Atomic dipole theory of photoemission from molecules adsorbed on surfaces

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Abstract
A theory for describing the angular dependence of the photoemission intensity emitted from small molecules adsorbed on single crystal surfaces is presented, in which the influence of the atomic potentials upon the excited electronic states is taken into account. It is assumed that the photoemission process is described by a coherent sum of spherical waves emanating from each of the atomic sites contributing to the initial state. The dependence of the photoemission intensity upon the direction of the electric field can be described in terms of a dipole vector associated with the symmetry of every atomic orbital in the initial state. The theory differs from those based upon a plane wave final state in that (i) the photoionisation process is governed by atomic-like dipole selection rules, and (ii) the phase of the outgoing wave from atom is perturbed by the atomic potential. Examples are discussed that illustrate how the details of the wavefunctions and structure of the molecules can be determined from the angle-resolved photoemission intensity.