Sodium dimers on the surface of liquid He-4

We have studied the structure of a sodium dimer interacting with liquid He-4. We calculated the equilibrium configuration and binding energy of a Na-2 molecule solvated in a bulk Liquid He-4 ''bubble'' and near the liquid-vapor interface ''dimple'' by using a density-functional approach. We find that the solvated molecule is a metastable state, while the the lowest energy bound state occurs when the molecule lies flat on the surface of the liquid. The binding energy for the ''erect'' dimer is only similar to 1 K higher than the flat dimer, with no potential energy barrier between the two orientations, implying relatively free rotations of the molecule on the surface. The small effects of the liquid environment on the vibrational properties of the dimer are investigated.