Energy Level Structure of Trapped Electrons in Methyltetrahydrofuran Glass from Photoconductivity and Optical Bleaching Studies

Abstract

Wavelengthselective bleaching of the optical spectra of trapped electrons in organic glasses. II.Electronelectron double resonance study of trapped electrons in γirradiated 2methyltetrahydrofuran glass: Magnetic energy transfer between two different spin systems Electrons are trapped in 'Y irradiated methyltetrahydrofuran (MTHF) glass at 77°K. Monochromatic photoexcitation produces photoconductivity and optical bleaching with a threshold near 780 nrn (1.6 eV) and a peak near 520 nm (2.4 eV). This transition is linear in light intensity and independent of temperature between 77 and 4.2°K, so it is interpreted as a one photon transition directly to the conduction band or to an autoionizing state. Effective double beam photoexcitation discloses a two photon transition which depends on the light intensity squared. The first photon corresponds to the well-known optical absorption of trapped electrons in MTHF at 1.0 eV. This transition is interpreted as a 1s~2p type. The 2p state then presumably crosses to a 2s-type state from which the second photon is absorbed. The wavelength dependence of the second photon transition shows a peak at 950 nm (1.3 eV) and a threshold near 1150 nm (1.1 eV). The temperature of both optical bleaching and photoconductivity under ir photoexcitation shows that population of the 2s-type state involves an activation energy ~O.OOl eV. The deduced energy level structure agrees well with theoretical calculations based on a semicontinuum model for trapped electrons in glassy matrices. Compared to more polar matrices like ice, both experiment and theory indicate that in less polar matrices like MTHF the short-range charge-dipole interactions are relatively less important compared to the long range polarization interactions and cause the bound excited states to be relatively more stable and the ground state to be relatively less stable.

Comparison of photoconductivity and optical absorption spectra of trapped electrons in γ-irradiated 2,2,4-trimethylpentane/2,2-dimethylbutane/2-methyltetrahydrofuran mixture glasses at 77 K

“…It is noted that the photocurrent in the neat MTHF glass is lower by five orders of magnitude than that in 3MHx. This contrasts with the published data (10) which report that the photocurrent in 3MHx is much smaller than that in MTHF, although the photocurrents are not measured with monochromatic light but by the differential filter method (8). This discrepancy might-arise partly from the difference in the ways of purification.…”

“…A number of studies have been made o n the optical absorption spectra of trapped electrons in a neat MTHF glass (6), but there have been only a few photoconductivity studies of trapped electrons in this matrix (7,8).…”

A photoconductivity study of trapped electrons in γ-irradiated hydrocarbon glasses at 77 K

“…Thus comparison of optical absorption and photoconductivity spectra is expected to provide information on the energy level structure of trapped electrons. For trapped electrons in polar glasses several photoconductivity studies have been made (2)(3)(4)(5)(6)(7)(8)(9) and theoretical approaches have been attempted to interpret these experimental results (10,11). On the other hand, only a few photoconductivity studies have been done for trapped electrons in nonpolar organic glasses (12,13).…”

Energy level structure and mobility of excess electrons in .gamma.-irradiated 5 M potassium carbonate aqueous glasses. Effect of ions on trapped and mobile electrons in aqueous glasses

“…The photoconductivity cell 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.…”