NONTHERMAL LASER-INDUCED DESORPTION OF METAL ATOMS WITH BIMODAL KINETIC-ENERGY DISTRIBUTION

Laser-induced desorption of metal atoms at low rate has been studied f or pulsed excitation with wavelengths of lambda = 266, 355, 532 and 10 64 nm. For this purpose sodium adsorbed on quartz served as a model sy stem. The detached Na atoms were photo-ionized with the light of a sec ond laser operating at lambda = 193 nm and their kinetic energy distri bution was determined by time-of-flight measurements. For lambda = 106 4 nm a distribution typical of thermal bond breaking is observed. If d esorption, however, is stimulated with light of lambda = 266 or 532 nm , the kinetic energy distribution is non-thermal with a single maximum at E(kin) = 0.16 +/- 0.02 eV. For lambda = 355 nm the non-thermal dis tribution is even bimodal with maxima appearing at E(kin) = 0.16 +/- 0 .02 and 0.33 +/- 0.02 eV. These values of the kinetic energies actuall y remain constant under variation of all experimental parameters. They appear to reflect the electronic and geometric properties of differen t binding sites from which the atoms are detached and thus constitute fingerprints of the metal surface. The non-thermal desorption mechanis m is discussed in the framework of the Menzel-Gomer-Redhead scenario. The transition from non-thermal to thermal desorption at large fluence s of the laser light could also be identified.