We calculate the electronic energy loss of protons and antiprotons in solids for a wide range of energies using a self-consistent method based on the extension of the Friedel sum rule to finite velocities. The calculations show the importance of nonlinear effects in the interaction of the moving particles with valence electrons in the intermediate range of energies around the stopping power maximum. The calculations give the magnitude and the velocity dependence of these effects, and provide a consistent theoretical explanation of recent experimental results for aluminum and silicon.

The Ll, $L\alpha ,$ $L\eta ,$ $L{\beta }_6,$ $L{\beta }_{2,4},$ $L{\beta }_{1,3},$ $L{\beta }_{9,10},$ and $L\gamma$ x-ray production differential cross sections in ${}_{92}\mathrm{U}$ have been measured using the 22.6- and 59.5-keV incident photon energies in an angular range 43°–140°. The measurements were performed using the ${}^{109}\mathrm{Cd}$ and ${}^{241}\mathrm{Am}$ radioisotopes as photon sources and a Si(Li) detector. Differential cross sections for various L x rays are found to be angle independent within experimental error. This is contrary to the strong angular dependence of measured photon-induced Ll and $L\alpha$ x ray production cross sections as reported by Kahlon et al. [Phys. Rev. A 43, 1455 (1991)] and M. Ertugrul [Nucl. Instrum. Methods Phys. Res. B 119, 345 (1996)]. Integral cross sections for production of the Ll, $L\alpha ,$ $L\eta ,$ $L{\beta }_6,$ $L{\beta }_{2,4},$ $L{\beta }_{1,3},$ $L{\beta }_{9,10},$ $L{\gamma }_{1,5},$ $L{\gamma }_{2,3,6},$ and $L{\gamma }_4$ x rays are also deduced in the present paper and are found to be in good agreement with those evaluated using the most reliable theoretical values of $L_i$ $\mathrm{}(i=1,2,3\mathrm{})\mathrm{}$ subshell photoionization cross sections, fluorescence yields, x-ray emission rates, and Coster-Kronig transition probabilities.

The optimization of the physical processes of an inner-shell ionization x-ray laser is carried out theoretically. Vapors of Na, Mg, and Al are adopted as a target material in order to avoid deleterious fast atomic processes except for the inner-shell ionization process. The effects of the vapor density and x-ray intensity are studied. Larmor x rays are suitable as a pump x-ray source because they have high brightness and short pulse. Their peak intensity and energy spectrum are appropriate for inner-shell ionization. In the Mg vapor the gain of 10 and ${10}^3{\mathrm{cm}}^{\mathrm{-}1}$ can be obtained with the use of ${10}^{11}$- and ${10}^{14}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2$ intensity x rays, respectively.

We extend the well-known O(4) group-theoretical method for the classification of doubly excited states to the two-electron continuum. The analytical continuation of the approximate dynamic symmetry group across the double ionization threshold is isomorphic to the homogeneous Lorentz group O(3,1). The infinite-dimensional irreducible representations of O(3,1) provide approximate wave functions, which incorporate electron-electron angular correlations. Moreover, the labels of the representations can be interpreted as approximate quantum numbers, which allow the extension of well-known propensity rules from double excitation to double ionization. Employing the dynamic O(3,1) symmetry group and a new method to calculate complex Wigner $\mathrm{}(9-j)\mathrm{}$ coefficients, we determine the angular distributions of the photoelectrons for double photoionization of helium near threshold. We find remarkable agreement with recent measurements. Similarities to and differences from other calculations are discussed.

By means of a time-dependent method, we have calculated final ionic state distributions after the decay of laser-prepared atomic high doubly excited, mutually penetrating $\mathrm{NLnl}$ states. The calculation is simplified by using two nondispersive radial wave-packet wave functions with the initial condition $〈r_1{〉}_{\mathrm{NL}}<〈r_2{〉}_{\mathrm{nl}}$ at $\mathrm{}(t=0\mathrm{}).$ Two decay processes have been investigated. The first is the autoionization of high doubly excited states (DES) with $N<~n.\mathrm{}$ Within our model it is described by two electrons moving like two “breathing” charge clouds exchanging energy until one of them escapes. The second decay process is the photoionization of the inner electron $(N>~2n$ including $E_N>~0),$ which leaves the final ion in a high Rydberg state. Our results are compared with data from experiments, in which DES are laser-prepared using isolated core excitation.

