Electron-impact ionization of hydrogen is investigated based upon ab initio quantal calculations. In the present calculations, strong interelectronic correlations are represented by the hyperspherical channel functions and accurate numerical solutions of the two-electron atomic Schrödinger equation are obtained by means of the smooth-variable-discretization method in combination with the $R$-matrix propagation method. The double-continuum boundary condition is represented by matching the numerical solutions to asymptotic solutions, which are described by superpositions of approximate asymptotic channel functions. We obtained the ionization threshold law, which is in good agreement with Wannier’s conjecture, and also an almost uniform energy distribution in double-continuum states at low energies, say $\sim 0.1$ a.u. At low energies, the angular distributions of the electrons in double-continuum states of ${}^1{S}^e$ localize where the interelectronic angular distance ${\theta }_{12}=\pi$. As the energy increases, the binary-encounter and the dipolelike transition mechanisms manifest themselves in the angular distribution. The spin-averaged total ionization cross section and the spin asymmetry from the present method agree well with experimental measurements as well as the convergent-close-coupling result, while for the spin asymmetry there is a noticeable disagreement with the hidden-crossing result at moderately low energies. An essential role of the potential ridge during the ionization process is apparent in the convergence of the present calculations.

Single-ionization cross sections and final-state momentum distributions have been calculated for the single-ionization collision reactions H${}^{+}$, $\overline{p}$, He${}^{2+}$, C${}^{4+}$, and C${}^{6+}$ + He at 100 keV/u. The classical trajectory Monte Carlo method has been used with a Wigner distribution to improve both the He electron classical radial and momentum distributions. Final-state ionized electron momenta are projected into a collision plane defined by the initial projectile momentum and the final transverse component of the He${}^{+}$ recoil momentum, with the results displayed differentially in various components of electron and recoil momenta. Positively charged projectile impact shows that the soft electrons drift with the recoil ion, while higher-energy electrons move with the projectile and away from the recoiling target. The electron capture to the continuum cusp is present for hard collisions, with asymmetry in both the collision plane and along the longitudinal momenta. Antiproton impact results in soft electrons drifting backward, away from the recoil ion, with the binary-encounter ring clearly observed.

Cross sections for excitation to low-lying states and ionization of ${\mathrm{He}}^{+}$ by antiproton impact are calculated in the energy range 1–500 keV by solving the time-dependent Schrödinger equation on a numerical lattice. The results are compared with those of other theoretical approaches. Such comparisons allow a strenuous test of the lattice Schrödinger-equation approach for this fundamental collision system.

Absolute total cross sections for electron-impact excitation of the ${3s}^2{}^{1}S\to 3s3p{}^{3}P$ and ${3s}^2{}^{1}S\to 3s3p{}^{1}P$ transitions in ${\mathrm{Si}}^{2+}$ were measured using the merged electron-ion beams energy-loss technique. The results are compared to $R$-matrix close-coupling theory, which predicts a strong resonance enhancement of the cross section near the threshold for excitation of the ${}^{3}P$ state and this is confirmed by the experiments. The observed disagreement between theory and experiment for the dipole excitation is suggested to be due to resonance interference.

We have carried out measurements of absolute photoionization cross sections of the excited ${\mathrm{nd}}^2{D}_{3/2,5/2}$ states of cesium for principal quantum number $n=7\mathrm{}$ and 8. Absolute cross sections have been determined experimentally using the technique of measuring depletion in the fluorescence signal from the excited states of atoms due to photoionization. The measurement was made with a linearly polarized ionization laser light at 532-nm wavelength. Photoionization cross sections have been determined as $(3.5\mathrm{}\pm 1.4)\times {10}^{-18}$ cm${}^2$ and $(3.1\mathrm{}\pm 1.2)\times {10}^{-18}$ cm${}^2$ for the ${7d}^2{D}_{3/2}$ and ${7d}^2{D}_{5/2}$ states, and $(2.4\mathrm{}\pm 1.0)\times {10}^{-18}$ cm${}^2$ and $(1.6\mathrm{}\pm 0.6)\times {10}^{-18}$ cm${}^2$ for the ${8d}^2{D}_{3/2}$ and ${8d}^2{D}_{5/2}$ states, respectively. We also propose an alternative method to measure the absolute cross sections for photoionization from highly excited Rydberg states, which is based on the methodology of the fluorescence depletion technique.

We present an easy to use expression for cross sections of electron-impact-induced ${1s}^2\mathrm{nl}\to {1s}^2{n}^{\prime }{l}^{\prime }$ excitation transitions with $2<~n<~n^{\prime }<~4$ in multiply charged ions of lithium isoelectronic sequence. This expression is based on our computations by convergent close-coupling (CCC) and Coulomb-Born with exchange and normalization (CBE) methods. We show scaling of the CCC and CBE cross sections with atomic number $Z$ and use this scaling for presentation of the cross-section data. For $6<~Z<~30$ the scaling is accurate to better than $\pm 20\%$ at any energy except in the vicinity of resonances. Contributions from indirect excitation channels do not scale with $Z;$ however, for calculation of excitation rates it is enough to average locally these contributions over energy and to take them into account in a frame of a general scaling-based expression for the cross sections. For excitation rates, total inaccuracy caused by all simplifications in the cross-section presentation is likely to be less than $\pm 30\%$ even for most risky cases. This assessment is based on comparison of excitation rates, computed using our scaling-based expression for the cross sections, with the excitation rates, computed using high-resolution (CCC and $R$ matrix) cross sections and experimental data.

