Non-thermal laser-induced desorption of metal atoms with bimodal kinetic energy distribution

Laser-induced desorption of metal atoms at low rate has been studied for pulsed excitation with wavelengths of j"266, 355, 532 and 1064 nm. For this purpose sodium adsorbed on quartz served as a model system. The detached Na atoms were photo-ionized with the light of a second laser operating at j"193 nm and their kinetic energy distribution was determined by timeof-flight measurements. For j"1064 nm a distribution typical of thermal bond breaking is observed. If desorption, however, is stimulated with light of j"266 or 532 nm, the kinetic energy distribution is non-thermal with a single maximum at E ,*/ "0.16$0.02 eV. For j"355 nm the non-thermal distribution is even bimodal with maxima appearing at E ,*/ "0.16$0.02 and 0.33$0.02 eV. These values of the kinetic energies actually remain constant under variation of all experimental parameters. They appear to reflect the electronic and geometric properties of different binding sites from which the atoms are detached and thus constitute fingerprints of the metal surface. The non-thermal desorption mechanism is discussed in the framework of the Menzel-GomerRedhead scenario. The transition from non-thermal to thermal desorption at large fluences of the laser light could also be identified. PACS: 61.80.Ba; 68.55.Jk; 79.20.Ds; 36.40 In recent years, laser-induced surface processes have received growing attention. Understanding of such reactions is not only of scientific interest but opens up the possibility of exploiting them for a large number of applications. An essential process among the variety of lasersurface interactions is non-thermal desorption of atoms, ions and molecules (see e.g. [1]) from the surface of semiconductors [2, 3], metals [4—17] or insulators [18]. Even though a large number of experimental and theoretical investigations on non-thermal desorption can be found in the literature, the present understanding of such light-induced reactions is still rather limited. In order to clarify in more detail how atomic or molecular species are liberated from a surface as a result of electronic excitation, experiments along the following lines are of particular value: i) The incident laser fluence should be kept extremely low so that only subtle changes of the surface morphology are induced. Desorption studies under such conditions of low reaction rate could open the door to correlate the desorption behavior with details of the electronic and geometric structure of different binding sites of the surface. Furthermore, light intensities not sufficient to remove large quantities of material per laser pulse ensure that gas phase collisions of the detached species can be avoided and their genuine kinetic energy distribution is measured. In addition, use of low light fluences minimizes the surface temperature rise and therefore the thermal desorption signal that could obscure observation of non-thermal processes. ii) It has been concluded from previous experiments that atoms or molecules are preferentially desorbed from ‘‘defects’’ of the surface, i.e. from sites with particularly low coordination numbers [4,19]. For this reason the preparation of surfaces with the largest possible number of such binding sites is essential. In other words, the surface under study should have a pronounced but reproducible atomic ‘‘roughness’’ to provide a detectable desorption signal even at low photon fluences. The present paper reports desorption studies along these lines with the objective to elucidate in more detail which role the surface structure plays in desorption on a microscopic scale. Surfaces with large roughness have been prepared by the deposition of metal atoms on dielectric substrates held at low temperature. For this purpose sodium adsorbed on quartz served as a model system. The deposited atoms form small particles, with the surface defects acting as nucleation centers, a process known as Volmer-Weber growth mode in thin film epitaxy [20]. If the deposition is continued further and further, the clusters finally grow together into a thin film. As will be described in more detail below, laser-induced desorption of atoms from small particles is particularly useful for the investigations reported here, since their surfaces can be highly corrugated offering a considerable number of sites with low coordination number from which desorption occurs preferentially. Another essential argument is that the relative number of such ‘‘defect’’ sites can be varied intentionally by changing the size and shape of the clusters. Furthermore, formation of small particles on the surface of a dielectric substrate turns out to ensure reproducible conditions, i.e. the roughness is such that reproducible desorption signals are detected in subsequent experiments. Unlike earlier measurements with continuous-wave laser light [4] the studies described here were also performed out of resonance of surface plasmon excitation with pulsed ultraviolet and infrared radiation. Atoms detached in non-thermal reactions are found and their kinetic energies are determined. Desorption being highly surface specific, it is shown that these kinetic energies essentially reflect the local electronic and geometric properties of different sites on the cluster surface.