EVIDENCE FOR HYDROGEN-BONDING IN THE SYSTEM HBR/LIF(001)

The absorption of HBr on LiF(001) single crystal surfaces was studied as a function of coverage by means of Fourier-transform infrare (FTIR) spectroscopy at temperatures less-than-or-equal-to 83 K. Three different spectral features could be distinguished. The absorption of the first layer was found to be redshifted by approximately 300 cm(-1) with respect to gas-phase molecules, its fwhm being 265+/-25 cm-1. The absorption intensity was more than one order-of-magnitude enhanced over that of gas-phase molecules. From these results we conclude that first-layer HBr is hydrogen-bounded to F-ions of the surface, the angle between the molecular axis and the surface being 21+/-5-degrees determined from experiments employing polarized infrared. The second layer (and at low enough temperatures also higher layers) were approximately 100 cm(-1) redshifted with respect to the gasphase absorption, fwhm = 75+/-10 cm(-1). With increasing coverage the doublet absorption characteristic of solid orthorhombic HBr could be observed.