This paper considers several implications of the approximation that the total amplitude for photoemission from an oriented molecule is the sum of the amplitude of coherent emission from spherically symmetric regions at the atomic centers. We show that this approximation, which is increasingly valid at high energies (accessible, for example, to synchrotron-radiation sources) is readily calculable using tabulated initial-state atomic functions and simple solutions of the radial Schrödinger equation. We illustrate the fact that in special but important cases (e.g., oriented benzine), one can obtain the angular distribution of photoemission at fixed final energy without any recourse to atomic-photoemission-amplitude calculations. Finally, we emphasize that fact that the orthogonalized-plane-wave approach, which is a special case of the independent-atomic-center approximation, fails in two regards—neglect of atomic phase shifts, and the neglect of initial-state core-region wave-function behavior, which is crucial at high energies. This latter point is illustrated by comparing orthogonalized-plane-wave calculations using Slater atomic functions (which emphasize the bonding-region wave-function behavior), and hydrogenic wave functions, which can have radial nodes and core-region behavior which can more closely approximate the behavior of the true initial-state wave functions.

• Received 19 October 1977

DOI:https://doi.org/10.1103/PhysRevB.17.4573