Two-vibron state dynamics in an anharmonic confined adlayer

The two-vibron dynamics of an anharmonic molecular monolayer confined between surface steps is investigated. Using the number states method, the equivalence between the two-vibron dynamics and the dynamics of a single fictitious particle moving in a three-dimensional lattice is established. This latter lattice contains local defects which characterize both the influence of the confinement and the effect of the anharmonicity, and give rise to localized states. The anharmonicity is responsible for the occurrence of localized two-vibron bound states, for which two quanta are located on the same molecule adsorbed close to the surface step. These states are more strongly localized than the corresponding single-vibron states. In addition, the sensitivity of the spectral response of the two-vibron states to the structure and the confinement size is demonstrated using a limited set of dynamical parameters (anharmonicity, lateral hopping constant, and internal vibration frequency). As a result, two-vibron spectroscopy appears as a powerful tool to investigate the growth of molecular monolayers adsorbed on stepped surfaces