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

Abstract

The adsorption of hydrogen azide HN 3 on NaCl(100) has been studied by means of polarization Fourier transform infrared spectroscopy PIRS and low energy 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 HN 3 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 quartet of strongly polarized sharp absorption lines for the asymmetric stretching vibration ν 2 at 2162.8, 2154.0, 2149.2, and 2138.3 cm −1 is obtained, the line at 2162.8 cm −1 being extremely weak in s-polarization. Saturation of adsorption, indicating the monolayer, is observed both upon dosing HN 3 in the low coverage range, where island growth is inferred, and desorbing HN 3 from multilayers. No domain orientation appears to be preferred. An activation energy of desorption of 43 ± 5 kJ/mol and a pre-exponential factor of 2.6 × 10 14±1.5 s −1 were determined.Isotopic mixture experiments with H 14 N 3 and H 14 N 15 N 14 N reveal that the four absorption lines of the ν 2 vibration are caused first by site splitting due to two energetically inequivalent HN 3 -species and second by Davydov splitting due to a strong correlation field between the dynamic dipoles. There are four molecules per adsorbate unit cell: two different pairs of energetically equivalent but translationally inequivalent molecules. Further, two different pairs of NH hydrogen bonds are observed. Tilt angles and intermolecular angles of the adsorbed molecules were determined. The translational symmetry of the adsorbate was derived by LEED. Simulation of the ν 2 quartet spectrum based on equations of classical electrodynamics was accomplished yielding the angles, lateral distances and dynamic vibrational couplings within the monolayer. Combining all results we propose a monolayer structure of angularly ordered zigzag chains of HN 3 molecules, where two molecules are linked by a hydrogen bond forming a dimer, while the other weaker hydrogen bond ties the dimers together or to a Cl − surface ion. Two HN 3 dimers make up the p(2 × 2) monolayer unit cell of 2D space group pg.