The Schwinger Variational Principle Applied to Molecular Photoionization

We have developed a method based upon the Schwinger Variational Principle to study molecular photoionization and electron molecule scattering. We obtain exact static-exchange solutions to the equations for the continuum orbitals within the Hartree-Fock approximation, and from these we derive cross sections and angular distributions for both of the above processes. We have applied this method to photoionization of the valence levels of three different systems. The first application of this method is in a study of the photoionization of the valence levels of NO. For photoionization of the 5σ level we find that the magnitudes of the cross sections for the resulting b3Π and A1Π states of NO+ are not in the statistical 3:1 ratio and that the position of the peaks in the shape resonances appear at significantly different photoelectron kinetic energies. The differences are a reflection of the sensitivity of the shape resonances to small changes in the exchange component of the molecular ion potential. We also examined photoionization of the 2π and 4σ levels of NO and found that these levels, along with the 5σ, exhibit shape resonances in the σ continuum. Vibrationally resolved branching ratios and vibrational state-specific asymmetry parameters for photoionization of the 5σ level of CO are presented. The agreement between these results and the measured data was obscured by autoionization peaks superimposed upon the region of the spectrum in which the experiment was performed. The limited experimental data which are not obscured by autoionization, agree quite well with our results. Finally, a study of the photoionization of the 5σ level of CO adsorbed on a nickel surface is reported. Approximating this system by the linear triatomic molecule NiCO leads to cross sections and angular distributions which are in good agreement with experimental data. The use of polarized, highly tunable synchrotron radiation along with detection at various angles yields far more information than can be obtained from the photoionization of the same molecule in the gas phase. With this technique it is possible to determine the adsorbate geometry, resolve partial channels, and resolve the photoemisison from two degenerate orbitals of different symmetries, simply by making appropriate choice of the polarization and collection angles. Examples of these techniques are discussed.