Laser bleaching of trapped electron optical bands in γ-irradiated alkaline ice

Inr. J. Radiat. Pkys. Chem. 1971, Vol. 3, pp. 193-199. Pergamon Press. Printed in Great Britain

LASER BLEACHING OF TRAPPED ELECTRON OPTICAL BANDS IN y-IRRADIATED ALKALINE ICE KEN K. Ho and LARRY KEVAN* Department of Chemistry, Wayne State University, Detroit, Michigan 48202, U.S.A.

(Received 27 October 1970)

Abstract-The maximum of the optical absorption band of trapped electrons in glassy alkaline ice (10 mol/dm-3 NaOH) at 77 K shifts to higher energy when the low-energy side of the band is optically bleached. This suggests that there is a distribution of ground-state energies for the trapped electrons and, consequently, a distribution of vacancy sizes in which the electrons are trapped. The fine structure of the broad absorption band has been probed by comparing laser bleaching at 633 nm with broad-band monochromator bleaching at 633 nm. Although the laser linewidth is 750 times smaller than that of the monochromator light, the trapped-electron band shifts were equivalent, and it was not possible to bleach a narrow portion out of the broad band corresponding to a single narrow component or resolved single-energy trap depth. It appears that the width of the component optical band corresponding to a particular ground-state energy is broad rather than narrow. Laser bleaching in the low-energy tail of trapped electrons in KC1 crystals caused uniform and symmetric bleaching consistent with a single trap depth. INTRODUCTION

RADIATION-PRODUCED electrons are trapped in a variety of glassy and crystalline solids including glassy alkaline ice (10moldmA NaOH) at 77 K(l), several organic Weiss has been glasses at 77 Kt2), and in alkali halide crystals at room temperaturet3). instrumental in developing an understanding of these electrons, particularly in alkaline icet4). The trapped electrons can be observed by their electron paramagnetic resonance or optical spectra. The optical spectrum is a broad band with a maximum in the visible or infrared regions. The band in alkaline ice is bleachable. Furthermore, if one bleaches on the low-energy side of the band that side is preferentially bleached, and the peak maximum shifts to higher energy c5r6). This effect suggests that there is a distribution of ground-state energy levels of the trapped electron and, consequently, a distribution of vacancy sizes in which the electrons are trapped. It therefore appears that the broad optical band of the trapped electron is composed of several overlapping narrower bands. The width of these narrower bands has been probed by comparing the bleaching characteristics of laser light with the much broader band obtained from a monochromator. Since the optical maximum is at 586 nm in glassy alkaline ice, the helium-neon laser line at 633 nm and the argon-ion laser lines at 472 and 466 nm are appropriate because they lie considerably to one side of the optical absorption maximum. The results in alkaline ice have been compared to laser bleaching of trapped electrons in crystalline KCl, where only one groundstate energy level is expected and where the optical band is expected to bleach uniformly, corresponding to a single component. EXPERIMENTAL

Alkaline ice samples were made by freezing thin discs of the glass between two liquid-nitrogen-cooled plates and were supported in a washer-like flat metal ring. The thickness of a typical sample was about 0.01 cm. Two or three samples were * John Simon Guggenheim Fellow. 193

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stacked together to obtain thicker samples when desired. The washer-type support design eliminates any windows that are subjected to y-irradiation from the optical path. Trapped electrons were produced in the alkaline ice by ““Co y-radiation at a dose rate of 0.4 Mrad/h. Electrons were also produced by photoionization of 5 x 1O-2 mol dm-3 ferrocyanide ion in the alkaline ice by 254 nm light. These samples were about 0.14 cm thick. Electrons were produced in KC1 only by y-irradiation. The bleaching was done with three sources. One source was a Spectra-Physics model 125 helium-neon laser with its output at 632.8 nm. A short-focus lens was used to disperse the beam so that it uniformly covered the 1.4 cm sample diameter. The intensity of the light at the sample position was about 13 mW. The second light source was a Spectra-Physics model 140 argon-ion laser with output at 472.7 nm or at 465.8 nm. Either of these lines could be selected by tuning the laser. The intensity of each line after dispersion through the lens was 4 mW at the sample position. The third source was a 500 W slide projector; this light was passed through a highintensity Bausch & Lomb monochromator. The optical absorption spectra were taken with a Cary 14 spectrophotometer. The samples were placed in a quartz Dewar with optically flat windows which fit in the sample compartment of the Cary 14. Care was taken to prevent ice formation in the Dewar or on the sample, and liquid nitrogen bubbling was prevented by cooling the liquid nitrogen with a stream of helium. The optical absorption of the unirradiated sample was measured in the Dewar at liquid nitrogen temperature to serve as one background baseline. The sample was then measured after irradiation and after different degrees of bleaching. Finally, the entire trapped electron band was completely bleached by using the slide projector without the monochromator. This returned the absorption of the sample to the background baseline taken before irradiation, if due care was taken to eliminate frost formation on the sample.

