We have recorded an angle-resolved photoelectron spectrum of the first four vibrational peaks of the $\mathrm{X}{}^2{\Sigma }_g^{+}$ state of ${\mathrm{H}}_2^{+}.$ In the spectrum, the individual peaks of the $\Delta N=0\mathrm{}$ rotational branch are completely resolved. From curve fitting of the rotational branches, we found the anisotropy parameter of the $|\overline{\mu }\mathrm{}(0,0)\mathrm{}{|}^2$ squared reduced multipole moment matrix element to be $\beta =1.85,$ with no significant difference between the vibrational states. For the $|\overline{\mu }\mathrm{}(2,0)\mathrm{}{|}^2$ squared reduced multipole moment matrix element, we found $\beta (\nu =0)=0.83,$ decreasing slightly at higher vibrational levels. The earlier observed increase in the $\beta$ parameter with vibrational quantum number is mainly explained by a decreased size of $|\overline{\mu }\mathrm{}(2,0)\mathrm{}{|}^2$ relative to $|\overline{\mu }\mathrm{}(0,0)\mathrm{}{|}^2,$ dominating the $\beta$ decrease of the $|\overline{\mu }\mathrm{}(2,0)\mathrm{}{|}^2$ squared reduced multipole moment matrix element itself. We have also determined the $\beta$ value of individual peaks in the $\Delta N=0\mathrm{}$ branch. The difference in $\beta$ between the peaks in this branch is small. In particular, the 0-0 transition does not seem to have a considerably higher $\beta$ value than the other members of the $\Delta N=0\mathrm{}$ branch, which is expected from theory.

We calculated the total electron-capture cross section for collisions of ${\mathrm{C}}^{4+}$ ions with atomic hydrogen in the energy range of 10–2000 eV/nucleon using the close-coupling two-center atomic orbital expansion method. Various basis sets have been used to check the convergence of the calculation and the results are shown to be in general agreement with existing experimental data. However, we found no evidence of a dip in the total electron-capture cross sections near 500 eV/nucleon as reported by Bliek et al. [Phys. Rev. A 56, 526 (1997)].

Doubly differential cross sections (DDCSs) for electron emission have been measured for collisions of 3.6 MeV/u ${\mathrm{Ne}}^{10+}$, ${\mathrm{Xe}}^{40+}$ and 5.9 MeV/u ${\mathrm{U}}^{29+}$ on neon, xenon, water, ethanol, methanol, propanol, ${\mathrm{C}}_2{\mathrm{F}}_6$, ${\mathrm{SF}}_6$, and ${\mathrm{C}}_3$${\mathrm{F}}_8$. Electrons ejected with emission angles between $0°$ and $180°$ with respect to the ion beam axis have been recorded simultaneously using a toroidal electron spectrometer. We analyze the singly differential cross section (SDCS) for binary encounter electron (BEe) production as a function of target electron number and laboratory emission angle. We find that there exists a linear scaling of the BEe SDCS with the number of electrons bound in the target with an energy lower than the reduced projectile energy. The enhancement of BEe production in the forward direction in collisions with partially stripped ions is studied for the different projectiles and targets and compared to theoretical calculations.

We solve a radial Schrödinger equation for the case of a multichannel square well plus an exponential potential in one of the channels. The solution is obtained by summing exactly the infinite terms of the perturbative series for the evolution operator of the system. Analytical expressions for the scattering matrix and the scattering length are given and a Deser-like formula that connects approximate ground energies and scattering length is obtained for a particular case. Possible physical applications of these results are discussed briefly.

