INDUCED INFRARED-ABSORPTION OF H2, HD, AND D2 PHYSISORBED ON NACL FILMS

Isotherms of H-2, HD, and D2 on annealed NaCl films in the temperature range 28 K to 33 K are reported with an isosteric heat of adsorption of -3.4+/-0.8 kJ/mole. Infrared spectroscopy of H-2, HD, and D2 induced by the strong electric fields at the NaCl film surface, reveals a number of features not reported before. Analysis of these spectroscopic features is consistent with molecule adsorption on defect sites and on smooth-face (100) surfaces of the film crystallites. The infrared bands of molecules adsorbed to smooth-face surfaces are red shifted by less than 1% from the gas phase fundamental vibrational frequencies for all hydrogen isotopes. A diffuse combination band is associated with the molecular tangential vibration along the surface. An analysis of both the thermodynamic and spectroscopic data with an extended Langmuir model of adsorption provides a consistent picture of the molecule physisorbed to the surface as a hindered rotor.