Photon-stimulated desorption of neutrals from silver and alkali halides

Abstract

Photon-stimulated desorption (PSD) from silver and alkali halides under valence excitation is studied by a sensitive method in which the desorption rate is measured by lock-in detection of a mass-spectrometer signal under chopped excitation. Main results are as follows. (a) The main desorption species are identified as halogen molecules in silver halides (AgBr and AgCl), and alkali-metal and halogen atoms in alkali halides (RbBr, KI, and RbI). (b) The decay time of the desorption rate corresponds to the lifetime $\ensuremath{\tau}$ of the relevant photoexcited states. The molecular desorption from silver halides, in contrast to the atomic desorption from alkali halides, results in a unique decay behavior in silver halides such as the decrease of $\ensuremath{\tau}$ with increasing excitation intensity. (c) Temperature dependence of the desorption yield under weakly absorbed photon excitation corresponds to that of ${(D\ensuremath{\tau})}^{\frac{1}{2}}$ (with $D$, the diffusion coefficient of the excited states), except at the low-temperature threshold (-85\ifmmode^\circ\else\textdegree\fi{}C) in AgBr, which is determined by the surface adsorption energy of ${\mathrm{Br}}_{2}$. (d) Desorption-yield spectra of silver halides are well described in terms of the diffusion of excited states and give the value of $D\ensuremath{\tau}$ at various temperatures. Combined with $\ensuremath{\tau}$, we can estimate $D$, which is 1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ ${\mathrm{cm}}^{2}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ for AgBr at room temperature. From these results, the diffusion of relevant excited states is shown to be important in the PSD under valence excitation, and models for diffusing species are proposed.