INFRARED PHOTODISSOCIATION OF SIZE-SELECTED METHANOL CLUSTERS

Size-selective infrared photodissociation spectra of (CH3 OH)n clusters from n = 2 to n = 9 were measured near the absorption band of the C-O stretching mode of the monomer at 1033.5 cm -1. The experiments were carried out in a molecular-beam apparatus in which the clusters are generated in a supersonic expansion and afterwards size selected in a scattering experiment with helium atoms. The internally excited clusters are dissociated by the radiation of a cw CO2 laser in a new antiparallel arrangement of the scattered cluster beam and laser beam. The observed spectra vary from the dimer, for which a double-peak structure appears, to the single-peak spectra of the trimer, tetramer, and pentamer which are continuously shifted to higher frequencies. A special transition is seen from the pentamer to the hexamer, for which again a double-peak structure is observed which continues to larger clusters. Applying an intermolecular model potential, a correlation between the observed spectra and the cluster configuration of minimum energy is derived. The line shifts of the dimer to the red and blue are caused by the nonequivalent position of the donor and acceptor in the hydrogen bond. The next three larger clusters are nearly planar rings, while from the hexamer onwards, only distorted rings and similar isomeric structures appear. These calculations together with the evaluation of the integrated dissociation cross section show that only internally excited dimers and trimers can be dissociated with one or two CO2 laser photons, respectively. © 1990 American Institute of Physics.