The percentage of bound–continuum character in the optical transition of trapped electron absorption bands, and the fate of quasifree electrons

The actual percentage of bound–free character in the optical transition of the trapped electron in 9.5M LiCl/D2O aqueous glasses at 77 K has been measured at 694 nm and found to be 3.7%±1.8%. Consequently, about 96% of the transition probability must be to bound excited states at 694 nm, which is 100 nm (or 0.31 eV) on the red side of the band maximum. This measurement has taken account of the number of photoreleased electrons which are retrapped and of those electrons which are bleached by indirect secondary processes. It differs, therefore, from previous measurements of the quantum yield of net bleaching as obtained by ’’steady-state’’ methods. The information was procured by evaluating both the quantum yield for net loss of trapped electrons (e−vis) and the quantum yield for the production of the infrared-absorbing electron state (e−ir). The latter measurement was used to estimate the fraction of photoreleased electrons which were retrapped as e−vis, by assuming that the relative probability of forming e−ir or e−vis was the same regardless of whether quasifree electrons were generated by photolysis or radiolysis. Samples of e−vis were produced by γ radiolysis at 77 K in glasses made from various concentrations of LiCl in D2O. They were bleached by 15 ns pulses of 694 nm light from a Q-switched ruby laser and the resulting absorption changes within 100 ns were analysed at 633 and 1850 nm. For LiCl concentrations between 6.5 and 15M only 20% to 35% of the quasifree electrons which were generated by 694 nm photolysis were lost by reaction in these γ-irradiated glasses. The distribution between trapping as e−vis or e−ir on the other hand was strongly dependent on LiCl concentration.