Abstract:
Using high-resolution electron energy loss spectroscopy we investigated the coverage-dependent vibrational properties of ammonia chemisorbed on Ru(001). With increasing coverage we find a drastic red shift of 100 cm(-1) for the ammonia symmetric deformation mode. Based on results for ammonia layers containing different isotopic species, such as NH3 as well as partially and totally deuterated NDxH3-x, this red shift can be identified as a dominantly static shift (no dynamical coupling). Based on DFT model calculations, the origin of this shift can be understood as a Stark shift due to the electric fields produced by the static NH, dipoles within the adlayer. Calculations for isolated ammonia and for ammonia adsorbed on small Ru clusters, both in the fields of neighboring electric dipoles, can well reproduce the sign and magnitude of the observed red shift and for this molecular system allow a clear distinction between pure electrostatic and adsorption-induced (chemical) effects. We show that as a consequence of the electric held, the ammonia molecule opens the H-N-H angle and a softening of the symmetric deformation mode occurs.