Resonant photoelectron spectra were measured in the photon energy range of Xe 4d to np excitations to study multielectron processes associated with the decay of these resonances. The results show a large fraction of shake-off electrons in the total decay rate and indicate an increase of this fraction with increasing principal quantum number n of the photoexcited electron. Higher resolution measurements reveal that this increase accompanies a decrease in the corresponding probability for shake-up transitions. The consequences of this behaviour regarding double excitations and double Auger decay above the 4d ionisation threshold are discussed.

Partial cross sections and angular-distribution asymmetry parameters were measured for subshell photoionization of xenon for photon energies between 40 and 1000 eV. These large-scale measurements show that the pronounced interchannel coupling between the valence and the 4d electrons persists beyond the 4d shape resonance in the subsequent Cooper-minimum region. Multielectron processes associated with 4d and 4p photoemission were measured directly for the first time over a broader energy range covering the near-threshold behavior up to the sudden limit. Comparing our experimental results with calculations based on the single-particle model shows that this theory, which fails to describe the intermediate energy range even qualitatively for the valence electrons, gives partial cross sections in reasonable agreement with experiment at higher photon energies, particularly beyond the 3d threshold. The same result is shown by the angular-distribution asymmetry parameter \ensuremath{\beta}, except for the photoionization of the ``4p'' subshell which resembles more the behavior of a 4d electron, corroborating the theoretical assumptions of core-hole fluctuations between these two subshells. In the shape resonance region the presented 4d partial cross sections are in reasonable agreement with theoretical results obtained recently by many-body perturbation theory.

Photoelectron spectra and zero-volt electron scans were measured following synchrotron-radiation excitation of argon, to elucidate the photon energy range between threshold and the Cooper-minimum region of the Ar 3s photoline. A number of the ``correlation satellite'' lines show a dramatic increase of fractional intensity, yielding a total of about forty observed photolines at threshold. This effect is interpreted as arising from strong interchannel coupling near threshold, yielding high--angular-momentum ionic states otherwise inaccessible. The distinction between ``intrinsic'' correlation effects due to discrete states and ``dynamic'' correlations arising through the photoemission process is emphasized.

Partial cross sections and angular-distribution asymmetry parameters were measured for Xe 4d and 3d subshell photoionization using photon energies of 250 to 1000 eV. Effects of interchannel coupling are observed at the onset of the 3d cross section. Outside this energy region our results agree qualitatively with the predictions of the Hartree-Fock theory if one takes into account al1 photoelectrons associated with a given subshell including multielectron processes.

Core level photoemission spectra of CO and N2 using snyhrotron radiation excitation have been measured in order to study the near threshold photon energy range of the K-shell satellites. The C 1s and N 1s π- π* satellites show very similar behavior characterized by increasing intensity of the first satellite and decreasing intensity of the second satellite towards threshold. In contrast to this characteristic behavior there is nothing comparable in the O 1s satellite spectrum. The results are discussed on the basis of the equivalent core model, potential theory and charge transfer effects.

Synchrotron radiation was used to excite an inner-shell electron into a Rydberg orbital at the Ar 2p to ns, nd, Kr 3d to np and Xe 4d to n p resonances. The resonant decay into shake-off channels was studied by three different electron measurements. Firstly, threshold electron scans were obtained over the resonances and thresholds. On the first resonance for each atom, photoelectron spectra were collected. The intensity distribution of low kinetic energy electrons was also determined for a few resonances. Finally, a shake calculation was carried out to compare with the experimental shake-off probabilities. Shake-off is observed to be a strong decay channel for these resonances.

The cross sections and asymmetry parameters of the two $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}^{2}\ensuremath{\Sigma}^{+}$ shakeup satellites on $\mathrm{C} 1s$ in the photoelectron spectrum of CO are found to differ significantly in the threshold region. The first satellite (due to triplet $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ coupling) shows intensity enhancement near threshold whereas the second satellite (singlet $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ coupling) decreases in intensity as expected for the adiabatic limit. Moreover, the data indicate that a shape resonance in the first-satellite channel is responsible for the observed effect rather than a conjugate shakeup process.

Photoionization of helium and neon to excited satellite states, ${\mathrm{He}}^{+}$ nl and ${\mathrm{Ne}}^{+}$ 1${s}^{2}$2${s}^{2}$2${p}^{4}$nl, was studied with synchrotron radiation and threshold electron analysis. Photoelectron satellites have been directly measured at threshold for the first time to our knowledge. The relative satellite cross sections were determined over the kinetic energy range from 0 to 1 eV. The angular distributions were also evaluated close to threshold. Strong correlation effects were observed in two cases. For He near threshold, the angular-distribution asymmetry parameter \ensuremath{\beta} is near zero for the n=2 satellite and is increasingly negative for the higher-n satellites, in agreement with the theoretical prediction of Greene. In the threshold photoelectron spectrum of Ne, many final states are present, some with quartet spin multiplicity and others with high-L values.

Photoelectron spectra of the neon valence satellites have been measured in their near-threshold region. Strong resonance enhancement of the satellite cross section is observed as a result of autoionization of doubly excited Rydberg states. The nonresonant cross section of most satellites follows approximately that of the ${2}_{p}$ main line as close as 0.5 eV above threshold. The resonant behavior is explained with respect to correlation effects showing the importance of interchannel coupling for the satellite production. The characteristic differences in the satellite threshold behavior are used to distinguish between relaxation effects and other electron correlations.

We report the first observation of electron-ejection asymmetries due to molecular orientation following $K$-shell photoexcitation in a free molecule. The angular distributions of electrons emitted by the decay of $K$-shell vacancies were measured in the vicinity of the ${\ensuremath{\pi}}^{*}$ discrete and the ${\ensuremath{\sigma}}^{*}$ shape resonance of CO. We see a sharp drop of the angular distribution asymmetry parameter to $\ensuremath{\beta}=\ensuremath{-}1$ at the ${\ensuremath{\pi}}^{*}$ resonance as predicted by theory, but no comparable effect across the ${\ensuremath{\sigma}}^{*}$ shape resonance. The results are discussed with respect to the intrinsic anisotropy of the $K$-shell vacancies.