Slow reorientation processes in irradiated frozen alcohols

Yolume 34, number 2

CHEMICAL PHYSICS LETTERS

SLOW REORIENTATION Ierzy

KROH and Elibieta

Institute

of Applied

Radtition

15 July 1975

PROCESSES IN IRRADIATED FROZEN ALCOHOLS ROMANOWSKA Ckmistty,

Technicul University, Zdd.2, Poland

Received 24 April 1975

It has been found, that in the irradiated, glassy alcohol matrices at 77 K the blue-shift

ofthe absorption

spectra of

trapped electrons takes place in the long time retion: up to tens of hours.

Pulse radiolysis

investigations

on electrons

trapped

(e;-) in polar alcohol glasses at 77 K revealed that the corresponding absorption spectra undergo shifts towards the shorter waves in the time period of several w [1,2] to seconds [3], depending on the matrix po-

larity. The above effect has been ascribed to the reorientation of the medium polar molecules in the coulombic field of the trapped electron [2,4] . It seems interesting to find out whether similar phenomena may be observed on 8 much longer time scale and con-

sequently Our

methanol, ene-glycol volume.

investigated

experiments

without

using pulse techniques.

were performed with irradiated

ethanol, r?-propanol, isopropanol at 77 K, each matrix containing All the matrices

were irradiated

and ethyl5% Hz0 by

with 60 ke\’

X-rays at a dose rate of 2.4 Mrad/h. The time of irra-

diation ranged between 3 and 5 min. The results for ethanol and isopropanol are shown in fig. 1. Siklar results were observed for methanol and n-propanol, but no shift occurs in the case of ethylene glycol.

Fig. 1. Absoption spectra of et in ethanol. f SW Hz0 @lass/Et) and in (Et)-24 h. X (Pr)-3 min. o(Pr)-24 h and s(Pr)-48, h after irradiation.

isopropanol f 5% Hz0 glass (Pr) at 77

K. d(Et)-3

min. 0

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Volume 34, number 2

CHEMICAL PHYSICS LETTERS

During storage of the sample in the dark at 77 K no decay of total absorption takes place since the decrease in the long wave part: of the spectrum is fairly well balanced by the increase of the short wave absorption., It has akc been ascertained that the spectral shifts are not due to photobleaching. For methanol, ethanol and n-propanol the spectral changes are practklly completed in ca. 20-30 hours, whereas for isopropandl the shift may be recorded up to ca. SO hours. Qualitatively the results are analogous to those obtained by pulse radiolysis [2] . One must of course realize that the long time shift represents only the tail of the phenomenon which predominantly occurs in much shorter periods (e.g. ils or s). According to expectation, the tail is long and prominent’for weakly polar matrices. For hig!!y polar ethylene glycol the tail is not detectable, since reorientation in this case is complete before the spectro-

.262

1.5 July 1975

photometric measurement may be under&en [2]. In contrast to this, for isopropanoi, characterized by the relatively lowest dielectric constant, ‘the shifts are long lasting and comparatively large. The general conclusion from our experiments is that the reorientation processes in the irradiated medium leading to the electrostatically equilibric configuration in the surrounding of e; may be quite slow especially for weakly polar systems.

References 111J.T. Riclu~ds rmd J.K. Thomas, J. Chem. Phys. 53 (1970) 2i8.

121L. Kevan, J. CAem. Phys. 56 (1972) 838. 131 J.H. Btiendale and P. Wardmzn, J. Chem. Sot. Faraday Trans. I69 (19733 584. Ann. Rept. Res. Reactor Inst. Kyoto Univ. [41 T. Higsshhura,

6 (1973) 38.