Theory of angle-resolved photoemission from the bulk bands of solids. I. Formalism

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Abstract
A theory for describing the angle-resolved photoemission spectra from the bulk bands of solids is developed which views the photoemission process as a single-step coherent emission of electrons from atomic sources into a final state of well-defined momentum. The final state is assumed to be free-electron-like except in the region near the strong atomic potential, where it is distorted into a spherical wave. While the momentum conservation, transport, and escape processes are free-electron-like in nature, the optical ionization process is governed by atomiclike dipole selection rules. The dependence of the peak intensities of the angle-resolved photoemission spectra upon the direction of the polarization of the electric field can be used to determine the orbital composition of the initial states in a very simple manner. The simple form of the theory presented here can be extended to materials having a hybridized final state by the use of a pseudopotential wave function modified by atomic-dipole selection rules. It is argued, however, that the single-unhybridized-final-state wave function is approximately valid for describing the angle-resolved photoemission spectra of all materials, and that the one-dimensional density-of-states model is probably not valid for any material with a lattice constant smaller than the mean free path. An experimental geometry is proposed in which the photoemission intensity is proportional to the charge density of the initial state in the direction of the final-state momentum, similar to the results of previous theories based upon the plane-wave final state.