Infrared Laser Spectroscopy of Molecules in Helium Nanodroplets

This project is centred upon the probing of the structures of molecular clusters in helium nanodroplets (HeDs) via infrared laser spectroscopy. Absorption of IR radiation by clusters inside HeDs causes helium to evaporate from the droplet, resulting in a lower ionisation cross section for the droplet and a consequent depletion of ion signals due to the absorbing clusters in mass spectra. This mass selectivity was used to assist with the assignment of specific IR bands to specific sizes of clusters. With this approach the first attempt to record IR spectra of the cyclic isomers of the water heptamer and octamer is reported. This work has been supported by ab initio calculations and evidence is presented for successful observation of these cyclic clusters. In addition, IR spectra of larger water clusters (H2O)n, where 11 ≤ n ≤ 21, were recorded. We found that the large water clusters are derived from 3-coordinated and 2-coordinated water monomers. IR spectra of methanol clusters have been recorded for the first time in the OH stretching region. We find that clusters such (CH3OH)4 and (CH3OH)5 are consistent with fully cyclic structures held together by hydrogen bonds. This conclusion is very different from earlier work in HeDs, which reported non-cyclic structure for the tetramer and pentamer. When water molecules were combined with methanol in HeDs IR spectra of clusters such as (CH3OH)2H2O and (CH3OH)3H2O clusters were observed for the first time. As with pure water and pure methanol clusters, the structures of these mixed clusters are consistent with cyclic structures. Finally, a short study of the IR spectroscopy of the I2(H2O) cluster is reported as part of a larger study which attempted to generate I atoms by thermal dissociation of I2 molecules prior to pickup by HeDs.