Induced infrared absorption of H2, HD, and D2 physisorbed on NaCl films

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Abstract
Isotherms of H2, 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 H2, 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.