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

We determine the profile of the vibrational band of an ordered monolayer ad sorbed on a clean surface corresponding to the infrared spectrum and to the resonant infrared-visible sum frequency generation spectrum. The theoretic al model is based on the renormalization of the monolayer and substrate Ham iltonians. The harmonic dynamics of the effective vibrons characterizing th e collective internal vibrations of the admolecules is written in terms of a complex dynamical matrix whose elements can be expressed as correlation f unctions of the external libron and phonon modes. The dephasing broadening is obtained by solving a master equation for the time evolution of the vibr on modes while the external dynamics of the layer is described by using mol ecular dynamics simulation. An application to the calculation of the profil e of the vibrational band of the low temperature (2 x 1) CO monolayer adsor bed on NaCl(100) is performed without any adjustable parameter by consideri ng a well established semiempirical potential to describe the molecule-surf ace and molecule-molecule interactions. The comparison with experimental sp ectra shows that the homogeneous phase relaxation due to the coupling betwe en 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 corro borates very well the temperature behavior of the monolayer structure and p rofile. (C) 1999 American Institute of Physics.