Spectroscopy of hydrogen and water clusters in helium droplets

Hydrogen and water clusters were studied by means of Coherent Anti-Stokes Raman Scattering (CARS) and infrared spectroscopy, respectively. The helium droplet encapsulation technique was used in these experiments.; The experiments with large para-hydrogen clusters of several thousand molecules per cluster embedded in helium droplets are presented. CARS spectra of hydrogen clusters with different ratios of ortho- and para- hydrogen are analyzed in terms of band intensities and frequencies. The CARS experimental setup and helium droplet apparatus are thoroughly described.; Small water clusters of 2 to 30 molecules per cluster as well as water monomers were studied in helium droplets. Rotational constants and rotational and vibrational relaxation times of single water molecules in helium droplets were deduced from the analysis of their depletion spectra. The method of studying the infrared intensities of molecules and molecular clusters was developed by using the helium droplet encapsulation technique. The method was applied to study the non-additive enhancement of the infrared intensity in water clusters of 2, 3 and 4 molecules. Gradual transformation of the spectra from a relatively narrow band of small clusters to a broad spectrum, characteristic for bulk ice, was observed.; The solid para-hydrogen Raman Shifter that was developed in our laboratory is described in one of the Chapters. This device utilizes the para-hydrogen crystal at T = 4 K as a medium for Stimulated Raman Scattering (SRS). The near-infrared laser was focused into the crystal and the red-shifted Stokes wave was obtained from the SRS process. This device provides mid-infrared laser radiation in the range of 4.4--8 mum to study heavy atom vibrations in molecules. The grow-from-the-liquid technique that was used here gives clear para- hydrogen crystals with a high laser damage threshold.