Relative photoionization cross sections for O + and O 2+ produced by the Auger decay of a 1s hole in atomic oxygen were measured by using synchrotron radiation between 525 and 553 eV. Energies and quantum defects of the members of the two Rydberg series converging to [1s]2s 2 2p 4 ( 4 P) and [1s]2s 2 2p 4 ( 2 P) ionization thresholds were determined. In addition, the 2 P and 4 P ionization thresholds were calculated from the two Rydberg series. The 182 meV resolution of the monochromator allowed a detailed study over both thresholds revealing evidence for post-collision interaction and allowing a comparison of the ionization continuum above both 2 P and 4 P thresholds with that of the ionization continuum above the Ar L 2-3 edges. This comparison indicates that the lifetimes of the Ar(2p) and O(1s) hole states are approximately the same.

The independent particle approximation is shown to break down for the photoionization of both inner and outer nᐉ ͑ᐉ . 0͒ electrons of all atoms, at high enough energy, owing to interchannel interactions with the nearby ns photoionization channels. The effect is illustrated for Ne 2p in the 1 keV photon energy range through a comparison of theory and experiment. The implications for x-ray photoelectron spectroscopy of molecules and condensed matter are discussed. [S0031-9007 (97)03382-6] PACS numbers: 32.80.Fb

Direct measurements of Ar^{+} 1s^{-1}2p^{-1}nl double-core-hole shake-up states are reported using conventional single-channel photoemission, offering a new and relatively easy means to study such species. The high-quality results yield accurate energies and lifetimes of the double-core-hole states. Their photoemission spectrum also can be likened to 1s absorption of an exotic argon ion with a 2p core vacancy, providing new information about the spectroscopy of both this unusual ionic state as well as the neutral atom.

A gas-phase time-of-flight ͑TOF͒ apparatus, capable of supporting as many as six electron-TOF analyzers viewing the same interaction region, has been developed to measure energy-and angle-resolved electrons with kinetic energies up to 5 keV. Each analyzer includes a newly designed lens system that can retard electrons to about 2% of their initial kinetic energy without significant loss of transmission; the analyzers can thus achieve a resolving power (E/⌬E) greater than 10 4 over a wide kinetic-energy range. Such high resolving power is comparable to the photon energy resolution of state-of-the-art synchrotron-radiation beamlines in the soft x-ray range, opening the TOF technique to numerous high-resolution applications. In addition, the angular placement of the analyzers, by design, permits detailed studies of nondipolar angular distribution effects in gas-phase photoemission.

Anion-yield spectroscopy using x rays is shown to be a selective probe of molecular core-level processes, providing unique experimental verification of shape resonances. For CO, partial anion and cation yields are presented for photon energies near the C K edge. The O- yield exhibits features above threshold related only to doubly excited states, in contrast to cation yields which also exhibit pronounced structure due to the well-known sigma* shape resonance. Because the shape resonance is completely suppressed for O-, anion spectroscopy thus constitutes a highly selective probe, yielding information unobtainable with absorption or electron spectroscopy.

Measurements of angular distributions of K-shell electrons photoejected from molecular nitrogen are reported which reveal large deviations at relatively low photon energies (Planck's omega < or = 500 eV) from emission patterns anticipated from the dipole approximation to interactions between radiation and matter. A concomitant theoretical analysis incorporating the effects of electromagnetic retardation attributes the observed large nondipole behaviors in N2 to bond-length-dependent terms in the E1 [multiply sign in circle] (E2,M1) photoelectron emission amplitudes which are indicative of a potentially universal nondipole behavior in molecular photoionization.

Second-order [ O(k(2)), k = omega/c] nondipole effects in soft-x-ray photoemission are demonstrated via an experimental and a theoretical study of angular distributions of neon valence photoelectrons in the 100-1200 eV photon-energy range. A newly derived theoretical expression for nondipolar angular distributions characterizes the second-order effects using four new parameters with primary contributions from pure-quadrupole and octupole-dipole interference terms. Independent-particle calculations of these parameters account for a significant portion of the existing discrepancy between experiment and theory for Ne 2p first-order nondipole parameters.

Electron spectroscopy, combined with synchrotron radiation in the photon-energy range hv=60 -190 eV, was used to measure the angular distributions of Xe 5p and 5s photoelectrons and of N4 500 Auger electrons. The branching ratios and partial cross sections for photoionization and Auger processes were also measured in certain cases. The measured asymmetry parameter P for Sp photoelectrons agrees well with many-electron calculations, which predict a pronounced oscillation in P5p above the 4d ionization threshold due to the 4d-5p intershell interaction. The N4 qOO Auger electrons are produced with photon-energy-dependent anisotropic angular distributions, resulting from alignment of Xe by photoionization. The theoretical analysis of Auger-electron angular distributions is described, and theoretical calculations are found to predict the measured asymmetries well. In addition, Auger-electron peaks were observed to broaden and shift at photon energies near the 4d ionization threshold because of postcollision interaction. The measured shifts of the N~0 lO' So line agree with previous measurements and theory. Electron spectra recorded through the energy region of the 4d~np Rydberg states show that they decay primarily by the Auger process, while the 5p and 5s partial cross sections are relatively weakly affected by autoionization. However, distinct resonance structure was observed in the P parameters for Sp3/2 and Sp~q2 photoelectrons. The measured results are compared with a theoretical calculation of resonant photoionization for the 4d~6p excitation.