PNNL

Abstract

The positive ion yield as a function of delay between ultraviolet femtosecond pulse pairs for four alkali halide single crystals has been measured. Two-pulse correlation allows direct observation of solid state and surface dynamics on an ultrafast timescale. The ion yield from 265nm irradiated NaBr, KC1, KBr, adn K1 depends critically on the time delay between the two sub-threshold pulses. Following irradiation of single crystal NaBr adn KC1, the positive ion desorption yield displays three distrinct features; a coherence peak, followed by rise, and decay features. In contrast, the yield of K+ from KBr displays only the coherence peak and picosecond decay features while the yield from K1 shows only the coherence feature. The data suggest that although the nanosecond ion desorption mechanism may be dominated by defect photoabsorption, significant electron-hole pair production may contribute to the desorption mechanism following femtosecond excitation.