The vibrational structure in the carbon $1s$ photoelectron spectrum of ethene has been measured with a total experimental resolution of about 55 meV, significantly better than has been previously reported. The spectrum has been calculated using ab initio electronic structure theory with a localized core hole and the equivalent-cores approximation. Agreement between the calculated spectrum and measurements (taken 29 eV above the carbon $1s$ threshold) is excellent. This agreement indicates that there is dynamic localization of the core hole, even though the $1{\sigma }_g-1\mathrm{}{\sigma }_u$ splitting is not small. The theoretical method provides an approach that is useful for analyzing such spectra and also shows some of the pitfalls of attempting to fit complex vibrational spectra without guidance from theory.

A treatment of continuum states in the application of diagrammatic perturbation theory to calculate the signal produced in two-color resonant, four-wave mixing (TC-RFWM) spectroscopy is developed. The third-order susceptibility is significantly modified from that obtained when considering only discrete states. To illustrate the contribution of continuum states, the line profile of a quasibound resonance arising from the configuration interaction of bound and continuum states is derived. Analytic expressions for line profiles are presented for two specific experimental implementations of TC-RFWM used in gas-phase spectroscopic studies. While the TC-RFWM line profiles are found to be very distinct from the line profiles measured in linear spectroscopic techniques, the results demonstrate the important capability to characterize the TC-RFWM line profiles in terms of the same fundamental and physically significant parameters.

Auger decay probabilities of metastable states in antiprotonic helium are derived using a minimal extension of the existing bound-state wave functions to account for the electron continuum. Calculations were performed for the Born-Oppenheimer wave functions of Shimamura [Phys. Rev. A $46$, 3776 (1992)] and the variational wave functions of Korobov [Phys. Rev. A $54$, 1749 (1996)]. Our results suggest that the overall accuracy of the Auger widths calculated from the presently available bound-state wave functions is not sufficient.

Using a Doppler-tuned fast ion-beam–laser technique, with co- and counterpropagating laser beams, the intercombination $1s2s{}^1{S}_0$–$1s2p{}^3{P}_{1,F}$ transitions in ${}^{14}{\mathrm{N}}^{5+}$ and ${}^{15}{\mathrm{N}}^{5+}$ have been measured to 0.7 ppm. This precision is equivalent to 20 ppm of the two-electron Lamb shift and is more than two orders of magnitude more precise than current relativistic and QED theoretical results at this $Z$. A comparison is made with theory for the hyperfine structure and isotope shift. Additionally, using a single copropagating laser beam, the differences between the $2{}^1{S}_0$–$2{}^3{P}_{1,F=1}$ and $2{}^1{S}_0$–$2{}^3{P}_0$ intervals in ${}^{14}{\mathrm{N}}^{5+}$ and between the $2{}^1{S}_0$–$2{}^3{P}_{1,F=3/2}$ and $2{}^1{S}_0$–$2{}^3{P}_0$ intervals in ${}^{15}{\mathrm{N}}^{5+}$ have also been measured. From the former we obtain an improved result for the ${}^{14}{\mathrm{N}}^{5+}$ $2{}^3{P}_1$–$2{}^3{P}_0$ fine structure.

An exchange-correlation functional is introduced that goes beyond the conventional gradient approximation by including contributions from the Laplacian of the density. The exchange part of this functional reproduces atomic exchange energies from the optimized potential model for main group elements (H-Xe) more accurately than other established exchange functionals. By construction, the exchange functional reproduces the small-gradient expansion for the exchange energy up to the fourth order and possesses the Coulomb asymptote for the exchange potential $v_x.$ In molecular applications, the exchange functional is combined with a modified version of a recently proposed nonlocal correlation functional based on a Coulomb hole model. Detailed results are reported for the molecules from the G2 database. In an overall assessment, the present results for the atomization energies and the molecular geometries for the G2 reference molecules seem comparable in quality to those from the B3LYP hybrid functional. Some illustrative results for transition-metal compounds are also given.

