Vibrational spectroscopy of single methanol molecules attached to liquid water clusters

Medium-sized water clusters [(H2O)(n), n = 15-20], produced by adiabatic expansion of water vapor into a vacuum, were doped with single methanol molecules by crossing the water cluster beam with an effusive methanol beam. The spectroscopy of the adsorbed methanol molecule has been studied, by exciting the C-O stretch with a CO2 laser (9.6 mu m) and the C-H or O-H stretch with a Nd:YAG-laser-pumped optical parametric oscillator (2.6-3.6 mu m) and monitoring the vibrational predissociation of the complex (infrared molecular beam depletion spectroscopy). It is found that the methanol molecule adopts a surface site and that it is bound to the water cluster by two or three hydrogen bonds, with the methanol molecule acting as proton donor and proton acceptor. As is evidenced by the essentially unperturbed C-H stretching spectrum, the methyl group is pointing away from the host cluster surface. Analysis of the dissociation products reveals that the water cluster does not adopt a rigid network structure. Instead it is composed of loosely bound subclusters which can be expelled from the host cluster by exciting the methanol chromophore with low-energy laser photons at moderate flux. The close resemblance to IR spectra of liquid water suggests that the doped water clusters are liquid. (C) 1998 Elsevier Science B.V. All rights reserved.