We derive a classical path expression for a pressure-broadened atomic spectral line shape that allows for an electric-dipole moment that is dependent on the position of perturbers. The theory is applied to the atomic hydrogen Lyman-$\alpha$ and Lyman-$\beta$ lines broadened by collisions with neutral and ionized atomic hydrogen. The far wings of the Lyman series lines exhibit satellites, enhancements that may be associated with quasimolecular states of ${\mathrm{H}}_2$ and ${\mathrm{H}}_2^{+}$. The sizes of these features depend on the values of the electric-dipole moments at the internuclear separations responsible for the satellites. Profiles are computed with and without spatial dependence of the dipole moment, and are compared with astronomical and laboratory observations. We conclude that in the present case the variation of the dipole moment is an important factor that cannot be neglected.

A benchmark ab initio and density-functional theory (DFT) study has been carried out on the electron affinities of the first- and second-row atoms. The ab initio study involves basis sets of $\mathrm{spdfgh}$ and $\mathrm{spdfghi}$ quality, extrapolations to the one-particle basis set limit, and a combination of the coupled cluster with all single, double (and triple) excitations [CCSD(T)], CCSDT, and full configuration-interaction electron correlation methods. Scalar relativistic and spin-orbit coupling effects were taken into account. On average, the best ab initio results agree to better than 0.001 eV with the most recent experimental results. Correcting for imperfections in the CCSD(T) method improves the mean absolute error by an order of magnitude, while for accurate results on the second-row atoms inclusion of relativistic corrections is essential. The latter are significantly overestimated at the self-consistent-field level; for accurate spin-orbit splitting constants of second-row atoms, inclusion of $2s,2p$ correlation is essential. In the DFT calculations it is found that results for the first-row atoms are very sensitive to the exchange functional, while those for second-row atoms are rather more sensitive to the correlation functional. While the Lee-Yang-Parr (LYP) correlation functional works best for first-row atoms, its PW91 counterpart appears to be preferable for second-row atoms. Among “pure DFT” (nonhybrid) functionals, G96PW91 (Gill 1996 exchange combined with Perdew-Wang 1991 correlation) puts in the best overall performance, actually slightly better than the popular hybrid B3LYP functional. B3PW91 outperforms B3LYP, while the recently proposed one-parameter hybrid functionals such as B1LYP seem clearly superior to B3LYP and B3PW91 for first-row atoms. The best results overall are obtained with the one-parameter hybrid modified Perdew-Wang (mPW1) exchange functionals of Adamo and Barone [J. Chem. Phys. 108, 664 (1998)], with $\mathrm{mPW}1\mathrm{LY}p$ yielding the best results for first-row, and mPW1PW91 for second-row atoms. Indications are that a hybrid of the type a $\mathrm{mPW}1\mathrm{LYP}+\left(1\mathrm{}-a\right)$ mPW1PW91 yields better results than either of the constituent functionals.

We have made high-accuracy measurements of the integrated Stokes parameters for resonance fluorescence from polarized electron-impact excitation of the ${4p}^{5}5p[5/2]{\hspace{0.5em}}^3{D}_3$ and ${4p}^{5}5p[5/2{]}^3{D}_2$ states of Kr. We report measurements in the region within 0.6 eV of threshold, which is below the first cascade threshold. We also present theoretical calculations of these Stokes parameters using a recently developed relativistic Breit-Pauli R- matrix code. In well $\mathrm{LS}$-coupled systems, nonzero values of the integrated Stokes parameter $P_2$ signal relativistic effects (like continuum spin-orbit coupling, i.e., Mott scattering). A single value of $P_2=4\mathrm{}\left(4\right)\mathrm{}\times {10}^{-3}$ at 12.0 eV was previously reported in this energy range [Furst et al., Phys. Rev. A 47, 3775 (1993)]. We have now measured $P_2$ at six different energies in this region to comparable precision. These results are consistent with $P_2=0\mathrm{}$ and with the theory. We discuss the effect of the electron-beam energy width on the accuracy of the measurements. Even when such effects are accounted for, serious discrepancies remain between theoretical and experimental results for excitation of the intermediately coupled ${}^3{D}_2$ state.

