The adsorption of ammonia on a Fe(110) single crystal surface studied by high resolution electron energy loss spectroscopy (EELS)

Abstract

EELS spectra of ammonia adsorbed on a Fe(110) single crystal surface at 120 K reveal four different molecular adsorption states: 1. At very low exposures (0.05 L) three vibrational losses at 345 cm −1, 1170 and 3310 cm −1 are observed which are attributed to the symmetric Fe-N stretching-, N-H 3 deformation and N-H 3 stretching modes of chemisorbed molecular ammonia, respectively. The observation of only three vibrational losses indicates an adsorption complex of high symmetry (C 3v). 2. Further exposures up to 0.5 L cause the appearance of additional losses at 1450 cm −1, 1640 cm −1 and 3370 cm −1. The latter two are interpreted as the degenerate NH 3 deformation and - stretching modes of molecularly adsorbed NH 3. The 1450 cm −1 loss is a combination of the losses at 345 cm −1 and 1105 cm −1. The observation of 5 vibrational losses is consistent with an adsorption complex of C s symmetry. 3. In the exposure range from 0.5 to 2 L adsorption of molecular ammonia in a second layer is observed. This phase is characterized by a symmetric deformation mode at 1190 cm −1 and by two additional very intense modes at 160 cm −1 and 350 cm −1 which are due to rotational and translational modes. 4. Exposures above 2 L cause multilayer condensation of ammonia characterized by translational and rotational bands at 190 cm −1, 415 cm −1 and 520 cm −1, and a symmetric deformation mode at 1090 cm −1. A broad loss feature around 3300 cm −1 is attributed to hydrogen bonding in the condensed layer. Thermal processing of a Fe(110) surface ammonia covered at 120 K leads to decomposition of the ammonia into hydrogen and nitrogen above 260 K. No vibrational modes due to adsorbed NH or HN 2 species were detected.