CONDENSATION OF SUPERSATURATED VAPORS ON BENZENE IONS GENERATED BY RESONANT 2-PHOTON IONIZATION - A NEW TECHNIQUE FOR ION NUCLEATION

A new technique is described here that allows the study of ion-induced nucleation by well defined ions. The technique is based on resonant two-photon ionization (R2PI) of a chromophore molecule present in small concentrations in a supersaturated host vapor. With this method it is now possible, for the first time, to selectively and unambiguously generate specific ions of interest and study their nucleating behaviors during the process of condensation of different supersaturated vapors. The new method is demonstrated by studying the ion-induced nucleation of supersaturated methanol and acetonitrile vapors by benzene molecular ions, The benzene ions are generated by R2PI using the benzene's A(1g) --> B-2u 6(0)(1) resonance transition. The measurements are carried out in a thermal diffusion cloud chamber. The nucleation count vs wavelength exhibits a characteristic peak at 258.9 nm, which matches the mass spectrum obtained by R2PI of benzene in a high-pressure mass spectrometric source. The observed nucleation count in supersaturated methanol vapor increases by about 20% for benzene cations as compared to negative ions, which suggests an apparent enhancement for the condensation of this supersaturated vapor on positive ions.