RESULTS

AND

DlSCUSSlON

Figure 1 shows the bleaching of the trapped electron optical band with monochromator light at 633 nm. The full width at half-intensity of the bleaching band was measured to be 29 nm. The optical absorption maximum shifts markedly to higher energy as the bleaching progresses. The band remains smooth, retains the same line shape and no evidence of structure appears in the band. Figure 2 shows the identical sample as in Fig. 1 which has been reirradiated to the same dose. The optical density is slightly smaller in this case. This small decrease generally occurs for successively irradiated and bleached samples. In Fig. 2 the bleaching was carried out with the 633 nm laser line which had a measured full width at half-height of 0.038 nm. The peak maximum shifts smoothly to higher energy in Fig. 2 as it does in Fig. 1. and there is no significant change in the line shape. Again there is no evidence of any structure in the broad optical band. In particular, no hole is bleached in the region ot the narrow laser line. Figure 3 shows a plot of the wavelength shift vs. the fraction of trapped electrons bleached. The relative electron concentration was taken as the optical density at the peak maximum for each curve minus the optical density of the background. Figure 3 shows that the magnitude of the peak shift is quantitatively equivalent for the laser

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FIG. 2. Laser optical bleaching of the same sample as in Fig. 1 reirradiated to 0.37 Mrad. The bleaching was done at 633 nm with a helium-neon laser with 0.038 nm full width at half-intensity. Curve 1, after 0.37 Mrad; 2,0.5 min bleach; 3,l.O min bleach; 4,1.5 min bleach; 5, complete bleach of e,-.

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FIG. 1. Monochrometer optical bleaching of er- in 10 mol dm-3 NaOH y-irradiated to 0.37 Mrad at 77 K. The bleaching was done at 633 nm with monochrometer light of 29 nm full width at half-intensity as indicated by W,. Curve 1, background absorption before irradiation; 2, after 0.37 Mrad; 3, 1.0 min bleach; 4, 1.5 min bleach; 5, 2.5 min bleach; 6, 35 min bleach; 7, complete bleach of e,-.

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KEN K. Ho and LARRY KEVAN

bleaching and the monochromator bleaching. It is also striking that the plot in Fig. 3 is linear and passes through the origin. Electrons may also be trapped in alkaline ice by photoionization of added ferrocyanide ion. The optical band of electrons produced in this way is identical to that of electrons produced by y-irradiation in peak maximum and half-width. Laser bleaching of this optical band is also identical with that shown in Fig. 3. Thus, no difference in trap depth distribution is revealed for electrons produced by y-irradiation compared to electrons produced by photoionization of a suitable solute.

A h.nm

FIG. 3. Plot of fraction of e, bleached vs. shift in r, maximum from Figs. 1 and 7: 0, 633 nm laser bleach; chrometor bleach.

optical absorption A. 633 nm mono-

Laser bleaching was also done with laser lines at 473 nm and 466 nm on the highenergy side of the band. In particular, it was of interest to see if any difference could be observed in bleaching at these two very close wavelengths which might indicate narrow components of the overall band. No difference was observed, and the peak seemed to bleach uniformly with a very slight shift in peak maximum to the lowenergy side of the band. Again no structure in the broad band was brought out. In the glassy matrix it seems reasonable to interpret the peak shift with bleaching on one side of the band as due to a number of overlapping electronic transitions from ground-state energy levels of slightly differing depths. The experiment was designed to see if the width of the optical band associated with a particular energy level was narrow or broad. If it were narrow, it was supposed that some structure would show up when laser bleaching was used compared to monochromator bleaching since the

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width of the bleaching light differs by a factor of 750. Since no structure is observed, we conclude that the band corresponding to a single electronic transition from a particular ground-state energy level is broader than 29 nm, which is the width of the bleaching light from the monochromator. For comparison with the results on glasses we show the laser bleaching of trapped electrons (F-centers) in y-irradiated crystalline KC1 of O-05cm thickness in Fig. 4. The 633 nm laser line is near the extreme of the low-energy tail of the optical band; yet the band still bleaches uniformly with no shift of the optical absorption maximum.

