Heterogeneous against homogeneous spectral response of polar molecules adsorbed on a real surface

We study the influence of point defects on the infrared response of polar molecules randomly deposited on a real ionic surface. When compared to the spectral response of the same species adsorbed on a perfect surface, the vibrational spectrum of physisorbed molecules perturbed by the defects displays shifts and broadenings characterizing the heterogeneity of the surface. This latter spectrum is expressed as a weighted sum of the spectral responses of individual molecules, each having specific shifts and broadenings which can be partitioned into (i) homogeneous contributions due to the interaction of the molecules with the perfect surface, (ii) defect contributions due to direct interaction with the defects, and (iii) mixed contributions due to the dynamical coupling between vibrational and external modes. Applications are carried out to the vibration of CO and to the ν1 and ν2 vibrations of NH3 molecules adsorbed on MgO(1 0 0) surfaces containing various concentrations of dipolar defects.