Near-IR absorption spectrum of the solvated electron in alcohols, deuterated water, and deuterated glasses: lack of observance of the near-IR spectrum in water glasses

Abstract

presolvated (IR) and fully solvated electron (~isible),~~J~-~ ~~~~ (b) two distinctly different in nature electronic species in structurally different traps,7JJ2*25 and (c) the solvated electron in an excited state (IR) and the solvated electron in the ground state (visible).22*23 Some arguments in favor of eIR- being the precursor of Q~- seem to arise from the time shift of the IR absorption to the visible range.9J7-22 Higasihimura et aL9J7 studied y-irradiated ethanol at 4 K and found a strong absorption band in the near-IR. On annealing to 77 K one observes an irreversible stepwise change in the IR band which finally disappears whereas a new band grows in the visible. However, if the initial trapping site is a statistically controlled inhomogeneity in the glass or liquid, the relaxation of this site resulting in a deeply solvated electron should surely be a continuous process rather than a stepwise one, at least in the framework of the theories belonging to group a. The results of most papers however give support for the stepwise ~hange.~~,~~ The other arguments against treating eIR- as the precursors of the fully solvated electrons were given by Ogasawa et al.7*8 They found that the absorption of y-irradiated alcohol glasses depends on the rate of freezing. When the sample is frozen quickly at 4 K, the infrared band appears. When the sample is warmed to 77 K, the IR absorption decreases as the visible band increases. When the sample is cooled slowly to 4 K, both bands can be observed. These results seem to support the idea of two distinct electronic species in the IR and visible regions. Differences between the two kinds of trapped electrons were further indicated by their different reactivities toward electron scavengem6 The spectra calculated by molecular dynamic simulations of the excess electron in liquid water show a continuous blue shift in the optical absorption maximum from initially formed IR band to fully equilibrated state in the vi~ible.*~,~~ Rossky et al. have postulated that in the first step (0-1 10 fs) the electron is solvated in an effective competition with nonradiative energy loss so that the electron is not in its ground state. The shifted IR spectrum observed experimentally at an early stage is thus characteristic of the solvated excited state. The second step process would then be characterized by the time scale for nonadiabatic transition between the ground state with the absorption band shifting to visible. The equilibrium absorption spectrum of the fully solvated electron is achieved when the solvent relaxes around this new ground state.