Adsorption of hydrogen azide on the NaCl(100) single crystal surface. orientational and translational ordering of the HN3 monolayer as revealed by PIRS, LEED and spectra simulation

The adsorption of hydrogen azide HN3 on NaCl(100) has been studied by means of polarization Fourier transform infrared spectroscopy PIRS and low energ y electron diffraction LEED. The well-defined surface was prepared in-situ under UHV by cleaving a single crystal with as few dislocations as possible . Ordered molecular adsorption of HN3 has been observed. Multiple absorptions of all four molecular vibrations with frequencies between 4000 and 1000 cm (-1), all nondegenerate in the gas, are found at 120 K. In particular, a qu artet of strongly polarized sharp absorption lines for the asymmetric stret ching vibration v(2) at 2162.8, 2154.0, 2149.2, and 2138.3 cm(-1) is obtain ed, the line at 2162.8 cm(-1) being extremely weak in s-polarization. Satur ation of adsorption, indicating the monolayer, is observed both upon dosing HN3 in the low coverage range, where island growth is inferred, and desorb ing HN3 from multilayers. No domain orientation appears to be preferred. An activation energy of desorption of 43 +/- 5 kJ/mol and a pre-exponential f actor of 2.6 x 10(14 +/-1.5)s(-1) were determined. Isotopic mixture experiments with (HN3)-N-14 and (HNNN)-N-14-N-15-N-14 reve al that the four absorption lines of the v(2) vibration are caused first by site splitting due to two energetically inequivalent HN3-species and secon d by Davydov splitting due to a strong correlation field between the dynami c dipoles. There are four molecules per adsorbate unit cell: two different pairs of energetically equivalent but translationally inequivalent molecule s. Further, two different pairs of NH hydrogen bonds are observed. Tilt ang les and intermolecular angles of the adsorbed molecules were determined. Th e translational symmetry of the adsorbate was derived by LEED. Simulation o f the v, quartet spectrum based on equations of classical electrodynamics w as accomplished yielding the angles, lateral distances and dynamic vibratio nal couplings within the monolayer. Combining all results we propose a mono layer structure of angularly ordered zigzag chains of HN, molecules, where two molecules are linked by a hydrogen bond forming a dimer, while the othe r weaker hydrogen bond ties the dimers together or to a Cl- surface ion. Tw o HN3 dimers make up the p(2 x 2) monolayer unit cell of 2D space group pg.