Monolayers of ortho-H-2, para-H-2, para-D-2 and normal-H-2 adsorbed on NaCl(001) single crystal surfaces

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 grea ter than or equal to 10 K and pressures between 1X10(-10) and 1X10(-5) mbar by polarization Fourier transform infrared attenuated total internal refle ction spectroscopy (PIRS-ATR). Single induced polarized infrared absorption s 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 re spect to the gas phase values, the wave numbers were measured to be redshif ted by 32.0 and 28.0 cm(-1) for pure (M=\1\) ortho-H-2 and para-H-2. In clo se agreement with potential calculations, these absorptions were assigned t o H-2 molecules adsorbed on the cation sites of the large NaCl(001) terrace s, while neither the absorptions associated with H-2 adsorption above defec ts nor the absorptions of (M=0) H-2 molecules above the anion sites, detect ed on microcrystallite surfaces at lower temperature, are observed under th e conditions set. The present experiments corroborate the fact that the NaC l surface discriminates between the (M=\1\) ortho-, (M=0) ortho- and para-s pecies. 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 transitio n dipole moments perpendicular to the surface for ortho-H-2, para-H-2 and p ara-D-2 were observed, while none parallel to the surface was detected. The se results are in accordance with the calculations showing that the s-compo nent of the polarization disappears due to free rotation of the H-2 (D-2) m olecules in a plane parallel to the surface, while the p-component gives a maximum intensity. The integrated cross sections <(sigma)over bar>(perpendi cular to) were measured and found to have the same order of magnitude: (6+/ -3)X10(-22) m/molecule for (M=\1\) ortho-H-2 and (4+/-2)X10(-22) m/molecule for para-H-2 within the error limits; the theoretical value is 4.5X10(-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\1) ortho-H-2 and para-H-2 adsorbed on the (001) terraces of NaCl(film) w as determined to be 0.6+/-0.2 kJ/mol, in agreement with the calculated valu e (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 ra tio s(ortho)/s(para) of 0.06+/-0.03 was obtained. (C) 1999 American Institu te of Physics. [S0021-9606(99)70605-7].