Comparing the short and ultrashort pulsed laser ablation of LiF

Pulsed laser ablation of LiF was studied using both nanosecond (ns) and femtosecond (fs) pulses at 248nm. Optical emission from electronically excited Li and F atoms in the plume of ejected material was investigated by wavelength, time and spatially resolved imaging methods. Careful analysis of images of species selected optical emission yielded estimates of the mean velocities of the Li+ ions arising in both excitation schemes (∼11 and ∼13km∕s, respectively), and highlighted the dramatic effects of radiation trapping, most notably by the reabsorption of Li(2p→2s) emission by ground state Li atoms in the ns ablation studies. Plumes formed by fs excitation are found to contain a higher fraction of energetic∕electrically excited components, including excited F atoms and ions, indicative of an explosive boiling mechanism, whereas the ablation plume resulting from ns ablation is deduced to arise primarily from thermal evaporation of the transiently heated target surface. The amount of target material removed ...