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 (W_nH)⁺ 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.