Quantum-state resolved products via vacuum ultraviolet photostimulated desorption from geologic calcite

We report the results of a photostimulated desorption PSD.study of neutral CO products from room temperature geologic calcite utilizing fluences-300 mJrcm2 of 157-nm excimer laser radiation. At this wavelength, we are coincident with an intense absorption peak absorption coefficients4.39=105 cmy1. such that most excitation occurs near or at the crystal surface. Detailed product state distributions are obtained using 2q1.resonance enhanced multiphoton ionization. The CO translational energy can be characterized by a temperature, Ts105"10 K, significantly lower than that of the substrate. The CO rotational 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 wavelengths and excitation occurs throughout numerous bulk crystal layers. In those studies, CO translational energy could be characterized by a temperature of Ts110 K with product internal energy distributions which showed both thermal and non-thermal components. This seemed to indicate a rapid, scattered release from the surface. Broadly similar PSD results over a range of excitation wavelengths and crystal absorptivities confirms a final surface state decay mechanism in the production of product CO. Subtle variations may reflect state differences in desorption for excitations occurring near or within the band gap and those occurring more deeply within the ‘charge-transfer’ band. q1998 Elsevier Science B.V.