Vacuum Ultraviolet Photoionization of Methane-Water Clusters Leads to Methanol Formation

We report on the photoionization
of a series of small methane-water
mixed clusters generated in a supersonic jet expansion. The resulting
clusters are investigated by synchrotron-based single photon vacuum
ultraviolet (VUV) ionization coupled with reflectron time-of-flight
mass spectrometry. Pure methane clusters (CH4)n+ and protonated water clusters (WnH)+ dominate the mass spectra, however
with judicious control of backing pressure and mixing ratio, mixed
methane-water clusters are generated. We gain unique insights into
the underlying dynamics and chemistry of methane and mixed methane-water
clusters through the analysis of fragment mass spectra, and photoionization
intensity curves, which provide appearance energies, which are then
compared to theoretically calculated ionization energies. In particular,
we provide experimental evidence of the formation of methanol upon
photoionization of the mixed clusters under cold conditions, which
is directly relevant to astrochemistry.