PNNL

Abstract

We have used femtosecond laser pulse pairs to measure the positive ion yield, from wide band-gap single crystals, as a function of time-delay between pulses. Two-pulse correlation allows direct observation of solid state and surface dynamics on an ultrafast timescale. The ion yield, from 265 nm irradiated MgO and KBr, depends critically on the time delay between two sub-threshold pulses. For example, the Mg+ desorption yield displays three distinct features; a coherence peak, followed by rise, and decay features. In contract, the yield of K+ from KBr displays only the coherence peak and picosecond decay features. The data suggest, that although the nanosecond ion desorption mechanism is dominated by defect photoabsorption, significant electron-hole pair production may contribute to the desorption mechanism following femtosecond excitation. Nanosecond photoexcitation of KBr near 6.4 eV leads to desorption of hyperthermal neutral bromine atoms without a significant thermal velocity component. Two-photon femtosecond excitation at 3.2 eV produces very similar results. Multiphoton femtosecond excitation provides an efficient excitation mechanism of the wide-gap material. These results are likely general for ionic crystals and are consistent with a recently described theoretical model.