Effects of rotational-symmetry breaking on physisorption of ortho- and para-H2 on Ag(111).

Quantum-state-selective thermal desorption of H2 weakly physisorbed on Ag(111) demonstrates significantly different desorption features between the nuclear-spin modifications. An energy shift due to the rotational-symmetry breaking induced by an anisotropic interaction affects not only the enthalpy but also the entropy of adsorption. The preexponential factor for desorption of the ortho-H2 is about three times as large as that of the para-H2. The entropy difference indicates a perpendicular orientation preference of anisotropic physisorption potential, which also suggests the importance of partial hybridization interaction for weak physisorption.