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

Abstract

We compare desorption of positive ions from lithium fluoride single crystals following pulsed laser excitation using either femtosecond (180 fs, 265 nm) or nanosecond (3 ns, 266 nm) sources. Following optical excitation, desorbed ions are mass analyzed using standard time-of-flight techniques. Several important differences between nanosecond and femtosecond excitation are revealed. Femtosecond excitation produces higher kinetic energy Li+ than does nanosecond excitation (10 eV vs. 5 eV) while nanosecond excitation yields significant quantities of impurity ions Na+ and K+, in addition to efficient Li+ emission. The Li+ desorption threshold is similar for both laser sources. This similarity is a surprising result, as sub-bandgap nanosecond pulses are only likely to excite defect states efficiently (via linear excitation), while the ultrahigh peak-power femtosecond pulses could in principle induce multiphoton and avalanche excitation. Femtosecond excitation results 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 function of time delay between two sub-threshold femtosecond laser pulses. We find that the majority of the Li+ yield decays rapidly, largely within the fs pulse duration. However, a weak but measurable decay component of approximately 2 ps is indicated.