Small (HCl) m (H2O) n clusters have been assembled in He droplets, and their spectra in the HCl stretch range (2500−3000 cm−1) have been obtained. In a recent He droplet study, a band at 2670 cm−1 was assigned to the dissociated H3O+(H2O)3Cl− ion pair. In this work, we have revised the assignment of this band to a cyclic hydrogen-bonded form of the (HCl)2(H2O)2 cluster based on careful measurements of the pickup pressure dependence as well as the transition moment angles associated with the HCl stretch vibrations. A number of vibrational bands due to small mixed clusters have also been observed. As the number of the captured water molecules increases, a broad feature appears that spans the 2550−2800 cm−1 range. The possible origin of this spectral broadening in large (HCl) m (H2O) n clusters is discussed.

This work studies the renormalization of the molecular moments of inertia I(G) in liquid helium. For this purpose we have measured the rotational-vibrational spectra of the nu(3) modes of a series of homologous light spherical top molecules such as CH(4), CD(4), SiH(4), and SiD(4) in He droplets. The spectra were fitted to an empirical gas phase Hamiltonian, yielding a set of spectroscopic constants. We found that the additional moment of inertia, DeltaI(He), scales approximately as square of I(G). This is in agreement with the theoretical model which assigns DeltaI(He) to coupling of molecular rotation with vibration of He in the molecular vicinity. Our results also indicate a large increase in the effective centrifugal distortion constants, which is another manifestation of the interaction of the molecular rotors with the He environment. Finally, the mechanism of the relaxation of rotational energy in liquid helium is discussed.

In this work, infrared laser spectroscopy in helium droplets is used to study the solvation of HCl with small water clusters. Clusters of HCl(H(2)O)(n) with n = 1-3 and (HCl)(2)H(2)O have been identified in the free OH stretching spectral range of 3680-3820 cm(-1). The assignment of the infrared vibrational bands of the clusters is aided by ab initio calculations.

Presented here are the results of the joint theoretical and infrared laser spectroscopic study of the hydrogen chloride monomer and clusters, (HCl)n (n=1-6), isolated in helium nanodroplets. The H-Cl stretching bands of the dimers and trimers show a large increase in the band intensity as well as low frequency shift with respect to that in a single HCl molecule. The average frequency of the bands for clusters larger than trimers remains approximately constant, which correlates well with the onset of the folded cyclic structure and the full development of the hydrogen bonding in larger clusters. The structure of the clusters was found to be cyclic planar for trimers, slightly twisted square planar for tetramers, envelope-like for pentamers, and folded pseudobipyramidal for hexamers. This change in structure upon an increase of the cluster size can be seen as an early stage of the structural transition to the HCl solid, which consists of zigzag chains of HCl molecules. Spectra of large clusters of about 500 molecules consist of a single band, which encompasses the same frequency range of trimers through hexamers.