Monolayers of ortho-H2, para-H2, para-D2 and normal-H2 adsorbed on NaCl(001) single crystal surfaces

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Abstract
Monolayers of pure ortho- H 2 , para- H 2 , para- D 2 and normal- H 2 adsorbed on NaCl(001) single crystal surfaces were investigated at temperatures T⩾10 K and pressures between 1×10 −10 and 1×10 −5 mbar by polarizationFourier transform infrared attenuated total internal reflectionspectroscopy (PIRS-ATR). Single induced polarized infrared absorptions of the fundamental vibrational modes of pure (M=|1|) ortho- H 2 and para- H 2 , as well as pure (M=|1|) para- D 2 and, slightly shifted, in the mixture of normal- H 2 were observed, allowing definite assignments of each. With respect to the gas phase values, the wave numbers were measured to be redshifted by 32.0 and 28.0 cm −1 for pure (M=|1|) ortho- H 2 and para- H 2 . In close agreement with potential calculations, these absorptions were assigned to H 2 molecules adsorbed on the cation sites of the large NaCl(001) terraces, while neither the absorptions associated with H 2 adsorption above defects nor the absorptions of (M=0) H 2 molecules above the anion sites, detected on microcrystallite surfaces at lower temperature, are observed under the conditions set. The present experiments corroborate the fact that the NaCl surface discriminates between the (M=|1|) ortho-, (M=0) ortho- and para- species. The lateral interactions have no influence on the stable sites and the discrimination, but they contribute by an additional shift of the peak frequencies of about 15 %. Moreover, with polarized light, induced transition dipole moments perpendicular to the surface for ortho- H 2 , para- H 2 and para- D 2 were observed, while none parallel to the surface was detected. These results are in accordance with the calculations showing that the s -component of the polarization disappears due to free rotation of the H 2 ( D 2 ) molecules in a plane parallel to the surface, while the p -component gives a maximum intensity. The integrated cross sections σ̄ ⊥ were measured and found to have the same order of magnitude: (6±3)×10 −22 m/molecule for (M=|1|) ortho- H 2 and (4±2)×10 −22 m/molecule for para- H 2 within the error limits; the theoretical value is 4.5×10 −22 m/molecule for both species. Applying the extended Langmuir equation, the difference between the desorption energies (E des ortho −E des para ) of (M=|1|) ortho- H 2 and para- H 2 adsorbed on the (001) terraces of NaCl(film) was determined to be 0.6±0.2 kJ/mol, in agreement with the calculated value (0.8 kJ/mol). Assuming that the sticking coefficients of (M=|1|) ortho- H 2 and para- H 2 are not dependent on temperature between 17 and 30 K, the ratio s ortho /s para of 0.06±0.03 was obtained.