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Photoionization unravels many interesting features in dynamic responses of
an atom to the electromagnetic fields. In this thesis, the
multiconfiguration relativistic iandom-phase approximation is applied to
the studies of photoionization of atoms with emphasis on double-electron
excitations and relativistic effects. We have chosen beryllium and
magnesium as cases for investigations. Photoionization calculations of
beryllium and magnesiumin in their ground states having 1So character are
perfomed. A three-configuration wavefunction
is adopted to decsribe the ground state, where n=2 and 3 for beryllium and
magnesium, respectively. For photon energies above the np3/2 ionization
threshold, the angular distribution and spin polarization of photoelectrons
from ns1/2,np1/2, and np3/2 subshells are obtained. In case of beryllium,
our results reveal that subshell cross sections near the threshold are
sensitive to the initial-state correlations arising from negative frequency
Feymann diagrams. In magnesium, large relativistic offects and very strong
channel interactions between 3p01(3s→εp)1p01(3p→εs)and 1p01(3p→εd)
are found in the energy region from the threshold to 0.8 a.u. Below the
np3/2 threshold, the npms 1p01 and npmd 1p01 Rydberg series of doubly
excited states manifest as autoionization resonances in the ns
photoionization cross section. The triplet 3pms 3p01 is resolved due to
gradual onset of the relativistic effects in magnesium. Theoretical
predications of the resonance positions are in excell ent agreement with
available experimental data.
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