Computational Study of the Vibrational Structure of the Ammonia Molecule Adsorbed on the fcc (111) Transition Metal Surfaces

We have computationally studied adsorption and vibrational energy levels of the ammonia molecule adsorbed on the fcc (111) transition metal surfaces Ni(111), Cu(111), Rh(111), Pd(110, Ag(111), Ir(111), Pt(111), and Au(111). Vibrational Hamiltonians are obtained by combining an exact kinetic energy operator for the isolated ammonia molecule with plane-wave density functional theory (DFT) potential energy surfaces. The resulting eigenvalue problems are solved variationally. This procedure gives us the anharmonic vibrational energy levels of the adsorbed ammonia molecule. The local density of the states (LDOS) analysis reveals that ammonia adsorbs to all studied surfaces through its lone pair orbital. It makes the symmetric bending potential asymmetrical around the planar structure, quenches inversion splittings, and blue shifts the symmetric bend wavenumber, in agreement with experimental observations. In this work, it has been observed that the magnitude of this blue shift depends almost linearly on the adsorption energy. The asymmetric bend and the stretches red shift, which indicates loosening of the NH bonds upon adsorption.