Effects of Electron Correlations on Photoionization and Multiphoton Ionization.

Photoionization processes in atoms are investigated with particular attention paid to the effects of electron correlations. Studies of electron correlations have been carried out in three different atoms: yttrium, argon and barium. Many-Body Perturbation Theory (MBPT) was used to calculate the photoionization cross section of the neutral yttrium atom from threshold at 6.52 eV to 100.00 eV. The resonance structure due to 4p excitations to 4d states was also calculated. The effects of electron correlations were found to be quite significant at low energies. MBPT was also used to calculate photoionization cross sections with excitation of the neutral argon atom from the single ionization threshold at 15.76 eV to 100.00 eV. Resonance structure due to both single electron excitations (3s to np) and double electron excitations (3s^23p^6 to 3s^23p ^4nm) was included. Calculated cross sections, leaving the ion in 3s^23p ^5, 3s3p^6, 3s^23p^4( ^1D)md(^2S) and 3s^23p^44p( ^2P) levels, the total cross section, and the angular asymmetry parameter for the 3p partial cross sections are compared with experiment and previous calculations. Calculated cross sections for 3s ^23p^6 to 3s3p^6epsilonp and 3s^23p^6 to 3s^23p ^4(^1rm D)md( ^2rm S)epsilonp, 3s^2 3p^44pepsilon d/epsilons were used to determine the satellite intensities, which were compared with photoelectron measurements and other calculations. The two photon ionization cross section for the neutral barium atom was also investigated. A double perturbation expansion in the Coulomb correlations and the atom-radiation field interaction was made, and the differential equation method was used for carrying out an infinite summation implicitly over the intermediate states. The effects of electron correlations were investigated.