Carbonyl sulfide-doped and cyanoacetylene-doped hydrogen clusters studied with rotational spectroscopy.

摘要

The weakly bound (H2)N -- OCS (N = 1-- 7) and (H2)N -- HCCCN (N = 1 -- 6) clusters were measured using a Fourier transform microwave spectrometer. Complexes and clusters containing both spin isomers of hydrogen (para-hydrogen and ortho-hydrogen) were observed. The nuclear quadrupole hyperfine patterns were observed for clusters containing 14N (I = 1), D (I = 1) and 33S (I = 3/2) nuclei. The spin-spin hyperfine patterns due to ortho-hydrogen molecules were measured and assigned.;For the orthoH2 -- OCS and orthoH2 -- HCCCN complexes, both a- and b-type transitions were observed and assigned to a distortable rotor model. For the paraH2 -- OCS and paraH2 -- HCCCN complexes, only a-type transitions were observed. Both dimers are floppy and near T-shaped. An ab initio intermolecular potential energy surface was calculated using the coupled cluster method for the H2 -- HCCCN system and three extreme orientations of the hydrogen molecule were considered. Bound-state rotational energy levels supported by the potential were obtained for the parallel orientations (orthoH2 -- HCCCN) as well as for the averaged potential (paraH2 -- HCCCN). Both potentials were scaled and the agreement with the experimental transition frequencies improved.;For most larger clusters (N = 2--7), only one rotational transition was observed. The spectra clusters with N > 2 were fit to a near prolate symmetric top model. Larger clusters can be pure paraH2 clusters, pure orthoH2 clusters, or mixed clusters with a combination of both spin isomers. Mixed clusters were observed and assigned based partly on the evolving hyperfine pattern due to the spin-spin coupling of the orthoH 2 molecules. Additionally, structural isomers of some mixed clusters were observed and assigned. The rotational constant of the clusters decreases as the size of the cluster increases. These studies provide a stepping stone for the study of larger doped hydrogen clusters and for the observation of possible onset of superfluidity.