The classical form factor is deduced from exact correspondence with a phase-space representation of the quantal form factor. Analytical expressions are provided for $n\overrightarrow{l}{n}^{\prime }{l}^{\prime },$ $n\overrightarrow{l}{n}^{\prime }$, and $\overrightarrow{n}{n}^{\prime }$ transitions in hydrogenic systems and for $\overrightarrow{n}{n}^{\prime }$ in the one-dimensional harmonic oscillator. An efficient procedure for calculation of quantal form factors as analytical functions of momentum transfer, for arbitrary quantum numbers, is presented. The classical approach has the ability to explain quite succinctly interesting trends and various important aspects which remain hidden within the quantal treatment of form factors. The classical-quantal comparison ranges from being qualitatively good for $n\overrightarrow{l}{n}^{\prime }{l}^{\prime }$ transitions to close agreement for $n\overrightarrow{l}{n}^{\prime }$ and $\overrightarrow{n}{n}^{\prime }$ transitions. Excellent agreement is obtained for the integrated form factor for all transitions.

The interpretation of resonant decay spectra of molecules resulting from an energetically unselective excitation may be very complex: a large number of core-excited states is accessible by broadband excitation and thus decay contributions of all of these states can be found in the observed spectra. For an understanding of the spectra a knowledge of the potential curves of all core-excited and final states involved as well as information about the populated intermediate states is necessary. In this paper, we demonstrate this by studying the broadband resonant Auger spectrum of hydrogen fluoride in detail. A comparison with the experimental spectrum obtained after electron-impact excitation will be drawn. For this paper the needed potential curves have been computed using extended ab initio multireference configuration-interaction calculations. The information on the core-excited states populated by electron impact is obtained by calculation of the electron energy-loss spectrum of HF and comparison with the corresponding experimental spectrum.

The influence of narrow-band excitation on the decay spectra of dissociative core-excited molecular states is investigated in detail. Surprising interference phenomena have been reported previously in the resonant Auger spectrum of HF of Pahl et al. [Phys. Rev. Lett. 80, 1865 (1998)]. Apart from a general quenching of the “atomic” contribution by detuning, pronounced interference patterns are found for small detuning above the resonance of the core-excited state. Utilizing the time resolution gained within the theory of wave-packet dynamics, the underlying interference mechanisms are analyzed thoroughly. Special attention is paid to the selection process of intermediate-state eigenfunctions accompanying narrow-band excitation. We present a method that enables the assignment of different contributions in the decay spectrum to the internuclear distances at which the corresponding transitions take place. In particular, information about the origin of the atomic transitions is obtained.

We present a quantum description for the evolution of atomic states of fast projectiles traveling through matter. Our approach is based on the solution of a quantum Langevin equation, i.e., a stochastic time-dependent Schrödinger equation that describes electronic excitations of atoms during their transport through solids. The present description can be considered the quantized version of a previously developed classical transport theory. We analyze in detail the correspondence between classical and quantum transport simulations. Applications to the stripping of relativistic ${\mathrm{H}}^{-}$ and H through thin carbon foils and a comparison with experimental data are presented.

Results of an investigation of collisional deexcitation of the metastable $6s6p{}^3{P}_0$ state in atomic ytterbium by helium and neon buffer gases are reported. We find upper limits for the quenching cross sections to be ${\sigma }_{\mathrm{He}}<13\mathrm{}\times {10}^{-22}{\mathrm{cm}}^2$ and ${\sigma }_{\mathrm{Ne}}<8\mathrm{}\times {10}^{-22}{\mathrm{cm}}^2.$ The small cross sections may allow an ytterbium parity nonconservation experiment to be performed in a vapor cell. We have also measured the pressure broadening and shift of the $6s6p{}^3{P}_0\to 6s7s{}^3{\mathrm{S}}_1$ and ${6s}^2{}^1{S}_0\to 6s6p{}^3{P}_1$ transitions by helium and neon.

We analyze the problem of suppression of reflections within the numerical propagation of wave packets using finite atomiclike pseudostate expansions. Artificial reflections at effective “walls” and reentering of probability flux into the interaction region represents a major limitation for the study of long-time evolution of atomic ionization processes. We propose two methods, the repetitive projection method (RPM) and Siegert pseudostate (SPS) propagation, and study their efficiency in suppressing reflections. It is shown that the quantum Zeno effect sets a limit on the efficiency of the RPM as well as of masking functions. For the exactly solvable propagation of a radial-free wave packet, we show that both the SPS and the RPM provide almost complete suppression of reflections without appreciable distortion in the physically relevant region of coordinate space.

