FUNDAMENTAL REACTIONS IN LASER-ABLATION OF METALS - DEFECT-INITIATED BOND BREAKING

The role of surface defects in laser stimulated ablation has been inve stigated. For this purpose, rough Na surfaces served as a model system . They were prepared by deposition of Na atoms on quartz substrates un der ultrahigh vacuum conditions and exposed to laser pulses with lambd a = 532 nm and 7 ns duration. In addition to atoms., Na dimers are det ached in large quantities. The time-of-flight distributions and the in tegral desorption signal of these dimers were measured as a function o f the laser fluence, and the desorption yield was determined for incre asing numbers of successive laser pulses. Measurements have also been performed after reducing the surface roughness by annealing at differe nt temperatures, We find that desorption of Na dimers occurs as a ther mal process for the fluence range and wavelength used in the experimen ts. The fluence dependence of the integral desorption rate exhibits a plateau which follows and precedes a sharp increase. For constant lase r fluence the integral desorption rate decreases as a function of the number of laser pulses. The results indicate that the Na dimers come o ff preferentially from special sites of low binding energy and low coo rdination number, Two of these sites, the reservoir of which is limite d, can be distinguished. They have different annealing behavior, diffe rent binding energies and can be depleted selectively by appropriately choosing the laser fluence. The sites with the lowest binding energy seem to be dimers which are adsorbed on terraces of the metal surface. (C) 1998 Elsevier Science B.V.