Thresholds for Laser-Induced Ion Formation from Aerosols in a Vacuum Using Ultraviolet and Vacuum-Ultraviolet Laser Wavelengths

Alpha-ray radiolysis of S02/H20/Nz gas mixtures is carried out to investigate the competitive processes of ion-induced and binary homogeneous nucleation in this system. Charged particles in the system are indicative of ion-induced nucleation and most likely involve water clusters. Charged particle neutralization was prevented by application of a dc electric field, therefore, uncharged particles in the system result from homogeneous nucleation. Hydroxyl (·OH) radicals are generated by the a-ray radiolysis. Oxidation of S02 by ·OH radicals leads to sulfuric acid (H2S04 ) vapor molecules and to subsequent binary homogeneous nucleation of H2S04/H10. At increased S02 concentrations the contribution of binary homogeneous nucleation increases relative to that of ion-induced nucleation. Electrical mobility distributions of bipolar ions and nanometer-sized charged particles were measured with a cluster-differential mobility analyzer combined with a Faraday cup electrometer. In this system, ion-induced nucleation occurred preferentially on negative rather than positive ions.