PHOTOELECTRON SPECTROSCOPIC STUDIES OF POLYATOMIC-MOLECULES - DETECTION-INTEGRATED CROSS-SECTIONS FOR IONIZATION IN FIXED TD SYSTEMS

We have, in this paper, derived expressions for the partial photocurrent produced by ionization in the electric dipole approximation in an orbital of an oriented polyatomic molecule. The cross-section formulas, which are integrated over all directions of ejection of the photoelectron but differential with respect to the direction of the fixed molecular axis, are obtained in their simplest possible forms by taking full account of the transformation properties of the point symmetry group of the target and are thus applicable to photoionization in any oriented molecule belonging to one of the 32 point groups. The theory, as an example, has been applied to photoionization in a 1 orbital of those oriented nonlinear systems whose point symmetry group is Td. This application shows that the singly differential, detection-integrated partial cross section for ionization by unpolarized, linearly or circularly polarized light in a1 orbital of a fixed Td molecule is (i) independent of the direction of its axis and (ii) equal to that averaged over all orientations of the target in space. Both conclusions are in agreement with recent experimental measurements on photoionization in 6a12 orbital of fixed CCl4. These results, in turn, are shown to mean that the variations with respect to the orientation of a molecule in space, found in theoretical calculations of photoelectron angular distribution for ionization in a1 valence orbitals of some of the Td targets, are due completely to the terms which stem from freezing both the molecular axis and the photoelectron detector in space. These terms completely vanish on integrating over all directions of propagation of the photoelectron, resulting in a current which is isotropic with respect to the orientation of the molecular axis in space. In such cases, it is therefore necessary to study the angular distribution of electrons ejected by photoionization in oriented molecules in order to obtain cross sections which will change with the direction of the target axis. © 1990 American Institute of Physics.