KINETICS OF PHOTOINDUCED DISSOCIATION OF NA CLUSTERS DEPOSITED ON MICA

Na clusters bound to mica surfaces have been irradiated with pulsed an d cw visible laser light. Kinetic energy and angular distributions of the Na atoms desorbing from the dusters have been determined using cw two-photon laser-induced fluorescence detection. In addition the depen dence of the desorption rate on laser power, wavelength and polarizati on has been measured. The most probable kinetic energy E(kin) of the p hotodesorbed atoms at the surface temperature T-s=300 K was found to b e E(kin)=18+/-5 meV, independent of laser irradiance (3 mu J/cm(2)...2 0 mJ/cm(2)) and wavelength (450 nm, 505 nm, 658 nm). With increasing o rientation angle between detection axis and surface normal (0 degrees less than or equal to Theta less than or equal to 90 degrees) E(kin) w as observed to decrease slightly, while it was nearly independent of s urface temperature between T-s=30 K and T-s=300 K. Also, with increasi ng radius of the Na clusters the desorbing Na atoms slowed down. The a ngular distribution of the Na atoms was of cos(2)-type with respect to the surface normal. These observations suggest that laser-induced des orption of Na from Na clusters bound to mica surfaces involves an init ial rate-limiting step of direct surface plasmon excitation followed b y a final step of delayed thermal desorption.