PHOTOIONIZATION ON INSULATOR SURFACES - DIFFUSE REFLECTANCE LASER FLASH-PHOTOLYSIS OF DISTYRYLBENZENES ADSORBED ON SILICA AND ALUMINA

The absorption and fluorescence of o-, m-, and p-distyrylbenzenes on surfaces of polycrystalline silica and alumina were investigated in the adsorbed state. Time-resolved diffuse reflectance transient absorption spectra were recorded following pulsed nanosecond laser excitation at 354 nm. In each case, a long-lived transient was detected and assigned to radical-cation absorption, and in two cases a short-lived transient was observed and assigned to triplet-triplet absorption. The radicals were also detected by ESR spectroscopy after laser excitation. Experimental observations of the laser fluence dependence of the radical-ion formation efficiency are consistent with a mechanism involving the sequential absorption of two photons during the same laser pulse. It is shown that model calculations for one- and two-photon excitation processes give predictions that are in good agreement with the measurements of the triplet-state and radical-ion production, respectively. The radicals decay by electron-ion recombination. The experimentally observed decay curves are well described by a temperature-activated diffusion model. Decay kinetics are discussed and tested in relation to this theoretical treatment.