LASER-ABLATION OF METALS - THE TRANSITION FROM NONTHERMAL PROCESSES TO THERMAL EVAPORATION

The mechanisms of laser-stimulated ablation of metal surfaces have bee n investigated for different regimes of incident photon fluence. For t his purpose, small Na particles served as a model system. They were he ld under ultrahigh vacuum conditions and exposed to laser radiation wi th wavelengths ranging from the ultraviolet to the near infrared spect ral range. The ablation products were photoionized by light of a secon d laser. Their mass as well as their kinetic energy distributions were determined by time-of-flight measurements. The results show that init ially, at low laser fluence, ablation occurs along one or even two non -thermal reaction pathways. At this stage, predominantly atoms come of f. The non-thermal mechanism can be understood within the framework of the Menzel-Gomer-Redhead scenario, As the: fluence grows thermal deco mposition gradually takes over and an increasing amount of dimers is f ound. Under certain experimental conditions the rate of dimers detache d from the surface per laser pulse can by far surmount the rate of ato ms. Finally, for even larger fluences, evidence for detachment of larg e particulates is found.