This paper reports the results of measurements of cross sections for electron excitation out of the ${1s}_3$ and ${1s}_5$ metastable levels of argon (the $J=0\mathrm{}$ and 2 levels, respectively, of the ${3p}^{5}4s$ configuration) into eight of the ten levels of the ${3p}^{5}4p$ manifold. The metastable atoms were generated by two methods: (a) an atomic beam emerging from a hollow-cathode discharge, and (b) charge-exchange collisions between a fast argon-ion beam and cesium atoms. The metastable argon atoms are excited by a crossed electron beam into the ${3p}^{5}4p$ levels and the emissions from these levels are utilized to determine the cross sections. Removal of the ${1s}_5$ atoms in the hollow-cathode discharge experiment by means of laser pumping allows us to determine the separate contributions from each metastable level to the observed fluorescence signal. The magnitudes of the cross sections for excitation out of the metastable levels into the different levels of the ${3p}^{5}4p$ manifold vary vastly. The patterns of the observed variations are interpreted by means of a multipole analysis. This multipole model is also used to discuss the comparison of excitation cross sections out of the metastable levels with those out of the ground level.

Cadmium $\mathrm{}(e,2e)\mathrm{}$ energy spectra have been measured for incident electron energy 150 eV and scattering angles 2°–18°, corresponding to momentum transfer 0.2–1 a.u. This choice of kinematics covers the transition from the dipole limit at 0.2 a.u. to the binary collisions regime at 1 a.u. The data are presented as the sum and difference of pairs of $\mathrm{}(e,2e)\mathrm{}$ spectra with ejected-electron directions 180° apart. This analysis reveals trends in the data and also provides a sensitive test of theory. Data sets for three special ejected-electron directions are compared with plane-wave Born approximation calculations. The calculated spectral shapes are in good agreement with all experiments when a simple phase correction is incorporated. The calculated difference/sum relative intensities are in good agreement with one data set but are in very poor agreement with the other two.

We present a method of deriving energy and angle-dependent electron-ion elastic scattering cross sections from doubly differential cross sections for electron emission in ion-atom collisions. By analyzing the laboratory frame binary encounter electron production cross sections in energetic ion-atom collisions, we derive projectile frame differential cross sections for electrons elastically scattered from highly charged projectile ions in the range between 60° and 180°. The elastic scattering cross sections are observed to deviate strongly from the Rutherford cross sections for electron scattering from bare nuclei. They exhibit strong Ramsauer-Townsend electron diffraction in the angular distribution of elastically scattered electrons, providing evidence for the strong role of screening played in the collision. Experimental data are compared with partial-wave calculations using the Hartree-Fock model.

Analytic solutions of the Schrödinger equation are presented for a repulsive ${1/r}^3$ potential. They lead to an in-depth understanding of scattering by a pure repulsive ${1/r}^3$ interaction including exact cross sections and phase shifts. Scattering by any potential which is asymptotically a repulsive ${1/r}^3$ is also discussed.

Relativistic calculations of the dielectronic recombination (DR) cross section and rate coefficient for Ne-like tungsten $\left({\mathrm{W}}^{64+}\right)$ in the ground state ${2s}^2{2p}^6$ are performed. The DR contributions of the most important Na-like doubly excited configuration complexes, namely, ${\mathrm{}\left(2s2p\right)\mathrm{}}^7{3ln}^{\prime }{l}^{\prime }$ ${(n}^{\prime }=3\mathrm{}–13)$ and ${\mathrm{}\left(2s2p\right)\mathrm{}}^7{4l4l}^{\prime },$ are calculated by level-by-level computations. Collisional transitions following the initial electron capture are neglected. The total DR cross section is presented in the form of 5-eV-wide peaks for the purpose of revealing the detailed atomic level structure, and in the form of 50-eV-wide peaks for the purpose of comparison with planned electron-beam experiments. The ${\mathrm{}\left(2s2p\right)\mathrm{}}^7{3l3l}^{\prime }$ complex is found to provide the dominant DR channel. The sum of the DR contributions of the ${\mathrm{}\left(2s2p\right)\mathrm{}}^7{3ln}^{\prime }{l}^{\prime }$ configurations with $n^{\prime }>13,$ evaluated by applying the complex-by-complex $n^{\prime -3}$ extrapolation method, is found to represent at most 3% of the total DR rate coefficient. The DR contribution of ${\mathrm{}\left(2s2p\right)\mathrm{}}^7{3ln}^{\prime }{l}^{\prime }$ configurations with $l^{\prime }>5\mathrm{}$ is found to be negligible (about 0.1%). The total DR rate coefficient is fitted to a convenient analytical expression which reproduces the original data with an accuracy of about 2% or better within a very wide electron temperature range.

