Spectroscopy of OCS-hydrogen clusters in He droplets

Clusters of para-hydrogen and ortho-deuterium have been assembled around (pH(2)) (oD(2)) an OCS chromophore molecule inside He droplets in a molecular beam and studied via IR diode laser depletion spectroscopy (v approximate to 2060 cm(-1)). The superfluid He-4 droplets provide a gentle host ensuring a constant low temperature of either T = 0.38 K for He-4 droplets or T = 0.15 K for both the pure He-3 and mixed He-4-He-3 droplets. The spectra show well resolved rotational structure of the vibrational bands for each attached hydrogen molecule in the range n = 1-8. With only one (n = 1) attached pH(2), HD or oD(2) an molecule the best fit rotational constants were used to determine the structure of the complex, which was found to be in surprisingly good agreement with quantum chemical calculations for the free complex. With n = 5 and 6 the Q-branch disappears for the pH(2) clusters but not for the oD(2) clusters which is consistent with a donut model. The moments of inertia of the pH(2) and the oD(2) complexes are explained by a new model in which each of the 18 attached helium atoms in a shell surrounding the OCS molecule are assigned a mass of 0.55, while each attached H-2 and D-2 molecule has an effective mass of about 10 and 12 u, respectively.