STRUCTURE AND VIBRATIONS OF ADSORPTION COMPLEXES NH3 RU(001) - DENSITY-FUNCTIONAL MODEL CLUSTER STUDIES/

The bonding and the vibrations of isolated NH3 molecules adsorbed in t he on-top and the three-fold hollow positions of a Ru(001) surface hav e been studied with the help of a density functional model cluster met hod. The calculated vibrational frequencies and adsorption energies ha ve been compared to experimental data for the stronger bound alpha(1) state and for the weaker bound alpha(2) state of NH3 on Ru(001). The d ensity functional results support the assignment of the alpha(2) state to surface complexes with on-top located adsorbates. The calculated a dsorption energy is smaller for NH3 in the three-fold hollow than in t he on-top sites. This finding is at variance with a conceivable assign ment of the alpha(1) state to NH3 in hcp three-fold hollow sites of Ru (001).