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 ls532 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. q1998 Elsevier Science B.V.