The electron loss of ${\mathrm{He}}^{+}$ projectiles in heavy targets is studied within the framework of the sudden approximation. The main focus of the present study is in the elastic (screening) contribution to the electron loss. The calculations presented here unitarize the final populations of the projectile and of the target, take into account the nonperturbative character of the collision, and include the possibility of multielectronic transitions in the target atom concomitantly with the loss process. An analytical estimate for the screening contribution is also obtained.

We present calculated cross sections and rates for dissociative recombination of the ${\mathrm{He}}_2^{+}$ molecular ion in the energy range 0.1 meV–2.0 eV. We used the multichannel quantum defect theory applied to the molecular case, with ionization and dissociation channels simultaneously included. A comparative study has been performed for different isotopic compositions of the ${\mathrm{He}}_2^{+}$ ion, different initial rovibrational states of the ion, and three dissociation paths: the lowest dissociative states ${}^3{\Sigma }_g^{+}{,}^1{\Sigma }_g^{+},$ and ${}^3{\Pi }_u$ of the ${\mathrm{He}}_2$ molecule for which autoionization widths are obtained by the complex Kohn variational method.

We have studied the emerging charge states $q_{\mathrm{out}}$ and energy loss of $300-{\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{u}\mathrm{}\mathrm{U}}^{73+}$ incident ions transmitted along a $〈110〉$ axis of a $120-\mu \mathrm{m}$-thick Si crystal. The emerging charge state distribution $F_C{(q}_{\mathrm{out}})$ for well-channeled ions is governed mainly by electron impact ionization (EII). The corresponding EII cross sections were obtained by fitting the experimental $F_C{(q}_{\mathrm{out}})$ with Monte Carlo simulations. For M shell ionization, they were found to be twice larger than those given by the binary encounter dipole approximation. The measured energy loss spectra were also compared to Monte Carlo simulations. The mean values and widths of these spectra increase with $q_{\mathrm{out}},$ reflecting the increase of the stopping power S with increasing transverse energy $E_{\perp }.$ The measured stopping for channeled ions with frozen charge state $73+$ and for nonchanneled ions with charge state close to $90+$ is in good agreement with theoretical estimates. Owing to the very high ion velocity, there is a significant contribution $\mathrm{}(25\%)\mathrm{}$ to the stopping from $\mathrm{Si}-L$ shell excitation even for the best channeled ions. The width and the asymmetrical shape (skewness $\mu )$ of the energy-loss spectra depend strongly on $q_{\mathrm{out}}(\mu >0\mathrm{}$ for very well-channeled ions, $\mu <0\mathrm{}$ for poorly channeled ions). For well-channeled ions, energy-loss spectra were reproduced by Monte Carlo simulations with the ${S(E}_{\perp })$ curve extracted from fitting the mean energy losses.

Fragmentation schemes of ${\mathrm{C}}_{60}^{4+}$ fullerene ions produced by bombardment of ${\mathrm{Ar}}^{8+}$ ions at low energies $(v\simeq 0.24\mathrm{a}.\mathrm{u}.)$ have been investigated. By analyzing the time of flight of the fragments in coincidence with the outgoing projectiles and the ejected electrons, several dissociation channels have been identified that involve the loss of small neutral as well as charged fragments. The probability of each channel has been measured experimentally and compared to the calculation based on a theoretical model including a statistical description of the evaporation, and a simulation of the multistep cascade decay for sequential losses of low mass fragments. The initial energy of the ${\mathrm{C}}_{60}^{4+}$ ion and the dissociation energy of each channel have also been evaluated.