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nm FIG. 4. Laser optical bleaching

of et- in KC1 y-irradiated to 0.83 Mrad at room temperature. The bleaching was done at 633 nm with a helium-neon laser with 0.038 nm full width at half-intensity. Curve 1, background before irradiation; 2, after 0.83 Mrad; 3 and 4, after partial bleaching; 5, after complete bleach of et-.

This is clearly the behavior characteristic of a single electronic transition associated with a single ground-state energy level. The full width at half-intensity of the trapped electron absorption band in KC1 is 86 nm or 0.3 eV. This, of course, is several times narrower than the 0.9 eV width of the electron band observed in alkaline ice. We conclude that the optical absorption band of trapped electrons in alkaline ice and other polar glasses is associated with several electronic transitions from a distribution of ground-state energy levels. This distribution is most likely due to slightly different geometrical configurations of the molecules surrounding the electrons. Since the charge-dipole short-range attractive interactions which contribute to the total trapping potential for the electron depend on the local molecular configuration”), a distribution of trapping potentials will arise. This will produce a distribution of ground-state energy levels. The results herein indicate that the width of the optical band for a transition from a single ground-state energy level corresponding to a single local molecular configuration is greater than 30 nm or 0.1 eV and probably approaches 0.3 eV.

KEN K. Ho and LARRY KEVAN

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Acknowledgemenls-This research was supported by the Air Force Office of Scientific Research under Grant No. AFOSR-70-1852, and by the U.S. Atomic Energy Commission under contracts AT(ll-l)-1852 and AT(ll-l)-2086. The experimental work was initiated at the University of Kansas. L. K. is indebted to Professor J. J. Weiss who first introduced him to the alkaline ice matrix, and thanks the John Simon Guggenheim Foundation for a fellowship and the Danish A.E.C. Research Establishment Risti for their hospitality and services.

REFERENCES 1. L. KEVAN, in Actions Chimiques et Biologiques des Radiations, Vol. 13, edited by M. HABSINSKY.

Masson, Paris, 1969. L. KEVAN, in Radiation Cherni.vtry of Aqueous Systems. pp. 21-71. Interscience, New York, 1968. J. E. Fundamental Processes in Radiation Chemistry, edited by P. Aust.oos, pp. 599-649. Interscience, New York, 1969. J. J. MARKHAM, F-centers in Alkali Halides. Academic Press, New York, 1966. P. N. MOORTHY and J. J. WEISS, Phi/. Meg. 1964, 10, 659; Adu. Chem. Ser. 1965. 50. 180. B. G. ERSHOV and A. K. PIKAEV, Adu. Chem. Ser. 1968, 81. 1. H. HASE and L. KEVAN, J. them. Phys. 1971, 54, 908. K. FUEKI, D. F. FENG and L. KEVAX, J. phys. Chern. 1970, 74, 1976. pp. 57-117.

edited

2. 3. 5. 5. 6. 7.

