Ultrafast and nanosecond laser-induced desorption of positive ions from lithium fluoride single crystals

We compare desorption of positive ions from lithium fluoride single crystal s following pulsed laser excitation using either femtosecond (180 fs, 265 n m) or nanosecond (3 ns, 266 nm) sources. Following optical excitation, deso rbed ions are mass analyzed using standard time-of-flight techniques. Sever al important differences between nanosecond and femtosecond excitation are revealed. Femtosecond excitation produces higher kinetic energy Li+ than do es nanosecond excitation (10 eV vs. 5 eV) while nanosecond excitation yield s significant quantities of impurity ions Na+ and K+, in addition to effici ent Li+ emission. The Li+ desorption threshold is similar for both, laser s ources. This similarity is a surprising result, as sub-bandgap nanosecond p ulses are only likely to excite defect states efficiently (via linear excit ation), while the ultrahigh peak-power femtosecond pulses could in principl e induce multiphoton and avalanche excitation. Femtosecond excitation resul ts in much less complicated time-of-flight spectra, as predominantly Li+ is detected with some H+ also observed. We have measured the Li+ yield as a f unction of time delay between two sub-threshold femtosecond laser pulses. W e find that the majority of the Li+ yield decays rapidly, largely within th e fs pulse duration. However, a weak but measurable decay component of appr oximately 2 ps is indicated.