Vibrational predissociation spectra of size selected methanol clusters: New experimental results

Abstract

Vibrational predissociation spectra of methanol (CH3OH)n clusters have been measured in the frequency range of the CO stretch mode for n=7 and n=8 and in the range of the OH stretch mode from n=4 to n=9. The clusters are completely size selected in a scattering experiment with helium atoms. For the IR excitation a line-tunable CO2-laser (1020 to 1080 cm−1) and an Optical Parametric Oscillator (3000 to 3800 cm−1) were used, respectively. The spectra of the CO stretch mode are slightly blue shifted by about 12 cm−1 and show a two peak structure for n=8 and a broad unstructured behavior for n=7 as is predicted in the theoretical calculations based on symmetric and asymmetric cyclic structures. In contrast, the spectra of the OH stretch mode exhibit large red shifts up to 500 cm−1, but also show the change of structured and unstructured behavior for even and odd sizes, respectively. The calculations based on the new potential model of the preceding paper are able to reproduce the general structures and the gross features of the spectra but fail for the absolute magnitude of the shifts.