Photoelectron spectroscopic studies of polyatomic molecules: Angular distributions for ionization in oriented T d systems

Publisher Website
Google Scholar
Abstract
A recently developed theory for angular distribution of electrons ejected by interaction of light with nonlinear molecules held fixed in space has been used to study ionization in a1 orbital of those systems which transform like the Td point symmetry group. Expressions for photoelectron angular distributions in the dipole approximation with the electric vector in the radiation beam both perpendicular and parallel to the molecular axis are derived. The properties of the two formulas, which are found to have completely different structures, have been studied. These expressions are shown to depend not only on all those quantities [i.e., the partial integrated cross‐section σ̄, the asymmetry parameter β̄, and the polar angle θ of the propagation vector k (k,θ,φ) of the ejected electron] which are present in photoionization of an unoriented molecule, but involve in certain cases also the azimuthal angle φ, phase shifts of the continuum waves representing the outgoing electron, and the phase of the dipole transition amplitudes. Such fixed‐molecule photoelectron angular distributions will therefore provide more stringent tests of theoretical models and probes of photoionization dynamics than the hitherto performed gas phase experiments on randomly oriented targets. They can also be used to identify the orientation of a molecule and/or geometry of a chemisorption site. The formulation presented here sets a frame work for the analysis of measurements and the calculations of spectra in those Td molecules which are fixed in space. We have applied it, as an example, to ionization in 4a1, 6a1, and 7a1 orbitals of oriented CF4, CCl4, and SiCl4, respectively. Without doing any dynamical calculations, using instead the experimentally measured values of σ̄ and β̄ as a function of the photon wavelength, the variations in the angular distributions with respect to the energy of the ejected electron as well as to the angles (θ,φ) and to the phases (which are treated as parameters) involved have been studied in detail. These distributions are found to have very rich and complicated structures arising from the spectral, angular, and/or phase variations of the angular momentum composition of the photocurrent.