Evolution of the spectrum of the solvated electron in BeF2 aqueous glasses at 76 K

Evidence is presented for a spontaneous shift from infrared-to- red-to-green in the overall spectrum of the electron trapped in an aqueous glass at 76 K following nanosecond pulse radiolysis. As with the alcohols, after the pulse the infrared absorption decays, there is a growth-then-decay of absorbance at intermediate wavelengths and net growth of absorption due to the equilibrated trapped electron state, e−vis. This progressive transition was not characterised by a single-step process and was discernible in aqueous glasses containing 7.5 or 10.7M BeF2 in D2O. At higher BeF2 concentrations, or with LiBr as glass-forming electrolyte, no growth of e−vis was observed, probably because of concurrent loss of e−vis in those glasses, as in LiCl and other glasses. BeF2 glasses of trapped electrons showed several other features of interest: (i) wavelength- selective bleaching of the e−vis band indicates that it is composed of a broad ’’green’’ band (λmax, 530–590 nm) which bleaches homogeneously, and a broad ’’red’’ region which bleaches inhomogeneously. (ii) λmax for e−vis changes with BeF2 concentration, being blue shifted with increasing concentration , with both green and red underlying structures shifting slightly. (iii) No absorption due to F−2 was found following pulse radiolysis in these glasses at 76 K. (iv) Several recent findings with LiCl are mimicked by F−2 glasses. For instance, G (e−vis) increases and G (e−ir) decreases as the electrolyte concentration is increased, while the combined yield remains constant at 1.8±0.1.