Femtosecond time-resolved laser-induced desorption of positive ions from MgO

We have used the pump-probe technique to measure the photostimulated positi ve ion yield as a funct:ion of time delay between two sub-threshold femtose cond laser pulses. We find that the ion yield from UV femtosecond irradiate d MgO depends critically on the laser pulse delay, Delta t, in two-pulse ex periments. In single-pulse experiments, excitation of MgO produces a variet y of ions including Mg+, MgO+, and a significant yield of H+. In contrast, if the femtosecond laser pulse is split into two sub-threshold beams and th en recombined with a variable time delay, the ion yield may be drastically altered depending on the delay between pulses. The Mg+ desorption yield dis plays three distinct lifetimes and persists for laser delays of over 100 ps . A pulse delay of only Delta t = 500 fs nearly eliminates ion desorption e xcept for Mg+. The use of a pair of delayed femtosecond laser pulses can th us control the species of the desorbed ion. The mechanism for femtosecond l aser desorption is clearly different from nanosecond laser desorption. We h ypothesize that the creation of electron-hole pairs by nonresonant two-phot on excitation contributes to the ultrafast desorption mechanism.