Time-resolved femtosecond laser-induced desorption from magnesium oxide and lithium fluoride single crystals

We have used the pump–probe technique to measure the photostimulated positive-ion yield, as a function of time delay, between two ultraviolet laser pulses. We find that the ion yield from 265 nm femtosecond-irradiated MgO and LiF depends critically on the laser pulse delay in two-pulse correlation experiments. For example, single-pulse excitation of MgO produces a variety of ions including Mg+, MgO+ and H+. If the femtosecond laser pulse is split into two sub-threshold beams and then recombined with a variable time delay, the Mg+ desorption yield displays three distinct time regimes and persists for laser delays of over 100 ps. A pulse delay of only 500 fs nearly eliminates ion desorption except for Mg+. In contrast, the Li+ desorption yield from LiF decays more rapidly, largely within the femtosecond pulse duration. A weak but measurable decay component of approximately 2 ps is indicated, however. We hypothesize that non-resonant excitation contributes to the ultrafast desorption mechanism, possibly through the creation of surface electron–hole pairs and allows direct observation of the surface dynamics.