INFRARED-SPECTROSCOPY OF HX (X = BR,CL) ADSORBED ON LIF(001) - ALIGNMENT AND ORIENTATION

The infrared spectra of HBr and HCl on LiF (001) single crystal surfaces were measured as a function of coverage at temperatures less-than-or-equal-to 83 K using Fourier-transform infrared (FTIR) spectroscopy. For each hydrogen halide three different spectral features could be distinguished. At low coverages broad absorptions centered at 2265 +/- 20 cm-1 (HBr) and at 2515 +/- 20 cm-1 (HCl) were observed. These absorptions were attributed to molecules hydrogen-bonded to F- anions of the surface, the angle between the molecular axis and the surface being 21 +/- 5-degrees for HBr and 19 +/- 5-degrees for HCl as determined from experiments employing polarized infrared radiation. Hydrogen bonding was evidenced by: (i) redshifts with respect to the gas phase (approximately 300 cm-1), (ii) broad infrared absorption (FWHM: 265 +/- 25 cm-1 for HBr, 295 +/- 15 cm-1 for HCl), and (iii) enhancement of the infrared absorption intensity compared to the gas phase by more than one order of magnitude for both HBr and HCl. With increasing coverage a second absorption was observed before the first one saturated (HBr:v = 2461 +/- 5 cm-1, FWHM = 75 +/- 10 cm-1; HCl:v = 2763 +/- 5 cm-1, FWHM = 80 +/- 10 cm-1). This absorption was attributed to molecules adsorbed in a second layer. The smaller redshift and spectral width for the second layer were consistent with weaker hydrogen bonding, probably to the halogen of molecules absorbed in the first layer. Further increase in coverage resulted in the appearance of the well-known doublet absorptions due to formation of solid. Coadsorption of HBr and HCl, as well as experiments under adsorption-desorption equilibrium conditions, confirmed that the first and second layers could coexist. The isotherms could best be understood on the assumption of a repulsive interaction within the first layer.