We propose a model to calculate in a quantum-mechanical way the screening and energy loss of swift ions in metals. The model is based on an extension of the Friedel sum rule to finite velocities, which allows us to describe the scattering and nonlinear screening of electrons in the field of the moving ion. The scattering process is formulated according to the partial-wave expansion, assuming an effective scattering potential that is adjusted in a self-consistent way using the phase-shift sum rule. We consider in particular the limits of low and high velocities, where the results of this model agree with those obtained using the density-functional and perturbation theories. We apply the method to the calculation of the mean energy loss (stopping power) on the whole range of velocities.

We have measured absolute differential cross sections (DCSs) for rovibrational excitation $(\overrightarrow{0}1,2,3,4)$ of the $\chi {}^3{\Sigma }_g^{-}$ electronic ground state of ${\mathrm{O}}_2.$ The scattered electron angular range was 10°–90°, while the cross sections were measured at ten specific energies in the 5–20 eV incident beam energy range. This energy regime encompasses the region where the ${}^4{\Sigma }_u^{-}$ and ${}^2{\Sigma }_u^{-}$ resonances of ${\mathrm{O}}_2^{\mathrm{-}}$ are known to significantly enhance the vibrational excitation process. The present data are found to be in generally good agreement with the more limited recent measurements of Allan [J. Phys. B 28, 5163 (1995)] and the earlier results of Wong et al. [Phys. Rev. Lett. 31, 969 (1973)]. Agreement with the data of Shyn and Sweeney [Phys. Rev. A 48, 1214 (1993)], however, was found to be only fair. Comparison of the present DCS with an available Born-closure Schwinger variational method calculation result is also made.

In this paper we study the single ionization of helium atoms by the impact of protons at intermediate energies. We present a continuum-distorted-wave–eikonal-initial-state calculation of the cross section doubly differential in the projectile scattering angle and the ionized electron energy. The internuclear interaction is incorporated by means of an eikonal phase approximation. This calculation includes a full account of the final-state interactions among the three collision partners, showing a much better agreement with the recent experimental data of Schulz et al. than other first-order theories. We also discuss the appearance of different structures associated with the electron capture to the continuum process and the binary-encounter projectile-electron collision.

State-selective electron-capture cross-section measurements in the energy range between 1 and 4 keV/amu are reported for collisions between N${}^{4+}$ ions and atomic and molecular hydrogen. The cross sections are measured in a crossed-beam experiment by means of photon emission spectroscopy. The singlet and triplet states are resolved. It is found that the capture probabilities are not statistically distributed over both spin states and depend strongly on the primary energy. For collisions with atomic hydrogen the agreement with fully quantum-mechanical calculations is in general good. The sums of the experimental results for capture into all the singlet and triplet states are in excellent agreement with previous total one-electron capture measurements. Additionally for molecular hydrogen we performed multichannel Landau-Zener calculations that are found to be in fair agreement with the experimental data except for the highest angular momenta states.

We discuss the possibility of zeros in the nonrelativistic radiative continuum-continuum matrix element for electron-atom (inverse) bremsstrahlung. As demonstrated earlier for upward transitions from bound states, the occurrence of different signs for the free-free matrix element in limiting cases, plus the requirement of continuity, implies the existence of zeros. Using knowledge of the sign of the dipole matrix element in the soft- and hard-photon limits with one continuum electron energy held fixed, we show that zeros can occur in the $s$-$p$ matrix element. We discuss the connection of our results to elastic scattering and to Ramsauer-Townsend minima. We consider the observability of zeros in this $(s$-$p)$ matrix element manifested as minima in the cross sections.

