THRESHOLDS FOR LASER-INDUCED ION FORMATION FROM AEROSOLS IN A VACUUM USING ULTRAVIOLET AND VACUUM-ULTRAVIOLET LASER WAVELENGTHS

The minimum laser fluence required for the formation of ions from aero sol particles in a vacuum as a function of a number of parameters has been investigated. Although the influences of various particle propert ies are complex, the threshold energy density does depend on light abs orption and lattice energy. Ionization thresholds were lower at 193 an d 157 nm than at 248 nm. The range in ion formation thresholds for dif ferent types of particles also decreased as the laser wavelength decre ased, consistent with the fact that all the substances used are likely to absorb vacuum-ultraviolet radiation fairly strongly. The threshold for negative ion formation was generally greater than that for positi ve ion formation. Pure sulfuric acid aerosols were ionized well only a t 157 nm. (C) 1997 American Association for Aerosol Research.