Ab initio calculations based on quantum chemical methods and the one center approximation have been carried out for the $L_{2,3}\mathrm{VV}$ normal Auger-electron spectrum of HCl. Spin-orbit and molecular field effects were explicitly included in the description of the intermediate $2p$ core hole state. The results are compared with the experimental spectrum obtained with synchrotron radiation. The corresponding atomic $L_{2,3}\mathrm{MM}$ Auger transitions are studied in the isoelectronic argon atom. The calculated Auger electron spectra are in good agreement with the experimental and to previous theoretical results. For HCl the calculations predict substantially different total Auger transition probabilities of 126, 99, and 113 meV for the three nondegenerate spin-orbit and molecular-field-split ${2p}^{-1}$ states: ${2p}_{3/2}^{-1}{(}^2{\Pi }_{3/2}),$ ${2p}_{3/2}^{-1}{(}^2{\Sigma }_{1/2}^{+}),$ and ${2p}_{1/2}^{-1}{(}^2{\Pi }_{1/2}),$ respectively. Furthermore, each of these core hole states gives rise to remarkably different intensity distributions. These effects are explained by (i) the partial orientation of the chlorine $2p$ core hole by the molecular field, (ii) a decrease of the net population of the chlorine $3p$ orbital in the bond direction due to the chemical bond, and (iii) the clear (97%) preference of the Auger decay of an oriented $2p$ core orbital to produce final states with at least one hole in the $3p$ orbital with the same spatial orientation.

Dielectronic recombination has been investigated for ground-state ${\mathrm{Li}}^{+}{(1s}^2)$ and metastable ${\mathrm{Li}}^{+}\mathrm{}\left(1s2s\right)\mathrm{}$ ions. Absolute recombination rates for the convoluted sum of these configurations are obtained and compared with theoretical calculations. Also, the effect of the space charge on the energy of the observed resonances is investigated as well as the effect of trapped residual ions in reducing the effect of the space charge.

An ultrafast inner-shell ionization process, together with x-ray emission stimulated by high-intensity short-pulse x rays, is studied. In this process it is found that the multi-inner-shell ionization predominates, leading to the formation of hollow atoms. These hollow atoms play an instrumental role in high brightness x-ray measurements of the ultrafast dynamics in chemistry, biochemistry, biology, and materials. As the pumping x-ray power and thus the density of hollow atoms are increased, the emission of x rays is not only of significance for the measurement but is good for x-ray lasing. This lasing mechanism has a clear advantage over the conventional inner-shell ionization method, yielding a gain value of more than ${10}^4{\mathrm{cm}}^{\mathrm{-}1}$ over a relatively long time. Classes of experiments of pump x-ray probe and x-ray laser are suggested.

We study retardation effects in the two-photon electron bremsstrahlung in a Coulomb field, for the electron energy range 1–10 keV. The calculation is based on an exact analytic expression for the matrix element of the process, in a nonrelativistic treatment including retardation. The comparison with previous results for the Coulomb case (exact Coulomb dipole approximation or Born approximation with retardation) confirms the importance of retardation effects in the electron energy range under investigation, as indicated before by the Born approximation.

The collisional dynamics of the Zeeman-split levels of $\mathrm{Rb}(5\mathrm{}{}^2{P}_{1/2},5{}^2{P}_{3/2})$ has been studied in the presence of a strong magnetic field (1.65 T) using steady-state, spectrally resolved laser-induced fluorescence techniques. State-to-state cross sections for transitions between $\mathrm{}(J,m)\mathrm{}$ levels ranged from $1.6\times {10}^{-15}$ to $2.5\times {10}^{-14}{\mathrm{cm}}^2$ for collisions with nitrogen and rare gases. The cross sections indicate a significant dependence on $\mathrm{}(J,m)\mathrm{}$ states and certain symmetry relations such as $\sigma (J,\overrightarrow{m}{J,m}^{\prime })=\sigma (J,-\overrightarrow{m}J,-m^{\prime })$ and $\sigma (J,\overrightarrow{m}{J,}^{\prime }m)=\sigma (J,-\overrightarrow{m}{J}^{\prime },-m)$ are demonstrated. The results also agree favorably with theoretical predictions of the relative decay rates for orientation, alignment, and octupole moment. The cross sections for rare-gas collision partners scale with polarizability as predicted.

Isolated core excitation was used to produce low-energy continuum electrons in the $l=3\mathrm{}$ angular momentum state of barium. Data were taken over a region of energy that coincided with the energy of the ${6p}_{3/2}\mathrm{nf}$ doubly excited states. Analysis of the data using multichannel quantum defect theory allowed the measurement of the widths of the ${6p}_{3/2}\mathrm{nf}$ states and the energy-dependent phase of the continuum electronic wave functions due to interaction with the doubly excited states. The phase of the continuum electrons is shown to vary continuously with energy, due to the anomalously broad widths of the double excited states.