High-resolution measurements of the $K{\beta }_2$ diagram line and its $M^1$ satellite were performed for zirconium, molybdenum, palladium, and praseodymium. The x-ray spectra were induced by impact with 28-, 40-, 65-, and 100-MeV α particles and measured with a transmission-type bent crystal spectrometer in modified DuMond slit geometry. From the relative yields of the $K{\beta }_2{M}^1$ satellites the experimental average $M$-shell ionization probabilities for nearly central collisions were deduced. They are compared with theoretical predictions from the semiclassical independent particle approximation using either hydrogenlike wave functions (SCA-HWF) or Dirac-Hartree-Fock wave functions (SCA-DHF), pointing out the relevance of the use of DHF wave functions instead of hydrogenlike wave functions in the SCA model. Moreover, the experimental energy shifts of the satellite transitions with respect to the diagram lines are compared to results of extensive multiconfiguration Dirac-Fock calculations.

A theoretical model for electron-impact ionization cross section has been applied to Rb and the theoretical cross section (from the threshold to 1 keV in incident energy) is in good agreement with the recent experimental data obtained using Rb atoms trapped in a magneto-optical trap. The theoretical model, called the binary-encounter–dipole (BED) model, combines a modified Mott cross section with the high-energy behavior of Born cross sections. To obtain the continuum dipole oscillator strength $df/dE$ of the $5s$ electron required in the BED model, we used Dirac-Fock continuum wave functions with a core polarization potential that reproduced the known position of the Cooper minimum in the photoionization cross section. For inner-shell ionization, we used a simpler version of $df/dE$, which retained the hydrogenic shape. The contributions of the $4\overrightarrow{p}4d$, $5s$, and $5p$ autoionizing excitations were estimated using the plane-wave Born approximation. As a by-product, we also present the dipole oscillator strengths for the $5\overrightarrow{s}{\mathrm{np}}_{1/2}$ and $5\overrightarrow{s}{\mathrm{np}}_{3/2}$ transitions for high principal quantum numbers $n$ near the ionization threshold obtained from the Dirac-Fock wave functions with the same core polarization potential as that used for the continuum wave functions.

We report experimental rate coefficients for the reverse heteronuclear energy-pooling collisions $\mathrm{K}\mathrm{}\left(5D\right)+\mathrm{Na}\mathrm{}\left(3S\right)\mathrm{}\to \mathrm{K}\mathrm{}\left(4P\right)+\mathrm{Na}\mathrm{}\left(3P\right)\mathrm{}$ and $\mathrm{K}\mathrm{}\left(7S\right)+\mathrm{Na}\mathrm{}\left(3S\right)\mathrm{}\to \mathrm{K}\mathrm{}\left(4P\right)+\mathrm{Na}\mathrm{}\left(3P\right)\mathrm{}$ at thermal energies. Both reactions are exothermic and very high rates were observed showing that reverse exothermic energy-pooling is an order of magnitude more efficient than the corresponding forward endothermic energy-pooling reactions. This is in accordance with the general behavior of the exothermic and endothermic energy-pooling rate coefficients in alkali-metal atoms. In the experiment the potassium atoms were excited in two steps to either the $5D$ or $7S$ state via the $4P$ level using two broadband cw dye lasers. A double-modulation technique has been used to select the fluorescence contributions at the $\mathrm{Na}{(3P}_J)$ exit channels due only to the above reactions. The ground-state sodium and potassium atom densities were measured by the absorption of lines from a K-Na hollow-cathode lamp. The measured densities and fluorescence intensities have been used to obtain absolute reverse energy-pooling rate coefficients. The contribution to the rate coefficients from other processes are discussed.

To experimentally determine electron-impact excitation cross sections with the optical method, it is necessary to measure all transitions out of a level (the apparent cross sections), as well as the cascades into the level. In the case of the ten ${3p}^{5}4p$ levels of argon, the emissions to lower levels lie in the visible and near infrared (660–1150 nm) and are hence observable with a monochromator–photomultiplier-tube (PMT) system. A Fourier-transform spectrometer (FTS) allows us to measure the previously uninvestigated cascades that lie in the infrared. For the incident electron energy range between onset and 300 eV, we have measured the apparent cross sections with a monochromator-PMT system, and the cascade cross sections with a weak emission FTS system. The magnitude of both the apparent and cascade cross sections increases with target gas pressure due to radiation trapping effects. By subtracting the cascade contributions from the apparent cross sections, we have determined the direct cross sections and verified that they do not vary with pressure in the 0.5–4-mTorr pressure range considered here.

