Fundamental reactions in laser ablation of metals: defect-initiated bond breaking

Abstract

The role of surface defects in laser stimulated ablation has been investigated. For this purpose, rough Na surfaces served as a model system. They were prepared by deposition of Na atoms on quartz substrates under ultrahigh vacuum conditions and exposed to laser pulses with λ=532 nm and 7 ns duration. In addition to atoms, Na dimers are detached in large quantities. The time-of-flight distributions and the integral desorption signal of these dimers were measured as a function of the laser fluence, and the desorption yield was determined for increasing numbers of successive laser pulses. Measurements have also been performed after reducing the surface roughness by annealing at different temperatures. We find that desorption of Na dimers occurs as a thermal process for the fluence range and wavelength used in the experiments. The fluence dependence of the integral desorption rate exhibits a plateau which follows and precedes a sharp increase. For constant laser fluence the integral desorption rate decreases as a function of the number of laser pulses. The results indicate that the Na dimers come off preferentially from special sites of low binding energy and low coordination number. Two of these sites, the reservoir of which is limited, can be distinguished. They have different annealing behavior, different 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.