Infrared and infrared-visible sum frequency generation spectroscopic response of harmonic monolayer vibrons: Homogeneous profile

Abstract

We determine the profile of the vibrational band of an ordered monolayer adsorbed on a clean surface corresponding to the infrared spectrum and to the resonant infrared-visible sum frequency generation spectrum. The theoretical model is based on the renormalization of the monolayer and substrate Hamiltonians. The harmonic dynamics of the effective vibrons characterizing the collective internal vibrations of the admolecules is written in terms of a complex dynamical matrix whose elements can be expressed as correlation functions of the external libron and phonon modes. The dephasing broadening is obtained by solving a master equation for the time evolution of the vibron modes while the external dynamics of the layer is described by using molecular dynamics simulation. An application to the calculation of the profile of the vibrational band of the low temperature (2×1) CO monolayer adsorbed on NaCl(100) is performed without any adjustable parameter by considering a well established semiempirical potential to describe the molecule-surface and molecule-molecule interactions. The comparison with experimental spectra shows that the homogeneous phase relaxation due to the coupling between the vibrons and the monolayer phonons-librons accounts for the most part of the peak width and interprets the temperature dependence of this width. Extension of the calculations to the interpretation of the orientationally disordered phase obtained for CO at higher temperature is done which corroborates very well the temperature behavior of the monolayer structure and profile.