INFRARED-SPECTROSCOPY OF METHANOL CLUSTERS ADSORBED ON LARGE ARX HOSTCLUSTERS

The pick-up technique has been employed to deposite single methanol mo lecules as chromophores on the surface of large Ar(x) host clusters. A t higher densities of the guest molecules in the interaction region, m ore than one methanol molecule are adsorbed on the host cluster and, h ence, dimers or even larger complexes can be formed. The methanol comp lexes are carried by the Ar(x) substrate clusters to the detector and are analyzed with a mass spectrometer. It turns out that the commonly encountered protonation reaction leading to the fragmentation of the i onized methanol complexes is appreciably quenched. The technique is th en combined with molecular beam depletion experiments employing the in frared radiation of a pulsed CO2 laser. In the region between 1000 and 1100 cm-1, which covers the nu8 CO stretch and the nu7 CH3 rock monom er vibrations, absorption spectra have been measured for the methanol monomer as well as the methanol dimer, trimer, and tetramer adsorbed o n large Ar(x) substrate clusters. For the monomer, both vibrations are observed. The dimer spectrum features three peaks and resembles very much the spectrum obtained in a previous study of the free gas-phase d imer, except that all lines are shifted to the red by approximately 2 cm-1 From this observation it is concluded that the methanol dimer on the Ar(x) host cluster adopts the same structure as in the gas phase. Whereas this observation has also been made for the trimer, the tetram er spectrum is distinctly different.