Laser-induced molecular coulomb explosion of ${\mathrm{N}}_2$ and ${\mathrm{CO}}_2$ is studied experimentally using linearly and circularly polarized laser light in the ${10}^{15}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2$ intensity range at $\lambda =800\mathrm{}\mathrm{nm}.$ The fragmentation kinetic energy releases are found to be identical in both cases using two different experimental methods. These results show that the molecular relaxation is independent of the respective directions of the laser electric field and of the internuclear axis. These results are confirmed by varying the linear laser polarization direction relative to the explosion direction of ${\mathrm{N}}_2$ and ${\mathrm{CO}}_2.$ Moreover, since in the linear case the molecular reorientation occurs along the laser electric field direction, these results show that inertial effects are not responsible for the possible elongation of the internuclear distances.

Negative-ion detachment in a bichromatic laser field is considered within the adiabatic theory. The latter represents a recent modification of the famous Keldysh model for multiphoton ionization (L. V. Keldysh, Zh. Éksp. Teor. Fiz. 47, 1945 (1964) [Sov. Phys. JETP 20, 1307 (1965)]) that makes it quantitatively reliable. We calculate angular differential detachment rates, partial rates for particular above-threshold detachment channels, and total detachment rates for the ${\mathrm{H}}^{-}$ ion in a bichromatic field with a 1:3 frequency ratio and various phase differences. The reliability of the present, extremely simple approach is testified to by comparison with much more elaborate earlier calculations.

Complete population transfer between nonresonant tunneling states in a double-well system is predicted to occur when a train of bright or dark laser pulses drives transitions to an excited state in the system. The transition induces the phase shift of the wave packet involved in the transition and alters successive evolutions of the wave packets of the nonresonant states as a result of quantum interference, resulting in complete tunneling.

We report the observation of probe pulse multiphoton ionization (MPI) of dissociating ${\mathrm{D}}_2^{+}$ molecular ions. ${\mathrm{D}}_2^{+}$ is formed by a pump light pulse through MPI of ${\mathrm{D}}_2$ at the equilibrium internuclear separation. The ions then dissociate in a Floquet one- and two-photon channel. MPI of ${\mathrm{D}}_2^{+}$ is observed as a pump-probe delay time-dependent structure in high-resolution ${\mathrm{D}}^{+}$ ion kinetic energy distributions. The dependence on delay time is caused by Coulomb repulsion of the two bare deuterons after ${\mathrm{D}}_2^{+}$ MPI which changes with the internuclear separation where the ions are hit by the probe light pulse. A model calculation shows that in the experiment we probably only observe those ions dissociating in the two-photon channel.

We perform a nonperturbative study of the multiphoton above-threshold detachment of ${\mathrm{H}}^{-}$ in the presence of $1.640-\mu \mathrm{m}$ and $1.908-\mu \mathrm{m}$ laser fields by means of the non-Hermitian Floquet formalism. The laser parameters used are related to the recent experiments [L. Præstegaard, T. Andersen, and P. Balling (unpublished)] on the two-photon detachment of ${\mathrm{H}}^{-}$ near the one-photon threshold. The total and partial (above-threshold) detachment rates as well as the electron angular distributions are calculated for the laser intensities from ${10}^9{\mathrm{W}/\mathrm{c}\mathrm{m}}^2$ to ${10}^{12}\hspace{0.5em}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2.$ It is found that at the weaker intensities (below ${10}^{11}\hspace{0.5em}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2),$ the perturbation theory provides a reasonable description of the two-photon detachment process and the detached electrons are largely in the d state. For higher intensity, however, the process becomes highly nonperturbative in nature. To perform the calculations, we have introduced an exterior complex-scaling–generalized pseudospectral (ECS-GPS) technique for the discretization and solution of the non-Hermitian Floquet Hamiltonian. The ECS-GPS procedure is accurate, simple to implement, and computationally more efficient than the basis-set expansion–variational methods for resonance-state calculations. It also provides a simpler procedure than the uniform complex-scaling method for the calculations of partial rates and electron angular distributions.

We present extensive and accurate calculations for the excited-state spectrum of spin-polarized neutral helium in a range of magnetic field strengths up to ${10}^{12}\hspace{0.5em}\mathrm{G}.$ Of considerable interest to models of magnetic white dwarf stellar atmospheres, we also present results for the dipole strengths of the low-lying transitions among these states. Our methods rely on a systematically saturated basis set approach to solving the Hartree-Fock self-consistent field equations, combined with an “exact” stochastic method to estimate the residual basis set truncation error and electron correlation effects. We also discuss the applicability of the adiabatic approximation to strongly magnetized multielectron atoms.