<title>Pulse-width influence on laser-induced desorption of positive ions from ionic solids</title>

Abstract

We have compared the desorption of positive ions, including Mg<SUP>+</SUP> and MgO<SUP>+</SUP>, form ionic magnesium oxide single crystals following pulsed laser excitation using either nanosecond or femtosecond sources. Following optical excitation, desorbed ions are rapidly extracted and mass analyzed using standard time-of-flight techniques. Ion yields and velocities are determined as a function of laser fluence. The threshold similarity is a surprising result, as sub-band gap nanosecond pulses are only likely to excite defect states efficiently, while the ultrahigh peak-power femtosecond pulses could in principle induce multiphoton and avalanche excitation. We argue that at least in this specific case, the important factor appears to be merely the number of photons and not the pulse duration. However, it is observed that femtosecond excitation yields considerable H<SUP>+</SUP> and less interference from impurity alkali ions than does nanosecond excitation. The source of the protons is presumably the hydroxylated MgO surface.