Probing Methanol Cluster Growth by Vacuum Ultraviolet
Ionization
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Abstract
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