K-shell ionization cross sections of Nb by electron impact in the energy range 20–34 keV have been measured. The influence of electrons reflected from a backing on the measurement was corrected using an electron transport model. For comparison several calculations using theoretical and empirical expressions have been performed.

Auger decay from the C $1s$ core-ionized state of CO has been studied with vibrational resolution both at the C $1s$ shape resonance and far above. Variations in the vibrational intensity distribution with photon energy are observed. This observation is explained solely by the modified vibrational envelope of the core-ionized intermediate state at the shape resonance maximum, as demonstrated in a numerical simulation. This implies that the temporary trapping of the photoelectron at the shape resonance does not significantly affect the deexcitation process. Thus the deexcitation step is described by the Franck-Condon principle even at the shape resonance. The angular anisotropy of the Auger decay at the shape resonance has also been studied with vibrational resolution. Weak dependence upon the final state and no detectable influence from the vibrational quanta is observed.

Charge-exchange cross sections and beam lifetimes are studied for decelerated bare uranium ions at the ESR storage ring. By deceleration from the initial energy of 358 MeV/u down to various energies as low as 49 MeV/u, i.e., far below the production energy of bare ionic species, the electron pickup cross sections were obtained for collisions with ${\mathrm{N}}_2$, Ar, ${\mathrm{CH}}_4,$ and Kr gaseous targets. The measured cross sections and beam lifetimes are compared with the theoretical results for radiative and nonradiative electron capture. The present data, along with the theoretical approximations discussed, provide a solid basis for the estimation of beam lifetimes for decelerated bare high-$Z$ ions. Moreover, a normalization procedure is proposed, in which absolute total charge-exchange cross sections are derived by normalizing the simultaneously measured yield of radiative electron capture photons to rigorously calculated relativistic cross sections. This method, along with the unprecedented beam conditions at the ESR storage ring, allows a significant improvement of the accuracy of cross-section data.

A tunable infrared laser source is utilized to study the photodetachment threshold spectra of the transition-metal negative ions ${\mathrm{Co}}^{-},$ ${\mathrm{Ni}}^{-},$ ${\mathrm{Rh}}^{-},$ and ${\mathrm{Pd}}^{-}.$ The binding energies of the respective ionic ground states are found to be 663.3(6), 1157.16(12), 1142.89(20), and 562.14(12) meV. In addition, a ${}^3{F}_4{-}^3{F}_3$ fine-structure splitting of 875(15) ${\mathrm{cm}}^{-1}$ is measured for ${\mathrm{Co}}^{-},$ and the ${}^2{D}_{5/2}{-}^2{D}_{3/2}$ splitting of ${\mathrm{Ni}}^{-}$ is determined to be 1485(3) ${\mathrm{cm}}^{-1}.$ For ${\mathrm{Pd}}^{-}$, the electronically excited ${4d}^9{5s}^2{}^2{D}_{5/2}$ state is found to lie 1127(4) ${\mathrm{cm}}^{-1}$ above the ${4d}^{10}{5s}^2{S}_{1/2}$ ionic ground state, i.e., it is bound by 422.4(5) meV. The results agree with previous values obtained from laser photodetached electron spectra, but constitute improvements in accuracy of up to two orders of magnitude.

The time-dependent many-electron problem encountered in the theory of energetic ion-atom collisions can be approached systematically within an effective single-particle picture, based, e.g., on the time-dependent density functional theory. In the present contribution we investigate ionization and electron loss from the target for the collision systems p, $\overline{p}$, ${\mathrm{He}}^{2+}+\mathrm{Ne}$, and $p+\mathrm{Ar}$ in the energy range 10 keV/amu–1 MeV/amu. The results presented are based on a static approximation to the exchange-correlation contribution of the effective potential. The corresponding time-dependent single-particle equations are solved with the continuum distorted-wave with eikonal initial-state approximation and the basis generator method, a coupled-channel approach in terms of structurally adapted basis functions. We demonstrate that in either case the results depend strongly on the explicit form of the local static exchange potential. An orbital-dependent analysis of electron removal in ${\mathrm{He}}^{2+}+\mathrm{Ne}$ collisions shows that the quality of the exchange potential is decisive for the suppression of artificial reaction channels.

