Fourier transform infrared spectroscopy (FTIR) experiments together with interaction potential calculations are performed to interpret the adsorption–desorption properties of para and ortho H2 molecules physisorbed on the (001) faces of NaCl crystallites. Isotherms of the adsorption process show preferential adsorption of para-H2 from gaseous normal-H2 at surface temperature around 10 K. Isobars of the desorption process show preferential desorption of para-H2 from a mixed ortho-H2 and para-H2 adsorbate. A quantum description of the molecular motions corroborates preferential adsorption of p- and (M=±1) o-H2 on the cation sites, as proposed in previous classical studies. It is shown that the adsorption energy is smaller for the para than for the (M=±1) ortho species in agreement with experimental measurement of desorption energy, while (M=0) o-H2 is only slightly bound to the substrate. This indicates that the ionic surface can discriminate all the H2 species. The determination of the bar spectrum for a set of independently adsorbed H2 species can interpret most of the infrared signals assigned to adsorption on the terrace sites. More specially the experimental behavior, with temperature and coverage, of the frequencies and relative intensities of the pure vibrational peaks and of the translation–vibration combination bands is very well explained in terms of hindered external motions of the admolecule.

Articles you may be interested inThe structure of N2 adsorbed on the rumpled NaCl(100) surface-A combined LEED and DFT-D study J. Chem. Phys. 137, 174705 (2012); 10.1063/1.4764299The monolayer structure of CO 2 adsorbed on the NaCl(100) single crystal surface: A tensor low-energy electron diffraction analysis A two-dimensional quantum crystal: H 2 on NaCl (100) Experimental and theoretical studies of the monolayer structure of OCS adsorbed on NaCl(001): Coexistence of orientationally inequivalent phases Monolayers of pure ortho-H 2 , para-H 2 , para-D 2 and normal-H 2 adsorbed on NaCl͑001͒ single crystal surfaces were investigated at temperatures Tу10 K and pressures between 1ϫ10 Ϫ10 and 1ϫ10 Ϫ5 mbar by polarization Fourier transform infrared attenuated total internal reflection spectroscopy ͑PIRS-ATR͒. Single induced polarized infrared absorptions of the fundamental vibrational modes of pure (M ϭ͉1͉) ortho-H 2 and para-H 2 , as well as pure (M ϭ͉1͉) 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 Ϫ1 for pure (M ϭ͉1͉) 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 NaCl͑001͒ terraces, while neither the absorptions associated with H 2 adsorption above defects nor the absorptions of (M ϭ0) 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 ϭ͉1͉) ortho-, (M ϭ0) ortho-and paraspecies. 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 Ќ were measured and found to have the same order of magnitude: (6Ϯ3)ϫ10 Ϫ22 m/molecule for (M ϭ͉1͉) ortho-H 2 and (4Ϯ2)ϫ10 Ϫ22 m/molecule for para-H 2 within the error limits; the theoretical value is 4.5ϫ10 Ϫ22 m/molecule for both species. Applying the extended Langmuir equation, the difference between the desorption energies (E des ortho ϪE des para ) of (Mϭ͉1͉) ortho-H 2 and para-H 2 adsorbed on the ͑001͒ terraces of NaCl͑film͒ was determined to be 0.6Ϯ0.2 kJ/mol, in agreement with the calculated value ͑0.8 kJ/mol͒. Assuming that the sticking coefficients of (M ϭ͉1͉) ortho-H 2 and para-H 2 are not dependent on temperature between 17 and 30 K,...