QUANTUM-STATE RESOLVED PRODUCTS VIA VACUUM-ULTRAVIOLET PHOTOSTIMULATED DESORPTION FROM GEOLOGIC CALCITE

We report the results of a photostimulated desorption (PSD) study of n eutral CO products from room temperature geologic calcite utilizing fl uences < 300 mu J/cm(2) of 157-nm excimer laser radiation. At this wav elength, we are coincident with an intense absorption peak (absorption coefficient = 4.39 x 10(5) cm(-1)) such that most excitation occurs n ear or at the crystal surface. Detailed product state distributions ar e obtained using (2 + 1) resonance enhanced multiphoton ionization. Th e CO translational energy can be characterized by a temperature, T = 1 05 +/- 10 K, significantly lower than that of the substrate. The CO ro tational and vibration state distributions appears thermalized to the surface. This maybe contrasted with our previous study of CO PSD from room temperature calcite done using 193-nm and 213-nm excitation. Bulk UV absorption decreases by two orders of magnitude at these wavelengt hs and excitation occurs throughout numerous bulk crystal layers. in t hose studies, CO translational energy could be characterized by a temp erature of T = 110 K with product internal energy distributions which showed both thermal and non-thermal components. This seemed to indicat e a rapid, scattered release from the surface. Broadly similar PSD res ults over a range of excitation wavelengths and crystal absorptivities confirms a final surface state decay mechanism in the production of p roduct CO. Subtle variations may reflect state differences in desorpti on for excitations occurring near Or within the band gap and those occ urring more deeply within the 'charge-transfer' band. (C) 1998 Elsevie r Science B.V.