INFRARED MOLECULAR-BEAM DEPLETION SPECTROSCOPY OF SIZE-SELECTED METHANOL CLUSTERS

Molecular beam depletion spectroscopy has been employed to study the dissociation of small methanol clusters in the spectral region between 1000 and 1100 cm-1 which covers the nu8 CO stretch (1033.5 cm-1) and the nu7 CH3 rock (1074.5 cm-1) monomer vibrations. Size selection has been achieved by dispersing the (CH3OH)n cluster beam by a secondary He beam. Aside from the recently published CH3OH dimer absorption bands at 1026.5 and 1051.6 cm-1 which are assigned to the excitation of the CO stretching vibrations in the non-equivalent subunits of the hydrogen-bonded complex, a previously unobserved band was found at 1071.3 cm-1. This absorption band is attributed to the excitation of the CH3 rocking vibration in the dimer. It appears that this transition which is very weak in the free methanol monomer receives substantial oscillator strength due to the intermolecular interaction in the complex. A splitting of this band could not be observed. The trimer and tetramer spectra feature single peaks for the CO stretching vibration being centered at 1042.2 cm-1 and 1044.0 cm-1, respectively. This observation is consistent with the cyclic structures of these species. The trimer and tetramer rocking vibrations are observed near 1060.5 cm-1 but cannot be localized exactly, due to a gap in the CO2 laser tuning range.