Interaction of wide-band-gap single crystals with 248-nm excimer laser radiation. XI. The effect of water vapor and temperature on laser desorption of neutral atoms from sodium chloride

We show that low partial pressures of water vapor $(10−5Pa)$ dramatically increase the intensity of neutral Na and Cl emissions from cleaved, single-crystal NaCl during pulsed laser irradiation at 248 nm (KrF excimer). The time-of-flight distributions of these emissions are consistent with thermal desorption from laser-heated surfaces. Significantly, introducing water vapor lowers the particle velocities and thus the effective surface temperature during emission. Transmission measurements confirm that laser absorption is reduced in the presence of water vapor. The Arrhenius analysis of the emission intensities and effective temperatures show reduced activation energies in the presence of water vapor, which more than compensate for the vapor-induced reduction in laser absorption and surface temperature. Atomic force and scanning electron microscopy of the irradiated surfaces show evidence for accelerated monolayer-scale erosion in the presence of water vapor. A mechanism for the effect of water on these emission and erosion processes is proposed and discussed.