G B Armen

Valence Auger and X-ray participator and spectator processes for neon and argon atoms

The rates of decay, both radiative and non-radiative, have been calculated for the valence shake-up states (vs, vp)nl of Ne(v = 2) and Ar(v = 3) for the five lowest n values. The various decay channels are discussed and trends for large n are examined. The calculations are performed at the single-configuration non-relativistic Hartree-Fock level. Inner-valence and valence-multiplet Auger channels are shown to be completely dominant, with maximum fluorescence yields of 2.7% in Ne and 0.2% in Ar. The total decay rates can be larger than the corresponding atomic K-shell rates for certain states, an extreme example being the Ar (3s, 3p)(1P)3d(2P) rate of 0.148 au. The calculated widths are shown to be consistent with the experimental results of Svensson et al (1988). The valence-multiplet participator Auger transition is a mode of decay involving a change of multiplet state for the same initial and final two-hole valence configuration, and has been shown to account for the difference in width between certain photoelectron satellites in Ne. The experimental spectrum of Becker et al (1989) is discussed in the light of present results. The valence decay mechanisms identified are expected to have general applicability to light atoms and to molecules containing first or second row atoms.

Calculation of the shake-up satellites in the 1s and 2s X-ray photoelectron spectra of neon

The intensities of satellites accompanying photoionization of Ne 1s and 2s shells and the total spectral functions were calculated using second-order perturbation theory. The problem of a suitable choice of potential for the calculation of satellites was investigated. The theoretical intensities of satellites are in agreement with experiment for the satellites 1s(2s to ns) and for the satellites 2s(2p to np). It was shown that the presence of the underlying continuum from another channel broadens asymmetrically the lineshapes of discrete satellites. These lineshapes can be calculated by the spectral function method in second order perturbation theory. The interchannel interactions which are represented by higher order corrections may result in further modifications of these lineshapes.

Quantum theory of post-collision interaction in inner-shell photoionization

We have performed fully quantum-mechanical and relativistic calculations of the post-collision-interaction effect in x-ray-induced argonK−L2L3(1D) Auger and xenonL2−L3N4(J=3) Coster-Kronig-electron emission. The Dirac-Fock computations include a complete integration over intermediate one-hole continuum states in the lowest-order expression of the resonant double-photoionization cross section. The results are in excellent agreement with synchrotron-radiation measurements of the post-collision-interaction shifts. We have also made nonrelativistic Hartree-Fock test calculations of the argonK−L2L3(1D) and xenonL3−M4M5(1G4) Auger-electron line shapes. The quantum-mechanical results are compared with rigorous semiclassical calculations which have been made without using the stationary-phase approximation. The results of this computational analysis are interpreted in terms of an analytical line-shape formula based on asymptotic Coulomb wave functions. As a consequence the most salient features of the post-collision interaction in inner-shell photoionization are explained.

Spectator excitation accompanying core Auger decay in atoms: a hydrogenic model

The response of an electron in a hydrogenic nl-orbital to a sudden change in central charge is investigated. This system is taken as a model for the interaction encountered by a Rydberg (spectator) electron when the core atomic system undergoes Auger decay. The most likely outcome is found to be a shake-up to final-state orbitals of quantum number . Calculations are carried out for values of n up to 50. The probabilities for spectator shake-up, shake-down, and no-shake tend to constants for larger n, and are found to be 90.3%, 8.7%, and 1.0% respectively for the Z = 1 case. The probability of shake-off is found to be small, less than 0.3% for low n, and decreasing as for larger n. The trends found in the hydrogenic case are shown to be in close agreement with Hartree - Fock calculations for the spectator Auger transitions .

Linewidths and intensities of satellites in photoelectron spectra in the presence of an underlying continuum

The theory of lineshapes of satellites in photoelectron spectra in a simplified fourth-order perturbation theory is developed. The calculation of the monopole satellites of the 2p ionization of the neon atom has been carried out in second- and simplified fourth-order perturbation theory. The theoretical linewidths and the relative intensities of the satellites are in agreement with experimental photoelectron spectra taken at h nu =130.1 eV.

Photoionization cross-section calculations for atomic lithium

Calculations based upon relaxed Hartree-Fock functions for the 1s diagram and satellite photoionization cross sections for atomic lithium are presented and compared with current experimental results. Configuration interaction in the initial and final ionic-core states is included in the calculation. While the results for the 1S and 3S multiplets are promising the present theory is shown to be limited in providing accurate cross sections for the 1P and 3P multiplets related to conjugate processes. The existence of continuum configuration interaction is demonstrated to be likely, and the influence of the lithium optical final state is shown to be of importance.

Interchannel interaction in the K-shell photoionization of lithium

An extension of previous single-channel calculations is presented for the photoionization cross sections of Li 1s diagram and conjugate shake-up satellite lines. The extension includes the effects of interchannel interaction to first order in the channel-channel interaction matrix. The calculations demonstrate the large role played by continuum configuration interaction in the problem. Interchannel interaction is shown to transfer intensity from the diagram channels into the conjugate shake-up channels. In particular the 1s2p(3P) conjugate shake-up state, which was previously predicted to have no intensity, is now calculated to have an intensity as large as 28% that of the 1s2s(3S) diagram transition. The present results are discussed with reference to current experimental results and contrasted to the previous calculations.

Atomic Inner‐Shell Threshold Excitation with Synchrotron Radiation

Monochromatized synchrotron radiation from a focused 8‐pole wiggler beam line has been used to excite the L2 and L3 holes of atomic Xe and the K hole of atomic Ar near their photoionization thresholds. We have measured the Xe L3‐M4M5 lpar;1G4rpar; Auger line, the Xe L2−L3N45 Coster‐Kronig line, and the Ar K−L23L23 Auger line with incident x‐ray energies from ∼20 eV below threshold to ∼200 eV above threshold. The spectra allow accurate determination of spectator‐ satellite energy shifts, postvcollision interaction (PCI) shifts in the diagram‐line energy, and spectral changes associated with the resonant Raman Auger (RRA) effect. The energies of satellites corresponding to photoelectrons promoted to bound states exhibit linear dispersion as a function of the initial photon energy. The measured post‐collision interaction energy shifts of the diagram lines agree qualitatively with semiclassical PCI theory above threshold. In all cases, the energy of the diagram line is observed to decrease below threshold. Th...

Double K‐Vacancy Production by X‐Ray Photoionization

We have studied double K-shell photoionization of Ne and Mo (Z = 10 and 42) at the Advanced Photon Source. Double K-vacancy production in Ne was observed by recording the KK-KLL Auger hypersatellite spectrum. Comparison is made with calculations using the multiconfiguration Dirac-Fock method. For Mo, double K-vacancy production was observed by recording the K{alpha}, {beta} fluorescence hypersatellite and satellite x rays in coincidence. From the intensities of the Auger or x-ray hypersatellites relative to diagram lines, the probabilities for double K-vacancy production relative to single K-vacancies were determined. These results, along with reported measurements on other atoms, are compared with Z-scaling calculations of the high-energy limits of the double-to-single K-shell photoionization ratio.

Interpretation of X-ray emission from lithium-like ions in collisions with helium

We consider the continuous X-ray distribution on the low-energy side of the Kalpha line in projectile spectra coincident with single-electron loss in collisions of lithium-like ions with helium. We demonstrate that the observed distributions are due to two-photon emission rather than to the radiative Auger effect.