Absolute total cross sections for electron-impact excitation of the $2s{}^2\overrightarrow{S}2p{}^{2}P$ transition in ${\mathrm{C}}^{3+}$ were measured from 7.35 eV to 8.45 eV using the merged electron-ion-beams energy-loss technique. The results settle the discrepancy between two previous experiments using the crossed-beams fluorescence method, being in very good agreement with the older results [P. O. Taylor, D. Gregory, G. H. Dunn, R. A. Phaneuf, and D. H. Crandall, Phys. Rev. Lett. 39, 1256 (1977)] but less so with the more recent ones [D. W. Savin, L. D. Gardner, D. B. Reisenfeld, A. R. Young, and J. L. Kohl, Phys. Rev. A 51, 2162 (1995)]. The present measurements are also in good agreement with unitarized Coulomb-Born and close-coupling calculations.

Electron-ion coincidence spectroscopy of energy-analyzed electrons and charge-separated ions is used to investigate complex decay processes of atoms after inner-shell photoionization with monochromatized synchrotron radiation. Improvements in the experimental technique allow a complete electron spectrum resolved into the different contributions of the final ion-charge states to be measured. This method of final ion-charge resolving electron spectroscopy is demonstrated by the example of xenon for different photoionization processes in the region of the $4\overrightarrow{d}\epsilon f$ resonance.

We have measured the ratio of differential cross sections for the production of doubly and singly ionized He by fast protons at energies between 4 and 10 MeV. The ratio was measured as a function of proton scattering angle, from which the energy transfer was calculated using binary kinematics. The ratio is found to be near 1.25%, nearly independent of either proton energy or energy transfer. This value is lower than that reported previously for a similar experiment, but in good agreement with recent Compton scattering and theoretical results.

High-resolution $\mathrm{Si}-2p$ photoabsorption spectra of $\mathrm{Si}{X}_4$ molecules $(X$=H, D, F, Cl, Br, ${\mathrm{CH}}_3)$ are presented. High-resolution spectra were obtained for ${\mathrm{SiBr}}_4$ and ${\mathrm{S}\mathrm{i}(\mathrm{C}\mathrm{H}}_3{)}_4,$ and they are compared to the $\mathrm{Si}-2p$ spectra of the other $\mathrm{Si}{X}_4$ molecules studied before. In all cases, vibrational fine structure of the Rydberg states could be resolved, allowing a derivation of vibrational energies and intramolecular distances of the core-excited molecules from Franck-Condon analyses. The results obtained for the various ligand atoms are compared with each other, and are discussed in the context of the equivalent-cores approximation. A detailed analysis of the core-to-valence excitations in all $\mathrm{Si}{X}_4$ molecules studied is also given.

Dipole series in H${}^{-}$ and in the positron-hydrogen system are studied in a coupled-channel calculation, in which the short-range part of the interaction between the electron or positron and the excited hydrogen core is modeled by a local potential, adjusted to reproduce low-lying states of the respective series. Consideration of the fine-structure splitting and radiative corrections enables us to describe the termination of the series towards the nondegenerate threshold. For the ${}^1{S}^e$ series below the $N=2\mathrm{}$ threshold we give quantitative predictions for the number of resonances and their energies.

For 0.25–0.75-MeV ${\mathrm{He}}^{2+}$ on He collisions we have measured total state selective double capture cross sections and cross sections differential in projectile scattering angle. For 0.25 MeV we present also state-selective scattering-angle-dependent double-capture cross sections. The projectile energy loss (the final electronic state) as well as the transverse momentum transfer (i.e., the projectile scattering angle) have been obtained by measuring the momentum vector of the recoil ion using cold target recoil ion momenum spectroscopy. The resonant transfer to the ground state is found to be by far the dominant double-capture channel. Capture to nonautoionizing excited states is smaller by about a factor of 7, and results in larger scattering angles than the ground-state double capture.