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

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Abstract
Electrons are trapped in γ irradiated methyltetrahydrofuran (MTHF) glass at 77°K. Monochromatic photoexcitation produces photoconductivity and optical bleaching with a threshold near 780 nm (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 → 2 p 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 ∼ 0.001 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.