Vibrational spectroscopy of methanol and acetonitrile clusters in cold helium droplets

Infrared molecular beam depletion spectroscopy of small methanol and aceton itrile clusters embedded in large helium clusters has been studied in the s pectral region of the CO stretch and the CH3 rock mode from 1023 to 1059 cm (-1). The results are compared with the experimental spectra of the corresp onding free clusters generated in adiabatic expansions and calculations bas ed on density functional theory or empirical potential models. For methanol clusters, the two types of experimental results are the same for the dimer and trimer structure. Different isomers are found in cold helium for the t etramer and pentamer, namely a monomer and dimer attached to a cyclic trime r. For acetonitrile clusters in helium, aside from the dimer, different str uctures are observed. The spectra from the trimer to the hexamer are domina ted by structures which contain the antiparallel dimer as building block wi th D-2d symmetry for the tetramer. They do not correspond to the minimum co nfigurations observed for the free clusters. The fragmentation of the two c luster groups in helium droplets by electron impact ionization is discussed . (C) 1999 American Institute of Physics. [S0021-9606(99)01030-2].