Journal of MOLECULAR STRUCTURE Journal of Molecular Structure 349 (1995) 69-72 .Neutron spectroscopy and Car-Parrinello simulation at adsorbates on magnesium oxide surfaces. Walter Lange1 Fachbereich Chemie, Universitat Siegen, D 57068 Siegen, Germany The spectra of dry MgO and of MgO covered with hydrogen bonded hydroxyl groups have been recorded by inelastic neutron scattering in an energy range up to 1000 meV. In contrast to IR absorption spectra the scattering function consists of a broad intensity extending from 10 to 800 meV. At the position of the O-H stretching vibration no distinct peaks are seen. The neutron spectrum of the hydrogen bond is dominated by multiphonon transitions of the proton which vibrates in phase with the MgO lattice phonons. This experimental results confirms a model emerging from recent Car-Parrinello simulations. There the characteristic shape of the O-H stretching band had been derived from adiabatic coupling of the hydrogen bond to the substrate phonons. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO 1. INTRODUCTION Surfaces of metal oxides are the subject of intense research, since these materials have important applications as high temperature ceramics and as catalysts. A prototype substance is magnesium oxide. The adsorption of water is of considerable interest for catalytic processes on its surface, which under standard conditions is covered with physi- and chemisorbed water. It is well known that above a certain coverage hydrogen bonding of the surface hydroxyl groups to a second 0 atom occurs [ 11. Hydrogen bonds of this type are found in such a number of systems that the treatment of their physical principles is of general interest. The characteristic feature of the corresponding infrared spectra is a broadened and red shifted O-H stretching band. The broadening indicates that a simple harmonic model will fail. Standard quantum chemical methods evaluate the total potential energy of the system considered as a function of the configuration and vibrational frequencies have to be calculated in the harmonic approximation from the curvatures of potential surfaces. These static methods do not allow to describe the dynamics of line broadening. For an adequate treatment the quantum chemical approach has to be combined with the simulation of the dynamics of the system. This is possible in the framework of the Car-Parrinello method [2]. It is currently assumed that low frequency modes which affect the distance of the two 0 atoms of the O-H..0 group [3] are at the origin of the line shape, The O-H stretching frequency strongly depends on the O..O distance and is modulated by its fluctuation. Recent simulations [4,5] directly evidenced this so called adiabatic coupling of the stretching mode of hydrogen bonded hydroxyl groups to the lattice phonons of the MgO substrate. However, in the small supercells employed for such calculations no realistic phonon distribution can be 0022-2860/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0022-2860(95)08711-7