Temperature dependence of scavenger effect on trapped electron photobleaching in 8 M NaOH and 5 M K2CO3 glasses

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PHYSICS LIFkS ,,

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OF SCAV&GElX EFFECT QN .TRAPPED ELECTRO,N PHOTQBLEACH’ENG Jlrl8 M MaOH AND-$ Ii4 ‘K&O3 GLASSES L

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.. Jekzy KRGH and Andriej PEONKA ‘Institute of Applied

Radtirion

Chemisrry;

Tebmical

University, 2dd.2, Poland

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Received 21 June 1!274

Upon phbtqbleaching,

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have reported [I] that & the &me range 6f temperatures one can obtain eit!!er negative br positive values of the apparent activation energy foi trapped.electrori dzczy &alkaline ice upon photo-

bleaching. .Thus trapped electrons produced by y. uradjation at the upper limit of Ihe temperature range tider i&estigation de,cay upcn photobleaching with 2 positive value of the apparent activation energy while those produced by &radiation at the lower limit of the same temperature range detiay, under the same &xp&imental co!Sitiori;, with a negative value df the apparent &$ivatio.n energy.‘The alkaline matrix however, some pkculiaai features. In y-irradiated

has,

to a delocalized.state

Fol-

of conduction

band.type.

lowing this c&clusion the threshold of the optical abs@thn band at ab.but 830 rm (1.5 eV) is the threshold of phoIbconductitity and iS the &rgy differ- : ence between the ground state &the $bttom of the, conduction band. The maximum in the photocurrent ” ._response, coinciding with the.maximun in the,optical absorp&on band; corresponds to the mkimuti densi&‘hf states in th’e‘conduction b&d which is’located ‘. : :, .-.s&&OA eV abbLe.its bottom..L&king for a different, r&&kwith &stinct‘feature:;,.tie.have,.chos’en 5 M .l :!&CC+ ‘glass. In that’glass r-irradiated at 77 I$ the : “..._ ._.. _- .,

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phbto~:urreni.~?velenglh dependence does not coincidl: with the optical &sorptioh band and it seems [3]

that ?he optical absorptidrLband corresponds to a transition from a bound ground it:!? to a localized excited state. FurU&rriore, from extrapo!ation of photocurrent wavelength dependence one obiains [3].only d ‘small, if tiy, energy difference between the localized excited state and a conduction band state. ThuS, undertaking the.present .comparative investigations we might hope that,temperature d!anges will affect rather the localizdd excited state matching the bottom

10 M .NaOH glass at 77 K +he waveiength debendence of the photccpndtictivity matches the absorption spectrum of trapped electrons. [2]. 31 ‘
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molecules

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of the conduction

band,-as predicted

for

5 NI K2C03 glass, than the delocalized states will1 ma-rimurn density s6me 0.6 eV above the bottom.of -he conduction band, as predicted for alkaline ice. The same shift in energy which in 5 M K2C03 glass’ may transfer electrons into or out of the conduction bared should not affect electrons in the conduction band of 8 M h:aOH glass. 1 tinspite of the above.expectations sirni!ar effects are observed for.bo:h r&rices, cf. figs: ,! and 2, when trapped.electrons are prodbced by r-irradiation at the l&,er limit of ,the,temp,erature ranges under present _ i+stigations. The .only,difference foi the above two ma:tjces; cf. fig. 1 and 2, is observed when trapped -. eler,trom

are produced

by -p-irradiation

at the upper

limits_$the t&-r@erat~re~rangcs. n!cse upper limits, i7.i: and 173,S, are,the temFe&W absve which fkie. rapM’therm~l’de~yof:trappe’d e!ectrop$ in 5 M.- : 9CO3 kFt_ c&tairGna .,,’ E M.yy0.H &&es .. :, _, ,? an effect& :

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CHE&IICar:kHYSICS

28, number 2

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LETTERS

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Fig. 2. Trapped eiectxon, ec, decay, as observed by ESk p.x%a, in +radiated (1.3 hfrrd/b, 1 hr) 5 hf. K9203 glass, containing 5 X IO”’ M of KaFe(CN)e, upon ~hotabl~~in~ with broad band visibbte tight: 77/77 - ~irradiation at 77; photobleaching at 77 K; 77/123 - +rradbtion at 77, photobleaching at 123 K; 123/123 - -@radiation at 123, phot+ bleaching at 123 K; 123177 - rirradiation at 12.3, Rhotobleaching at 77 K. Fig. 1. Trapped zfectron, e;,.declly, as observed by ESR spec,tra, in_+rradiated (1.3 hf.rad/hr, 1 hr) 8 M NaOH glass, con.*?zGning5 X 10e3 M of KsFe(CN)e, upon photobleaching with Jxoad band visible light: 77177 - +rradiation at 77, photobla*l: at 7.7 K; 77/173 - +mdiation at 77, photobkaching a7Q 173 K; 173/173 - I-irradhtion ai 173 Kphotobleaching at.173 K; 173/77 - +.r.radiation at 173, photobleaching at 77 K. The mow above the curfe 173177 indicates the moment at which the temperature was incrcascd from 77 to 173 K.

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scavenger like K3Fe(CN)6starts..Here, in contrast to the alkaline ice, the decrease of tekpexature has no affect on trapped electron~dlcay upon photobleaching’ ’ iri5MK2C03gla~:.This is an unexpected re&t’ if one aiees that upon phqtobleaching the mobile electrons react with a scavenger. The negative value of the apparent activation enirgy for trapped electron de&ty upon phorobleach-: ..,..: ing. ‘observed for the’inatdces YGrradiated at the lower limit of tempera’& ranges, could be explained by an ‘increased retrapping at higher tetiperatures; duet’0 .,‘. ; fnatrii relaz$ion [4] or some kind of &+liov&dek~ ., - .., :: ‘... . : : : ‘,:.

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trap reaction [S]. The changes of decay rate at Iower temperatures, observed for.the matrices r-irradiated at +Aeupper limit of temperature ranges remain, however, obscure. The maxt-rium of the optical absorption spectrum for trapped electrons in 8 M NaOH glass (580 nm) nearly matches that for trapped electrons in 5 M K2CO3 glass (540 run) at 77 K. In both matrices, yirradiated at the tipper temperature Limits, nearly the same, insignifIcant, blue-shifts are obsar
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