Identification of a vibrational Stark Shift within an adsorbate layer: NH3on Ru(001)

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 t he 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 dominant ly static shift (no dynamical coupling). Based on DFT model calculations, t he origin of this shift can be understood as a Stark shift due to the elect ric fields produced by the static NH, dipoles within the adlayer. Calculati ons for isolated ammonia and for ammonia adsorbed on small Ru clusters, bot h in the fields of neighboring electric dipoles, can well reproduce the sig n and magnitude of the observed red shift and for this molecular system all ow a clear distinction between pure electrostatic and adsorption-induced (c hemical) effects. We show that as a consequence of the electric held, the a mmonia molecule opens the H-N-H angle and a softening of the symmetric defo rmation mode occurs. (C) 1999 Elsevier Science B.V. All rights reserved.