Shoji Noda and Larry Kevan
2454 "ICndbodc of Chemlstry and Physics," 54th ed, Chemical Rubber Publishing Co.. Cleveland, Ohio, 1973-1974,p D-131. (IO)I. M. Kolthoff, J. fhys. Chem., 35, 2732 (1931). (11)M. Eigen and L. DeMaeyer, "TBchnique of Organic Chemistry." Vol. 8, Part 2. L. Frlass and A. Weissberger, Ed., Intersclence, New York, N.Y., 1963,Chapter 18.
(12)H. Eyring and E. M. Erying, "Modern Chemical Kinetics," Reinhold, New York, N.Y.. 1983,p 100. (13)0. Schwarzenbach and 0. Haggar, Helv. Chem. Acta, 20, 1591 (1937). (14)W. Liptay, Angew. Chem.. 81, 195 (1969). (15)A. A. Frost and R. G. Pearson, "Kinetics and Mechanism," 2nd ed, Wiley. New York, N.Y., 1981,p 150.
Energy Level Structure and Mobility of Excess Electrons in y-Irradiated 5 M Potassium Carbonate Aqueous Glasses. Effect of Ions on Trapped and Mobile Electrons in Aqueous Glasses Shojl Noda and Larry Kevan" hpartment of Chemktry, Wayne State University, Detroit, Michigan 48202
(Received May 16, 1974)
Publication costs assisted by the U.S. Atomic Energy Commission
The energy level structure of trapped electrons (et-) in y-irradiated 5 M K2C03glassy ice a t 77 K has been investigated via the wavelength dependence of the photocurrent. Scavenger effects demonstrate that the photogenerated charge carrier is an electron. The photocurrent wavelength response per unit incident photon does not coincide with the optical absorption band of the et- and can be phenomenologically resolved into two bands. The optical absorption band is a symmetrically bleached with monochromatic light and can also be resolved into a t least two subbands. The photocurrent band a t longer wavelength coincides with the longer wavelength absorption band. Thus the et- in the shallower potential wells appear to have no bound excited state as is the case for et- in 10 M NaOH glassy ice. For et- in the deeper potential wells, the shorter wavelength absorption and photocurrent bands do not coincide and indicate a bound excited state as i s found for et- in crystalline ice. The difference between the energy level structure for et- in different aqueous matrices can be understood in terms of a semicontinuum model of electron trapping and the modification of the polarization interactions by the ions present in the matrix. The analysis is supported by the photocurrent wavelength dependence in 4.5 M D-glucose glassy ice. The field and temperature dependence of the photocurrent and the electron drift mobility were also investigated. Superohmic photocurrents are observed in certain dose ranges a t 77 K. The electron drift mobility is inversely proportional to field a t high fields and exhibits a negative temperature dependence from 80 to 103 K. The dominant scattering mechanism near 77 K for photoexcited mobile electrons is lattice phonon scattering. In contrast to et- in 10 M NaOH ice, the mobility is independent of the hole (COB-) concentration and there appear to be no shallow traps; relative to the conduction band associated with COB-. These differences are explained in terms of the different degree of lattice polarization induced by COB- and 0- ions in aqueous matrices.
Introduction Excess elect,rons produced by high-energy radiation or by photoionization of a suitable solute are stabilized in a variety of aqueous' and organic2 glasses. The trapped electrons exhibit diverse energy level structure depending on the nature of the glassy matrix. The trapped electron may also be photoexcited to a mobile state in which their transport mechanisms in the glassy matrix can be probed through their mobility characteristics. In 10 M NaOH glassy ice, the trapped electron appears to have no bound excited state and the optical absorption spectrum is assigned to bound-free t r a n ~ i t i o n s .Drift ~ and Hall mobility experiments have demonstrated that the photoexcited electron in this glass is quasi-free and that its transport mechanism is well described by a band model.4 On the other hand, in less polar organic glasses, such as 2-methyltetrahydrofuran (MTHF), the trapped electron has bound excited states Tho Journal of Physical Chemistry, Vol. 78,No. 24, 1974
and can be photoexcited to a mobile state by both 0ne-5,6 and photon processes. The mobility of photoexcited electrons in MTHF glass is several orders of magnitude smaller than in 10 M NaOH glassy ice and appears to be best explained by a hopping model.7~~ Electrons in alkane glasses, such as 3-methylhexane, seem to have similar energy level structures and mobility characteristics to electrons in MTHF. The influence of matrix polarity on these characteristics of excess electrons has been emphasized.2 It is of importance to determine whether 10 M NaOH glassy ice is typical of aqueous glasses with regard to stabilization of electrons. Previously, it has been suggested that the highly alkaline ice may represent an extreme in effective matrix polarity.2i8 In this work we have studied excess electrons in a different aqueous glass, 5 M K2C03. By photoconductivity and optical bleaching studies we have found that trapped electrons of at least two different types are
Effect of lens or Trapped and Mobile Electrons
2455
formed which are dirrtinguished by their energy level structure, Mobile electrons produced by photoexcitation have a relatively large drift mobility that exhibits characteristics consistent with a band model. Thus it is tentatively concluded that a quasi-free electron state is typical for aqueous glasses.
76-
2"
'2 5 x
I--
xperimewtal Soctiion The p h o t o c o n ~ u c ~ i vcell ~ t y and the set-up for the photoconductivity measurements have been described in an earlier work.5 The light beam from a 500-W slide projector was directed through semitransparent electrodes. For the wavelength dependence of the photocurrent a Bausch & Lomb high- intensity monochrometer with a dispersion of 20 nm was used. 't'hmnocouple measurements showed that the sample tempei ature increased
