Abstract
A new technique is described 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 a small concentration 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 behavior. The new method is demonstrated by studying the ion-induced nucleation of supersaturated methanol and acetonitrile vapors by benzene, toluene, and p-xylene molecular ions. The measurements are carried out in a diffusion cloud chamber. The nucleation rate vs. wavelength exhibits the characteristic resonance peak for the R2PI of the chromophore molecule. A preference for the nucleation on positive ions is observed.