Spectroscopy of OCS–hydrogen clusters in He droplets

Abstract

Clusters of para-hydrogen (pH₂) and ortho-deuterium (oD₂) have been assembled around an OCS chromophore molecule inside He droplets in a molecular beam and studied ia IR diode laser depletion spectroscopy (ν≈2060 cm⁻¹). The superfluid ⁴He droplets provide a gentle host ensuring a constant low temperature of either T = 0.38 K for ⁴He droplets or T = 0.15 K for both the pure ³He and mixed ⁴He–³He 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 pH₂, HD or an oD₂ 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 pH₂ clusters but not for the oD₂ clusters which is consistent with a donut model. The moments of inertia of the pH₂ and the oD₂ 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 H₂ and D₂ molecule has an effective mass of about 10 and 12 u, respectively.