PHOTODESORPTION OF NA ATOMS FROM ROUGH NA SURFACES

We investigate the desorption of Na atoms from large Na clusters depos ited on dielectric surfaces. High-resolution translational energy dist ributions of the desorbing atoms are determined by three independent m ethods, two-photon laser-induced fluorescence, as well as single-photo n and resonance-enhanced two-photon ionization techniques. Upon variat ion of surface temperature and for different substrates (mica vs lithi um fluoride) clear non-Maxwellian time-of-flight distributions are obs erved with a cos(2) theta angular dependence and most probable kinetic energies below that expected of atoms desorbing from a surface at the rmal equilibrium. The half-width of the time-of-flight distribution de creases with increasing surface temperature. A quantitative descriptio n of the energy and angular distributions is presented in terms of a m odel which assumes that following the initial surface plasmon excitati on neutral atoms are scattered by surface vibrations. Recent experimen ts providing time constants for the decay of the optical excitations i n the clusters support this model. The excellent agreement between exp eriment and theory indicates the importance of both absorption of the laser photons via direct excitation of surface plasmons as well as ene rgy transfer with the substrate during the desorption process. (C) 199 7 American Institute of Physics.