by G. STEIN, WILLARD, in

R&sum&Le maximum de la bande d’absorption optique des electrons pi&g& dans la glace alkaline (NaOH, 10 mol dm-3) a 77 K se d&place vers les tnergies plus &vt-es lors d’un blanchiment optique du c&e des basses Cnergies de la bande. Cc fait suggtre. pour les tlectrons piCgCs, l’existence d’une distribution d’Cnergies de l’ttat fondamental ainsi que, par consCquent. celle d’une distribution des dimensions des lacunes dans lesquelles les electrons sont pitgt%. La structure fine de la large bande d’absorption a Ctt CtudiCe en comparant le blanchiment par un laser & 633 nm & l’effet obtenu avec un monochromateur a bande passante large centree sur 633 nm. Quoique la largeur de la raie du laser soit 750 fois infbrieure ?t celle de la lumi&-e du monochromateur, les d&placements de la bande des Clectrons piCg6s Ctaient equivalents et il n‘a pas tt8 possible de blanchir une portion Ctroite de la bande large, qui aurait correspondu a une seule composante ttroite ou B une diversification de la profondeur de pikge. II semble que la bande optique d’une composante correspondant & une tnergie particuliere de I.&at fondamental soit large plut6t qu’ttroite. Le blanchiment par un laser dans la partie du spectre de basse knergie des Clectrons pi&g& dans les cristaux de KC1 causait un blanchissement uniforme et symmCtrique s’accordant avec une profondeur de pitge unique. Pe3IOMe MaKCaMyM IlOJIOCbI OuTR~eCKOrO flOrJlOUeHIIR ‘3aXBaYeHHblX B IUCJIOYHOM JIbA> (10 MOJI/~M-~ NaOH) 3neKTpoHoB rlp~ 77 K c4BRraeTcfl a cropo~y 6onee BblcoKRX 3HeprMii np~ OnITMSeCKOMOT&eJIaBaHHH CTOpOHbl nOJIOCb1C HM3KOii 3HeprHeti. ‘%O n03BOIIReT npeAuO_?O~MTb HaJIwIHe pacnpeAeneHwi 3Heprlla OCHOBHbIXCOCTOaHHii Ann 3aXBaYeHHblX ~~CKT~OHO~ M, CneAoBaTeJIbHO, paCupeAeJIeHlia BaKaHTHbIX MeCT, B KOlOpblX OHM 3aXBa~blBaIO7CII. TOHKaR CT,,yKry,,a ILlFipOKOii nOJIOCbl nOrJIOUIeHIiR MOXET 6bITb OupeAeneHa CpaBHeHAeM JIa3ep”OrO OT6enHBaHWn IlpFi 633 HM C UIApOKOnOJIOCHbIM OT6eJIllBaHHeM MOHOXpOMaTAYeCKMM CBeTOM npU 633 HM. XOTn "IIlpHHa JIEiHIiM JIa3epa B 750 pa3 MeHbUle UrHpAHbI nllHHH MOHOX,,OMaTM’EeCKOrOCBeTa, CMeLUeFuill nOJIOC 3aXBaSeHHbIX 3JIeKTpOHOB OKaSaJIMCb OnAHaKOBbIMH M HeBOJMOXHO 6bIno BblCBeTllTb y3KylO TaCTb nOJIOCb1,COOTBe-rCTByIOlUyHJHJIM OTAeJIbHOti yJKOi2 KOMuOHeH7c, MJIMpa3peLJIeHHOii OAHO3HepreTEYeCKOti rny6aae JIOByLUKLi.O’IeBriAHO, lUapMHa nOJIOCb1COCTaBJE-IlOmei?,COOrBeTCTBy,Ou(eii 0npeAenetiHoti 3Heprm4 ocHoaHor0 COCTOIIHIIII, cKopee y3Ka YeM umposa. OT6ennBamie nasepoh4 HK3K03HepreTH’feCKOrO NXBOCTaD 3neKTpOHOB, 3aXBaYeHHblX B KpHcTannax KC1 06ycnoBneBae I paBHOMepHOe N CUMMeTpMYHOe or6enaBaHFie, B COOTBeTCTBHHC OAHHaKOBOfi rny(iaao8 JIOByllIeK.

Zusammenfassung-In glasigem alkalischem Eis (NaOH, 10 mol dm-3) bei 77 K verschiebt sich das Maximum der optischen Absorptionsbande der eingefangenen Elektronen zu hiiheren Energien, wenn die niederenergetische Flanke der Bande optisch gebleicht wird. Dies fiihrt zu!Vermutung, dass es eine Verteilung der Grundzustandsenergien fiir eingefangene Elektronen gibt und folgedessen such eine Verteilung der Dimensionen der Vakanzen, in denen die Elektrone eingefangen sind. Die Feinstruktur der breiten Absorptionsbande wurde untersucht indem die Bleichung durch einen Laserstrahl bei 633 nm mit der Bleichung mittels eines breitbandigen Monochromators bei 633 nm verglichen wurde. Obgleich die Linienbreite des Lasers 750 mal kleiner ist als die des Monochromatorlichtes, sind die Bandenverschiebungen der eingefangenen

Laser bleaching of trapped electron optical bands in alkaline ice

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Elektronen Pquivalent, und es erwies sich unmiiglich, eine enge Portion aus dem breiten Band auszubleichen, welches einer einzigen engen Komponente oder einer abgesonderten monoenergetischen Fallentiefe entsprechen wiirde. Es scheint, dass die optische Breite der Bandenkomponente, welche einer einzelnen Grundzustandsenergie entspricht, eher breit als eng ist. Das Bleichen mit Laserstrahl in der nieiderenergetischen Flanke der eingefangenen Elektronen in KCI-Kristallen verursachte eine gleichmlssige und symmetrische Bleichung, in tibereinstimmung mit einer einzigen Fallentiefe.