On the angular distributions of electrons photoejected from fixed-in-spaceand randomly oriented molecules

New theoretical expressions are devised from a dynamical perspective for mo lecular photoionization cross sections differential in electron ejection an gles which facilitate comparisons between theory and experiment and provide a convenient basis for ab initio calculations. The cross sections obtained for fixed-in-space molecules, including the lowest-order (nondipole) effec ts of retardation, are given in terms of invariant molecular body-frame tra nsition moments and related normalized angular-distribution amplitudes whic h can be calculated employing interaction-prepared states without reference to specific scattering boundary conditions. Corresponding expressions for molecular dipole and nondipole anisotropy factors appropriate for randomly oriented molecules are obtained in closed forms involving expectation value s of harmonic polynomials over the fixed-in-space body-frame angular-distri bution amplitudes. The expressions are seen to be in the spirit of correspo nding results for atomic photoionization anisotropy factors, to which they reduce in appropriate limits. Interpretations of recently measured angular distributions of photoelectrons from the K-shell of molecular nitrogen illu strate how the development relates measurements on randomly oriented molecu les to those performed on fixed-in-space molecules. The theoretical formali sm provides results in excellent accord with measurements of the molecular nitrogen K-shell dipole anisotropy factor, and accounts for the origins of large nondipole effects observed at relatively low photon energies ((h) ove r bar omega 5000 less than or equal to eV) in the measured angular distribu tions. (C) 2001 Elsevier Science B.V. All rights reserved.