainly distinguishable from the peak in the :-isible region (at 6300 a). The entire spectrum reaches the maximum at the end of the electron beam prtlse. The decay of the IR spectrum is faster than the visible spectrum, and seems to be non-uniform in wavelength. Similar spectra in D,O doped with 10el M NH,F are shown in fig. 2. The waveleng*h distribution at t = 224 :
5
6 Wavelength
Fig 1. Transient absorption spectra observed in pulse radioIysisof single crystals ofDl0 ice at -1jO’C. Spectrum at the end of the pulse, (0); spectru$ after 0.4 msec, (a); spectrum after 2 fisec, (x).
transmitted
I
I
4
(2,
7
8
9
IO ll(Xld,
(i,
Fig. 2. Transient absorption spectra observed in pulse radiocrystals of D20 ice doped with 10e2 hf NHJF. Spectrum at the end of the pulse, (0); spectrum after 0.4 msec, (0); spectrum after 2 msec, (x). The dot-dash line indicates Iysis of single
the assumed spectrum at f = - which is inferred from the stable absorption at -196’C IS.]. (The real peak height at - is not equal to that at r = 2 mscc.)
t=
0 is almost the same as that in the pure ice. The relative absorbance becomes about twice that in the pure crystal, and the decay becomes slower over the entire region. Especially, the decay of the absorption in the visible region becomes much slower. Probably, this is caused by limited recombination of the electron due to trapping of holes by fluorine ions. In the IR region the non-uniformity of the decay on wavelength becomes pronounced compared with that in the pure crystal. The IR absorption seems to be due to several types of shal!ow traps of various depths, especially in the doped sample. In the doped sample, about 10% of the absorbance in the visible region builds up slowly with a half life of about 0.1 msec. This building up observed at 6300 a is shown in fig. 3. It rather re-
sembles that reported by Kenny and Walker 161, whc found building up of 50 nsec at 6328 K in pure water and low3 M OH- aqueous solution by using a Febetron machine and a He-Ne laser. From the analogy to the case in ethanol, the IR
absorptions observed in the pure and the doped crystal are considered to be due to the electrons in shallow traps. The striking transformation from the IR region to the visible region, as is observed in ethanol, does not occur in crystalline ice. Mostof the electrons in deep traps seem to be produced independently of : those in shallow traps. However, there remains the possibility that the small building up in the visible :
Volume 14, number 2
< e
CHEMICAL PHYSICS LETTERS
PlJk 1
15 May 1972
References
5
Richards and J.K. Thomas, J. Chem. Phys. 53 (1970) 218. hf. Noda, T. Warclshina and H. Yoshida, PI T. Hipshimurn, J. Chem. Phys. 53 (1970) 1152; H. Hase, T. Warashina, >I. Noda, A. Namiti and T. Higashimura, Proceedings of the 14th Congress on Radiation Chemistry, Sapporo, Japan (197 1) p. 111. [31 H. Hase, hl. Noda and T. Higashimura, J. Chem. Phys 54 (1971) 2975. r41 V.N. Schubin, V.A. Zhigunov, V.I. Zolotarevsky and P.L. Dolin. Nature 212 (1966) 1002; LA. Taub and K. Eiben, J. Chem. Phys. 49 (1968) 2499; G. Nilsson, H.C. Christensen, J. Fenger, P. Pagobcrg and SO. Nielsen, Advan. Chem. Ser. 81 (1965) 7 1. [51 K. Rawrtbata, J. Chem. Phys. 55 (1971) 3672. [61 G.A. Kenney and D.C. Walker, J. Chcm. Phys. 53 (1970)
[II
‘D % ._
‘0 ‘I0 G
Time
Fig. 3. OsciJloscope trace showing the writion of transient absorption as a function of time after the pulse for single crystals of D20 ice doped with lO_’ hi NH4F. region of the doped crystal may be due to the retrapping of the electron thermally ejected frun the shallow trap.
J.T.
1282.
225