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A descriptive account is given of recent experimental and theo-retical studies of partial-channel photoionization cross sec-tions for production of parent molecular ions in specific finalelectronic states.Particular attention is focused on the va-lence-shell region of molecular nitrogen (-15 to 45eV),inwhich case channel coupling and final-ionic-state configuration-mixing effects are important.Time-dependent Hartree-Fock cal-culations reveal the presence of strong coupling between 3σ_→kσ_u and 1π_u→kπ_g ionization,in accordance with the familiarconfiguration-mixing between N→Vπ and N→Vσ intravalenceexcitations described by Mulliken.In this case it is found,however,that the σ→σ~* contribution appears above thresholdas a shape resonance in the photoionization continuum,whereasthe π→π~* contribution is in the discrete spectral region ofmolecular nitrogen.Calculations in the intensity borrowingand sudden approximation clarify the origins of the many ionicstates present in the inner-valence (2σ_g) region,where thereis a complete failure of the Koopmans approximation.Detailedcomparisons with recent synchrotron-radiation,(e,2e),and(e,e+ion) measurements indicate the present calculations ac-count satisfactorily for the experimental photoelectron spectraand pbotoionization cross sections.
A descriptive account is given of recent experimental and theo-retical studies of partial-channel photoionization cross sec-tions for production of parent molecular ions in specific finalelectronic states. Particular attention is focused on the va-lence-shell region of molecular nitrogen (- 15 to 45 eV), inwhich case channel coupling and final-ionic-state configuration-mixing effects are important.Time-dependent Hartree-Fock cal-culations reveal the presence of strong coupling between 3σ_ → kσ_u and 1π_u → kπ_g ionization, in accordance with the familiar configuration-mixing between N → Vπ and N → Vσ intravalenceexcitations described by Mulliken. in this case it is found, however, that the σ → σ ~ * contribution appears above thresholdas a shape resonance in the photoionization continuum, whereasthe π → π ~ * contribution is in the discrete spectral region of molecular nitrogen. Calculations in the intensity borrowing and sudden approximation clarify the origins of the many ionicstates presen where there is a complete failure of the Koopmans approximation. Detecting comparisons with recent synchrotron-radiation, (e, 2e), and (e, e + ion) measurements indicate the present calculations ac-count satisfactorily for the experimental photoelectron spectra and pbotoionization cross sections.