PNNL

Abstract

We have used the pump-probe technique to measure the photostimulated positive ion yield, as a function of time-delay between two sub-threshold femtosecond laser pulses. We find the ion yield, from 265 nm femtosecond irradiated MgO and LiF, depends critically on the laser pulse delay two-pulse 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 re-combined with a variable time delay, the Mg+ desorption yield displays three distinct lifetimes and persists for laser delays of over 100 picoseconds. A pulse delay of only 500 femtoseconds nearly eliminates ion desorption except for Mg+. In contrast, for LiF the majority of Li+ yields decays rapidly, largely within the femtosecond pulse duration. However, a weak but measurable decay component of approximately 2 picoseconds is indicated. We hypothesize the nonresonant two-photon excitation contributes to the ultrfast desorption mechanism through the creation of electron/hole pairs and allows direct observation of the surface electron/hole pair trapping dynamics and measurment of the surface free carrier lifetime.