| The work presented in this thesis corresponds to the use of infrared lasers to probe the rotational, vibrational, and solvation dynamics of molecules and molecular clusters embedded in liquid helium droplets. The formation of clusters of hydrogen fluoride (HFn, n ≥ 3) provides a compelling example of the unique cluster growth process that occurs when single HF molecules are sequentially added to a helium droplet. From the infrared spectrum in the H-F stretching region, it is apparent that the cold, liquid helium environment quenches the condensation energy so rapidly that the fifth HF molecule is kinetically trapped behind the small barrier to insertion into the cold, preformed tetramer ring. The energy available to the metastable "4+1" cluster is insufficient to surmount the barrier to the lower energy cyclic pentamer.; The isomerization dynamics of solvated molecular complexes are studied with infrared-infrared double resonance spectroscopy, consisting of two infrared lasers used in a pump-probe configuration. Binary systems with two minima on their potential surfaces are chosen such that the vibrationally excited state energy is larger than both the isomerization barrier and the dissociation energy. One of two possible isomers is vibrationally excited. After the absorption of the first infrared photon, the cold helium heat bath removes the excited vibrational energy on a time scale that is fast in comparison to the droplet flight time between the upstream pump and downstream probe laser interaction regions. The probe laser is tuned through the vibrational bands of the two isomers, and a negative (positive) signal in the double resonance spectrum corresponds to a reduction (increase) in the population of the probed isomer. We observe photo-induced isomerization of the binary HCN-HF, HF-N2O, and HCN-HCCH complexes, while the two isomers of HCN-HCCCN do not isomerize. The results are rationalized in terms of the ab initio intermolecular potentials. When photo-induced isomerization is observed, the downstream population ratios are observed to be strongly dependent on the identity of the upstream isomer and the excited vibrational mode. These observations provide strong clues regarding the mechanisms of the vibrational relaxation processes occurring for molecular complexes solvated in liquid helium droplets. |
