Time-resolved photobleaching of solvated electron absorption bands

Abstract

A review is given of studies on the time resolution of photobleaching of solvated (or trapped) electrons following excitation by a 20 ns Q-switched ruby laser pulse. Various transient and permanent bleaching effects are shown to evolve on distinct time scales ranging form nano-seconds to tenths-of-a-second. The systems studied were: photolytically produced hydrated electrons in liquid water at room temperature, γ-induced trapped electrons in a variety of aqueous and organic glasses at 77 K and γ- or additively produced F-centre electrons in KBr and KI crystals at room temperature and 77 K. A number of effects were observed which are pertinent to a basic understanding of the nature and origin of the optical absorption bands of solvated electrons. They are also pertinent to the interpretations to be placed on photobleaching and photoconductivity data under “steady-state”, low intensity illumination. In addition to transient and immediate bleaching effects arising directly from photoexcitation, there are important contributions to the overall net bleaching of trapped electrons in glasses which take place up to 1 s after the light pulse. These apparently arise from incipient but latent electron states or from a delayed response of the medium to multiple excitations (and de-excitation or retrapping) of e t -. In aqueous glasses transient species which absorb in the I.R. (1152 nm) are observed to arise from photoexcitation of e t − at 694 nm. Transient bleaching was seen for K 2CO 3 and NaClO 4 aqueous glasses at 77 K but only at the wavelength of excitation. For LiCl glasses at 77 K and F-centres at ≈ 295 K, bleaching of the whole band occurred and indicated the formation of long-lived photoexcited states. For e aq − in liquid water the ground state is repopulated too rapidly to observe non-linear absorption, consequently the excited state (whether bound or free) has a lifetime less than 3–5 ps. The absorption band of e t − in α-methylterahydrofuran glasses at 77 K must be “heterogeneous” on its high energy tail since none of the permanent bleaching at 694 nm is accompanied by bleaching on the same time scale at 633 or 442 nm. Variations of the integrated Beer-Lambert absorption law are presented for use with high light intensities. The appropriate equations are developed to show how the absorbance at a monitoring wavelength changes as a result of concurrent intense illumination at another wavelength. It is also shown that for absorption bands which are heterogeneously broadened, due to the presence of underlying discrete bands, the extinction coefficients will be larger than those calculated on the basis of the band being homogeneous. When very narrow discrete bands exist the absorption spectrum observed is merely the profile of the population distribution of traps.