Photochemistry of Electrons Trapped in Glasses of Methyltetrahydrofuran at 77°k

Abstract

I n the radiolysis of methyltetrahydrofuran glass a t 77 "I< electrons are produced with a high yield. These are immobilized in traps in the glass and do not recombine with positive ions. They can be released from their traps by illumination with visible and infrared light and may then diffuse to positive ions and be captured. The quantum efficiencies for bleaching have been measured as a function of wavelength. This quantum efficiency falls as bleaching proceeds and depends on the fraction of the electrons bleached and not on their concentrations. About half the electrons are trapped close to positive ions and, on release, have a high probability of diffusing to the positive ion. The remaining half are trapped hon~ogeneously. On release from their traps, few of these electrons reach positive ions although they diffuse over long distances before being retrapped in the glass. This behavior is compared to that discussed by Samuel and Magee for the thermalization and capture of electrons in liquid water.The rate of recombination of an electron and positive ion in a condensed phase is an alinost unltnown quantity and the uncertainties which remain about this process lie a t the root of inany fundainental problems of radiation chemistry. Important advances, permitting a direct study of the process under special conditions, have, however, recently been inade by W. H. Hamill who has extended to radiolytic reactions the techniques of studying the photocheinistry of organic glasses which were pioneered by G. N. In an organic glass a t low teinperature reactive interinediates forined in radiolytic or photolytic decoinposition are held iininobile in a rigid solvent cage. They have long lifetimes and can be observed, a t leisure, by conventional optical techniques. Ronayne, Guarino, and Hamill (2) (henceforth R. G. and H.) found that, on radiolysis of glasses of 2-methyltetrahydrofuran (i\lITHF), a strong optical absorption is observed in the near infrared kvhich is due to electrons trapped in the glass. In glasses containing biphenyl as a solute the intensity of the electron band is reduced and a new absorption due to the biphenyl anion 4~ appears.'The electron band is photosensitive, being bleached by visible and near infrared light. If biphenyl is present in a bleaching experinlent the intensity of the biphenyl anion band increases as the intensity of the electron band is diminished. Electrons released froin their traps in the the glass inay be captured by positive ions or by biphenyl molecules.In a sense this technique allows one to observe, in slow motion, the recombination of electrons and positive ions. This paper describes a study of the photocheinistry of the bleaching of trapped electrons in i\lITHF glasses which shows that thermal electrons have a high mobility in the glass. While few electrons recoinbine with the positive ion before being trapped, they prefer trapping locations in the vicinity of positive ions. E X P E R I M E N T A LThe experimental techniques for purifying M T H F , making optical cell...

Optical Absorption Characteristics and Photobleaching Behavior of Trapped Electrons in γ-Irradiated Alkaline Ice

“…The sample was simply transferred from liquid nitrogen to liquid helium and photobleaching data was recorded to give cf>575 575 = 0.15 at t=O at 4.2°K. This behavior parallels that found for et-in MTHF 10 and in 3MP,9 but in methanoPO and in n propanoP2 cf > remains constant during bleaching. Figure 5 shows cf>575 590 vs fraction of electrons bleached for samples with two different initial concentrations of et-.…”

Matrix photodissociation and photoionization of carbon tetrahalides with noble gas resonance radiation

“…The decrease of absorptions upon exposure to the Nernst glower was explained by a photobleaching mechanism where electron traps (anions) were excited and electrons were transferred to isolated cations, a common phenomenon in radiation chemistry. 19 Accordingly, the stable 1037 cm-1 absorption was attributed to a stable ion pair, CCl;Cl-, which could not attract electrons and bleach. 4 Hence, the identification of new absorptions is dependent on the mechanism of photolysis.…”

Energy Levels and Optical Properties of Trapped Electrons in 2-Methyltetrahydrofuran

“…Dyne and Miller 4 Let rand R be the distance from the center of the cavity and the distance between the positive ion and the center of the cavity, respectively. The potential energy in the direction joining the positive ion with the center of the cavity is, then, approximately expressed as follows:…”