We use multiple scattering photoelectron diffraction theory to calculate the angular distribution of electrons photoemitted from core levels of fixed-in-space molecules. Non-spherical scattering potentials, needed for low photoelectron energies (E<50 eV), are included in our formalism through non-diagonal scattering matrices; these fit directly into a new approach for multiple scattering theory originally based on spherical potentials. When the kinetic energy of the photoelectron is low its scattering cannot be adequately represented by spherically-symmetric potentials. In addition, we have considered the effect of the final-state hole, and found this to be important as well. As an example, we calculate the angular distribution of photoelectrons emitted from the K shells of C in oriented gas-phase CO molecules, as recently measured by several groups. Particularly important are the energies for which shape resonances appear in the continuum, with the angular distributions showing radical changes over such resonances.

This work was supported in part by the Director, Office of Science, Office of Basic Energy Sciences and Division of Materials Sciences under US Department of Energy Contract No. DE-AC03- 76SF00098. RDM acknowledges financial support by the Basque Government (Programa de Formación de Investigadores del Departamento de Educación, Universidades e Investigación). DR acknowledges help and support from the German Fulbright Commission. FJGA acknowledges partial support by the Spanish Ministerio de Educación y Ciencia (Ful-bright grant FU-98-22726216). We acknowledge the support of NSF (contract INT-9815358) and DAAD (contract 315/PPP) for international cooperation, and NERSC of US DOE for supercomputing.

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