Siegert pseudostates (SPSs) are defined as a finite basis representation of the outgoing wave solutions to the radial Schrödinger equation for cutoff potentials and the problem of their calculation is reduced to standard linear algebra easily implementable on computers. For a sufficiently large basis and the cutoff radius, the set of SPSs includes bound, weakly antibound, and narrow complex-energy resonance states of the system, i.e., all the physically meaningful states observable individually. Moreover, the set is shown to possess certain orthogonality and completeness properties that qualify it as a discrete basis suitable for expanding the continuum. We rederive many results of the theory of Siegert states in terms of SPSs and obtain some (to our knowledge) previously unknown relations. This not only makes the results practically applicable, but also sheds a new light on their mathematical nature. In particular, we show how the Mittag-Leffler expansions for the outgoing wave Green’s function and the scattering matrix can be obtained on the basis of very simple algebraic relations, without assuming them to be meromorphic functions. Explicit construction of these two fundamental objects completes the SPS formulation of scattering theory for the one-channel case. The computational efficiency of this approach is illustrated by a number of numerical examples.

Argon photoionization is studied using the $R$-matrix technique. Total photoionization cross sections, as well as partial cross sections for emission of a $3s$ electron, are obtained and compared with experimental results in the photon energy range between 30 and 38 eV. As a by-product of the photoionization calculations, threshold cross sections have been estimated for ${\mathrm{Ar}}^{+}$ states up to 38 eV above the Ar ground state, and are compared with experiment. These comparisons show that the present approach can describe Ar autoionizing states reasonably well, even up to 38-eV photon energy. However, some modifications of previously used techniques have proven to be important in order to stabilize the calculation in this difficult energy range.

Elastic, diffusion, and viscosity cross sections for collisions of two hydrogen atoms in excited electronic states have been calculated by solving the Schrödinger equation with interaction potentials obtained by full configuration-interaction calculations. Viscosity and thermal conductivity have been reported as a function of the principal quantum number.

A general analytic formula for the total dielectronic recombination (DR) rate coefficients of all the ions along the NiI isoelectronic sequence with $34<~Z<~92$ is constructed on the basis of previously calculated DR data, which were obtained by relativistic ab initio level-by-level computations using the HULLAC code for ten Ni-like ions. The DR rate coefficients are given as a function of the electron temperature and of the ion charge. The results generated by this formula reproduce the previously calculated DR rate coefficients, generally to better than 5% accuracy for all electron temperatures ${\mathrm{kT}}_e>~{0.03E}_I$ ${(E}_I$ is the ionization energy of the Cu-like ion). As a test, the results of level-by-level DR calculations for two additional elements, barium and tungsten, are compared to those of the analytic formula. The agreement is found to be similar to that for the first ten ions. The analytic formula may be conveniently used for modeling the ion abundance distribution in hot plasmas.

We have made a spectroscopic study of the electrons photodetached from a fast beam of ${\mathrm{Ca}}^{-}$ ions by laser photons of energy 2.08 eV. The spectrum exhibits peaks associated with detachment via the $\mathrm{Ca}{(}^1{S)+e}^{-}\mathrm{}(ks,d)\mathrm{}$ and the $\mathrm{Ca}{(}^3{P)+e}^{-}\mathrm{}\left(\mathrm{kp}\right)\mathrm{}$ channels. The measured angular dependence of electron yields in each peak has been used to determine asymmetry parameters. The results are $\beta =1.21\mathrm{}\left(4\right)\mathrm{}$ and 1.60(4) for the ${}^{1}Sks,d$ and the ${}^3\mathrm{Pkp}$ channels, respectively. The measured saturation dependence of the electron yield on photon flux has been used to determine photodetachment cross sections for the same channels. The results are $\sigma =66\mathrm{}\left(7\right)\mathrm{}$ and 61(5) Mb, respectively. The latter values disagree significantly with the only available calculation.