Cross sections for the processes of projectile electron loss and single and double capture of ${\mathrm{C}}^{3+}$ and ${\mathrm{O}}^{5+}$ projectile ions impinging on He, Ne, Ar, Kr, and Xe targets were measured in the energy range of 1.0–3.5 MeV. The measured cross sections present a strong saturation as the target atomic number increases, for all the systems and collision channels studied. The single-capture data are compared with calculations based on a semiclassical model and on the eikonal approximation, both presenting a good general agreement with the experiment. In the case of electron loss, the observed saturation is in accordance with previous measurements for ${\mathrm{He}}^{+}$ projectiles, and is present in the first-order calculations for the antiscreening contribution but not in those for the screening. This is due to the fact that, for heavy target atoms, the screening mode can be highly nonperturbative. Calculations for the screening contribution to the electron loss, based on the free-collision model, together with first-order results for the antiscreening, are compared with the experimental data, presenting good agreement in most cases. However, this comparison also shows that one has to include other competitive channels in order to give a better description of the collision.

Over the past several years, another type of spectrometer has been developed that proves to be superior to conventional spectrometers. In this “momentum imaging” spectrometer, electrons and target-recoil ions produced in ionizing collisions are accelerated to opposing position-sensitive detectors by an external electric field. The momentum imaging spectrometer essentially projects (or images) the initial three-dimensional (3D) electron and recoil-ion momentum vectors onto the 2D plane of each corresponding detector. Because the spread in electron arrival time is quite small in comparison with the spread in recoil-ion arrival time, one can utilize the electron signal as a timing marker to extract the full 3D momentum vector of the recoiling ion. This technique has proven to be quite successful in cold-target recoil-ion momentum spectroscopy. Momentum imaging methods have also been recently utilized in the search for evidence of saddle-point electron emission. Experimental studies of ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{2+}$ incident on He were carried out by Abdallah et al. [Phys. Rev. A 56, 2000 (1997)]. Rather surprisingly, their results exhibited projectile-charge dependent shifts in the opposite direction than that implied by the saddle-point mechanism. However, as we shall demonstrate, proper saddle-point shifts may be observed if one takes into account the time of flight of the electron.

Electron energy and angular distributions are computed for a model of atom–negative-ion collisions. In this model, electron-atom interactions are represented by zero-range potentials in an approximation where two identical atoms move along straight-line classical trajectories in head-on collisions. Analytic expressions for the ionization amplitudes are interpreted in terms of Sturmian eigenvalues and eigenfunctions. At high velocity, the computed distributions exhibit direct excitation and continuum capture cusps in addition to the binary encounter ridge. At low velocities, a single feature corresponding to an electron distribution centered midway between the target and projectile emerges. For initial conditions corresponding to gerade symmetry a single broad peak appears, while for ungerade symmetry there is a node at the midpoint so that the peak splits into two parts. It is confirmed that the advanced adiabatic approximation gives an accurate description of the ungerade distribution at low and intermediate velocities.

We have measured the absolute cross section for electron-impact excitation (EIE) of ${\mathrm{Si}}^{2+}{(3s}^2{}^1\overrightarrow{S}3s3p{}^{1}P)$ from energies below threshold to 11 eV above. A beams modulation technique with inclined electron and ion beams was used. Radiation at 120.7 nm from the excited ions was detected using an absolutely calibrated optical system. The fractional population of the ${\mathrm{Si}}^{2+}\left(3s3p{}^3{P}^o\right)$ metastable state in the incident ion beam was determined to be $0.210\pm 0.018$ $\left(1.65\mathrm{}\sigma \right).\mathrm{}$ The data have been corrected for contributions to the signal from radiative decay following excitation from the metastable state to $3s3p{}^{1}P$ and ${3p}^2{}^{3}P,$ and excitation from the ground state to levels above the $3s3p{}^{1}P$ level. The experimental $0.56\pm 0.08-\mathrm{eV}$ energy spread allowed us to resolve complex resonance structure throughout the studied energy range. At the reported $\pm 14\%$ total experimental uncertainty level $\left(1.65\mathrm{}\sigma \right),$ the measured structure and absolute scale of the cross section are in good agreement with 12-state close-coupling R-matrix calculations.

