Laser ablation of metals: the transition from non-thermal processes to thermal evaporation

The mechanisms of laser-stimulated ablation of metal surfaces have been investigated for different regimes of incident photon fluence. For this purpose, small Na particles served as a model system. They were held under ultrahigh vacuum conditions and exposed to laser radiation with wavelengths ranging from the ultraviolet to the near infrared spectral range. The ablation products were photoionized by light of a second laser. Their mass as well as their kinetic energy distributions were determined by time-of-flight measurements. The results show that initially, at low laser fluence, ablation occurs along one or even two non-thermal reaction pathways. At this stage, predominantly atoms come off. The non-thermal mechanism can be understood within the framework of the Menzel-Gomer-Redhead scenario. As the fluence grows thermal decomposition gradually takes over and an increasing amount of dimers is found. Under certain experimental conditions the rate of dimers detached from the surface per laser pulse can by far surmount the rate of atoms. Finally, for even larger fluences, evidence for detachment of large particulates is found.