Probing methanol cluster growth by vacuum ultraviolet ionization.

The ability to probe the formation and growth of clusters is key to answering fundamental questions in solvation and nucleation phenomena. Here, we present a mass spectrometric study of methanol cluster dynamics to investigate these two major processes. The clusters are produced in a molecular beam and ionized by vacuum ultraviolet (VUV) radiation at intermediate distances between the nozzle and the skimmer sampling different regimes of the supersonic expansion. The resulting cluster distribution is studied by time-of-flight mass spectrometry. Experimental conditions are optimized to produce intermediate size protonated methanol and methanol-water clusters and mass spectra and photoionization onsets and obtained. These results demonstrate that intensity distributions vary significantly at various nozzle to ionization distances. Ion-molecule reactions closer to the nozzle tend to dominate leading to the formation of protonated species. The protonated trimer is found to be the most abundant ion at shorter distances because of a closed solvation shell, a larger photoionization cross section compared to the dimer, and an enhanced neutral tetramer precursor. On the other hand, the protonated dimer becomes the most abundant ion at farther distances because of low neutral density and an enhanced charged protonated monomer-neutral methanol interaction. Thomson's liquid drop model is used to qualitatively explain the observed distributions.