A simple nonadiabatic process, the collision between an atom and a diatomic homonuclear molecular ion giving rise to different electronic states of the ionic fragment, is considered for the case of the triatomic system ${\mathrm{N}}_2{\mathrm{H}}^{+},$ in the energy range of 3–40 eV. New correlated potential-energy surfaces, originating from a previous full-dimension multireference double configuration-interaction calculation, are employed and the dynamical calculations on these surfaces are performed by applying the semiclassical multichannel S matrix in its recent refinements. The collision is studied in a simplified approach where the system remains fixed in a collinear arrangement. The dynamical coordinate is the distance between the H projectile and the ${\mathrm{N}}_2^{+}$ center of mass while the ${\mathrm{N}}_2^{+}$ vibrations are included a posteriori in a parametric scheme. First the conditional probabilities for the nonadiabatic processes involved are extracted from the computed S matrix and then probabilities at different impact parameters are obtained. The corresponding total cross sections for the two main processes considered, ${\mathrm{N}}_2^{+}{(1\mathrm{}}^2{\Sigma }_g^{+})+\mathrm{H}{(1\mathrm{}}^{2}S)\to {\mathrm{N}}_2^{+}{(1\mathrm{}}^2{\Sigma }_u^{+})+\mathrm{H}{(1\mathrm{}}^{2}S)$ and ${\mathrm{N}}_2^{+}{(1\mathrm{}}^2{\Sigma }_u^{+})+\mathrm{H}{(1\mathrm{}}^{2}S)\to {\mathrm{N}}_2^{+}\left(2\mathrm{}{}^2{\Sigma }_u^{+}\right)+\mathrm{H}{(1\mathrm{}}^{2}S),$ are finally computed. The nonadiabatic cross sections reveal that the formation of ${\mathrm{N}}_2^{+}$ in its lowest excited state is largely preferred. An immediate application of this theoretical work is suggested in order to stimulate and direct further ion-molecule experiments on the $\mathrm{H}(1\mathrm{}{}^2{S)+N}_2^{+}$ system.

We have measured highly resolved time-of-flight spectra and concomitant angular distributions of slow ${\mathrm{Na}}^{+}$ ions produced by near-threshold dissociative ionization of ${\mathrm{Na}}_2.$ Using double-resonance excitation via the $A{}^1{\Sigma }_u^{+}$ state, ${\mathrm{Na}}_2$ molecules are promoted to rotational levels $J=14\mathrm{}$ or 16 and vibrational levels between $v=50\mathrm{}$ and 69 of the $4{}^1{\Sigma }_g^{+}$ state. Then, a third photon—from the second of two tunable dye lasers employed—causes the transition into the double continuum associated with the $1{}^2{\Sigma }_u^{+}$ potential of the ${\mathrm{Na}}_2^{+}$ cation. A detailed Monte Carlo simulation is employed to study the dynamics of this dissociative ionization process in detail. Based on this approach, we were able to subject recent calculations of this $1{}^2{\Sigma }_u^{+}$ potential to experimental scrutiny. We find our data to be compatible with a zero-energy crossing near 8.8 Å and a well depth of about 70 ${\mathrm{cm}}^{\mathrm{-}1}$, which agrees with most of the published calculations. Angular distributions for the vibrational level $v=60\mathrm{}$ are also reproduced well using the same potential parameters and an anisotropy parameter $\beta =2\mathrm{}$ modified by a semiclassical argument to account for parent molecule rotation.

