Size-dependent dynamics of a quantum solvent: Laser spectroscopy of HCN–(HD)n grown in helium nanodroplets

The progressive solvation of HCN by molecular hydrogen (HD) is studied by high-resolution laser spectroscopy. The [HCN–(HD)n] clusters are grown in helium nanodroplets, an ideal spectroscopic medium which at sufficiently low temperatures permits the study of systems containing n=2–14 HD molecules. A single isomer is observed for each cluster size, independent of the order in which the molecules are added to the helium droplets, indicating that these systems are rather “fluid.” Clusters with an incomplete first solvent shell (n=1–11) rotate as a unit, having rotational constants that steadily decrease with increasing size. At n=12, the HCN undergoes nearly free internal rotation within the solvent cage of the hydrogens. The fine structure observed in the corresponding “R(0)” transition suggests that the solvent cage is somewhat anisotropic. The further addition of solvent molecules results in a quenching of the HCN internal rotation at n=14, suggestive of a significant increase in the anisotropy of the solvent environment.