We investigate the effect of low partial pressures of water vapor and bulk temperature on laser-induced positive ion emission from cleaved, single-crystal sodium nitrate and sodium chloride. Both materials yield more intense Na+ emission in the presence of water vapor (P-H2O similar to 1 x 10(-5) Pa) than samples irradiated under UHV conditions (P-total < 1 x 10(-7) Pa). Emission intensities increase rapidly with increasing temperature in a fashion consistent with activation energies on the order of 0.07-0.08 eV, both with and without water vapor. We attribute Na+ emission to a defect-mediated photoelectronic process. Na+ adsorbed at or near surface anion vacancies (electron traps) is ejected when the underlying or adjacent vacancy is photoionized. Vacancy formation in both materials is expected to have activation energies on the order of 0.08 eV, suggesting that Na+ emission is rate limited by the production of surface anion vacancies. We attribute the effect of water vapor to defect-mediated sorption that increases the ion emission probability. (C) 2002 Elsevier Science B.V. All rights reserved.