Abstract
We present a theory which relates the line shape of infrared absorbtion by chemisorbed atoms or diatomics to the dynamics of lattice vibration. We start from the relationship between the line shape and the thermodynamic Green’s function and use Dyson’s equation to compute the latter with the aid of many‐body perturbation theory. The derivation includes local modes and surface states as well as anharmonic interactions. Illustrative calculations show that a variety of line shapes are possible and we identify their dynamical origin.