Radiation chemistry of low-temperature aqueous glasses. Part 4.—Distinguishable electron traps in neutral and alkaline glasses

Abstract

Extensive studies of the photobleaching kinetics of e; in aqueous alkaline glasses show (i) that electrons are trapped in potential wells of differing depths from which they can be selectively photobleached, and (ii) that trapping occurs within the Coulombic field of the positive charge produced concurrently with the electron. Examination of other aqueous glasses indicates that the trapped electron is located in a " water-walled " cavity.Electrons detached either photochemically or radiation chemically from molecules in neutral or alkaline aqueous glasses at low temperatures may, after deceleration, be trapped in the matrix in a form designated e;. At 77 K, e; is immobile, has a characteristic singlet e.s.r. spectrum and has an intense visible absorption band with Amax in the range 535-590 nm.I* Several models have been put forward to describe e; in aqueous glasses. Jortner and Scharf proposed that the electron was located in an expanded S orbital of the alkali metal cation, and Moorthy and Weiss that it was bound in an expanded 3s orbital on 0-, the anionic form assumed by the hydroxyl radical in alkaline glass. However, convincing arguments against these proposals are (i) that Schulte-Frohlinde and Eiben have shown that the optical spectrum of e; is unaffected by changing the cation, (ii) when e; is formed in 7 M NaClO, glass a virtually identical spectrum is observed,6 and (iii) that similar spectra displaced to shorter wavelengths are manifested by e; in glassy alcohols which initially contain no anions or cation^.^ Others * have suggested that e; is located in anion vacancies but Bennett, Mile and Thomas have shown that the same optical and e.s.r. spectra are produced when alkali metals and water react in the rotating cryostat when neither 0-nor anions are present. Arguing from the trapping of electrons in alcoholic glasses, Eiben and Schulte-Frohlinde l o have suggested that the electrons are held in pre-existing structural cavities in the medium, and Barzynski and Schulte-Frohlinde have demonstrated conclusively that G (e;) is much smaller in a polycrystalline alcoholic or alkaline aqueous matrix than in an amorphous one.Ershov and Pikaev regard e; as a " peculiar I;-centre ", i.e., the electron is immobilized in a cavity in the medium not far from the positive '' holes " formed during radiolysis. However, the identity of e; produced in alkaline aqueous glass by radiolysis and by photolysis of ferrocyanide ion,12 as well as some of the evidence cited above, does not support this conclusion.

Partial deconvolution of the absorption spectrum of trapped electrons in 15 M LiCl aqueous glasses at 77 K

“…Subsequent bleaching with red light (>620 nm) did not change the position and shape of the remaining part of the absorption band which will be referred to as the 'green' band. However, when light of wavelength shorter than h,,, was used, the band shifted to the red, in accord with the previous findings of photoshuffling in other aqueous glasses (2,17).…”

“…3 should give straight lines. Actually, the quantum yield decreases with the fraction of bleaching as observed in other aqueous glasses (2,18) and this fact is emphasized by the use of dependence of @, on the percentage of bleaching is identical for the four wavelengths within experimental errors; this shows that the quasi-free electron (e,,-) formed has the same fate even if it may have different kinetic energy upon being released from the trap.…”

“…absorption spectrum of the equilibrated solvated Experimental electron (e,,,-) in aqueous glasses at 77 K is the result of overlapping of narrower bands, each of which inay originate from a distinct species of electron traps (1)(2)(3)(4). The problem remains unsettled, however, whether there is a continuous distribution of substructures or only a small number of discrete overlapping bands contributing to the overall absorption band (5,6).…”

“…Many of the conclusions reached so far rely solely upon the results of steady-state bleaching (2,9) and this has been shown to introduce ambiguities (10). In the hope of resolving these issues more conclusively, both steady-state and time-resolved photobleaching experiments have been conducted on the same system, and this is the topic of this paper.…”

Electron localisation: evolution of optical spectra

“…When a sample of trapped electrons in aqueous or alcohol glasses produced by y irradiation at 77 K was partially bleached to rid it of trapped electrons which were close to potential reaction partners, photoinduced compensatory spectral shifts were observed (30). These were termed photoshuttling, or photoshuffling, since illumination with blue light caused loss of absorption in the blue edge of the band and a compensatory growth on the red side, and vice versa.…”