Experimental results are presented on the degree of linear polarization of the extreme-ultraviolet emission of neutral and ionized helium following electron-impact excitation and ionization excitation of helium. The polarization of the photon emission from the decay of He $\mathrm{}\left(1snp\right)\mathrm{}{}^1{P}^o$ states with wavelengths of 517 to 584 Å has been extended to electron-impact energies of 1500 eV, and compared with theory for electron-impact excitation of neutral helium. In addition, the polarization of a number of the ${\mathrm{He}}^{+}$ decays with wavelengths between 256 and 1640 Å have been measured. Particular attention has been paid to the $\mathrm{}\left(2p\right)\mathrm{}{}^2{P}^o\to \mathrm{}\left(1s\right)\mathrm{}{}^{2}S$ and $\mathrm{}\left(3p\right)\mathrm{}{}^2{P}^o\to \mathrm{}\left(1s\right)\mathrm{}{}^{2}S$ decays with wavelengths of 304 and 256 Å, respectively. These have been measured from threshold (66 and 73 eV, respectively) to 1500 eV using a characterized molybdenum/silicon multilayer mirror polarimeter whose reflection and polarization characteristics have been optimized at 304 Å ${\mathrm{He}}^{+}\mathrm{}\left(2p\right)\mathrm{}{}^2{P}^o$ results are compared with a recent threshold alignment measurement and distorted-wave Born-approximation calculation for the ${(e}^{-}{,2e}^{-})$ reaction of ionization excitation. Very good agreement with the threshold theoretical predictions is obtained. These results indicate that near threshold, partial waves with $L>\mathrm{}0\mathrm{}$ contribute substantially to the two-electron wave function of the escaping electrons. Measurements on the unresolved multiplets at 1215 and 1640 Å, made using a more conventional reflection-type polarization analyzer, indicate that relative fine-structure cross sections are quite different than predicted by some theories.

The Schwinger variational iterative method and the distorted-wave approximation are combined to study elastic electron scattering by ${\mathrm{O}}_2$ for incident energies ranging from 5 to 500 eV. Differential, integral, and momentum transfer cross sections as well as polarization fractions are reported. A complex optical potential, which accounts for the static, exchange, correlation-polarization, and absorption contributions is used to describe the electron-molecule interaction. Comparison of our calculated results with experimental and other theoretical results is encouraging.

The details of a recent theoretical formulation of electron-impact detachment of weakly bound negative ions are presented. The formalism is applied at low energies to find the behavior of the cross section near threshold. Also, the applicability of a recently proposed scaling law is discussed. The formulation uses different coordinate systems for the initial and final states. The initial state is a simple product of wave functions of each electron, while the final state is a simple product of wave functions in center-of-mass coordinates.

The ${2p}^{-1}\mathrm{ns}\left(\mathrm{nd}\right)\mathrm{}$ inner-shell photoexcited resonance states in argon are studied using a combination of optical potential, multichannel quantum defect theoretical, and R-matrix methods. Optical potential inclusion of the infinite number of core decay channels correctly accounts, in an implicit manner, for the physical broadening of resonances within the ${3p}^{-1}$ and ${3s}^{-1}$ cross sections, but highly asymmetric resonance features remain. Comparison with experimental results is made, showing good qualitative agreement in the resonance profiles. The quantitative differences are due to unconverged theoretical resonance energies and to discrepancies in the background cross sections. Fine-structure-resolved cross sections and angular distribution parameters are investigated in the present calculations, revealing major qualitative departure from nonrelativistic predictions; these features should be observable in high-resolution measurements.

We have obtained generalized oscillator strengths, in the framework of the first Born approximation, for the electron-impact excitation of ${\mathrm{H}}_2$ through the $X{}^1{\Sigma }_g^{+}\mathrm{}(v=0\mathrm{})\mathrm{}\to B{}^1{\Sigma }_u^{+}{(v}^{\prime }=0\mathrm{}-33),$ $X{}^1{\Sigma }_g^{+}\mathrm{}(v=0\mathrm{})\mathrm{}\to B^{\prime }{}^1{\Sigma }_u^{+}{(v}^{\prime }=0\mathrm{}-8)$ and $X{}^1{\Sigma }_g^{+}\mathrm{}(v=0\mathrm{})\mathrm{}\to C{}^1{\Pi }_u{(v}^{\prime }=0\mathrm{}-12)$ vibronic transitions. The target electronic wave functions, previously used to determine electron-impact dissociation cross sections for the same transitions [Phys. Rev. A 57, 1025 (1998)], were calculated at the configuration-interaction level for several internuclear distances. The discrete vibrational wave functions were generated by solving the vibrational Schrödinger equation in the Born-Oppenheimer scheme. The Franck-Condon approximation was not invoked. Optical oscillator strengths for the same transitions were also obtained and compared favorably with previous theoretical and experimental results. The calculated generalized oscillator strengths presented good agreement with recent measurements by Zhong et al. [J. Electron Spec. and Rel. Phen. 94, 127 (1998)] for the $B{}^1{\Sigma }_u^{+}$ and $C{}^1{\Pi }_u$ vibronic transitions.