Near-threshold photoelectron recapture due to postcollision interaction with a single Auger electron is examined within the quantum-mechanical framework of radiationless resonant Raman scattering. Model calculations using a hydrogenic approximation are performed, revealing general features of the escape probability as a function of inner-shell decay width Γ. The escape probability is useful in estimating diagram and spectator Auger intensities near threshold. The process of recapture produces a delay in the onset of double ionization to energies above threshold, which scales as ${\Gamma }^{2/3}.$ The present model is also applied to the cross section for ejection of photoelectrons with zero kinetic energy (ZKE), assuming a stable final state. A comparison with semiclassical results at small Γ suggests that existing semiclassical distributions are formally correct, with further convolution suggested by Thomas et al. [J. Phys. B 29, 3245 (1996)] not required. Results of the present quantum ZKE model, which does not include additional cascade effects, are in agreement with existing data. Quantities that describe the escape and ZKE-emission distributions are identified and their variation with Γ is examined.

The complete momentum spectra for single and double ionization of He by $1-\mathrm{GeV}/u$ $(\beta =0.88)$ ${\mathrm{U}}^{92+}$ have been investigated using a classical trajectory Monte Carlo method corrected for the relativistic projectile. The ${1/r}_{12}$ electron-electron interaction has been included in the post-collision region for double ionization to incorporate the effects of both the nuclear-electron and electron-electron ionizing interactions, and to access the effects of electron correlation in the electron spectra. Experimental measurements were able to determine the longitudinal momentum spectra for single ionization; these observations are in accordance with the theoretical predictions for the three-body momentum balance between projectile, recoil ion, and ionized electron. In particular, the Lorentz contraction of the Coulomb interaction of the projectile manifests itself in the decrease of the post-collision interaction of the projectile with the electron and recoil ion, causing them to recoil back-to-back as in the case for a short electromagnetic pulse. This feature is clearly displayed in both the theoretical and experimental longitudinal momentum spectra, and by comparing to calculations that are performed at the same collision speed but do not include the relativistic potentials. Moreover, collision plane spectra of the three particles demonstrate that the momenta of the recoil ion and ionized electron are preferentially equal, and opposite, to each other. The electron spectra for double ionization show that the inclusion of the electron-electron interaction in the post-collision regime partitions the combined ionization momentum of the electrons so that the electrons are preferentially emitted in opposite azimuthal angles to one another. This is in contrast to calculations made assuming independent electrons.

Electron-impact excitation of Sr from metastable states is studied in the framework of an elaborate close-coupling approximation, using accurate target wave functions. An interpretation is given of measured cross sections, including their near-threshold resonance structure. The cross sections for transitions induced from the $5s5p{}^{3}P$ metastable level to the $5s5d{}^{3}D,$ ${5p}^{23}P,$ $5s6s{}^{3}S,$ $4d5p{}^{1}D,$ $5s4f{}^{3}F,$ $4d5p{}^{3}F,$ and $5s6p{}^{3}P$ levels are calculated in Born-Ochkur and two-state and six-state close-coupling approximations. We also consider transitions from the ${5s}^{21}S$ ground state to these levels. The calculations are based on a nonrelativistic Hamiltonian. In the configuration-interaction approximation, accurate wave functions are used for the outer electrons, with a frozen core. The results are found to be in good agreement with the most recent experimental data.

Electron-impact excitation cross sections from the low-lying ${1s}^2{2s}^2{2p}^5$ state of F-like selenium to singly excited states have been calculated. Our relativistic distorted-wave Born procedures have been used for the present calculations. Instead of atomic structure code GRASP, the latest version GRASP${}^2$ code is used as multiconfiguration Dirac-Fock atomic structure calculations. The present results have been comprehensively compared with earlier calculations. One of the motivations for the present work is that there are some discrepancies between our results and those of others.

Detailed, materials-specific stopping calculations for ultrathin films exhibit a scaling with layer number $N$ which would be expected for asymptotic $N$ but not small $N$’s: a bulk contribution plus a term linear in $1/N$ fit the calculations extremely well. We derive this scaling for a jellium model with a general spatially varying density profile. The model gives the magnitude of the $1/N$ term correctly as well as the scaling with kinetic energy of that contribution to the stopping. We also show that the loss to surface excitations is larger than the $1/N$ term for $v>~0.7$ (atomic units) and calculate the projectile velocity dependence of the apparent position of the surface plane.

Slow ${(E}_{\mathrm{kin}}=4\mathrm{}\mathrm{}\mathrm{keV})$ highly charged ${\mathrm{Ar}}^{q+}$ $(6\mathrm{}<~q<~13)$ ions were incident at $25°$ on a Au(111) single crystal surface. The ions scattered at an angle of $75°$ were analyzed in energy and charge state. When electrons are removed from the $L$-shell $(q>~9)$ of the incoming Ar ion the yield of multiply charged scattered ions (MCSI) increases by about 3 orders of magnitude. The yield of MCSI increases monotonously with an increasing number of initial $L$-shell vacancies. The experimental results are compared with and interpreted by a model calculation including a side-feeding process into Ar inner shells, recapture to the surface and Auger transitions after the ion-surface interaction.

The stopping of swift nonrelativistic heavy ions has been analyzed theoretically as a function of the projectile charge $q_1.$ Excitation and ionization of target electrons are described within Bohr’s classical theory and the limitations toward Bethe’s quantal description are outlined. For oxygen ions in carbon the Bethe regime is found to be confined to the velocity range where the ion is essentially stripped in charge equilibrium. The effect of projectile electrons is taken into account mainly via screening of the Coulomb interaction, but excitation of projectile electrons is discussed briefly. The significance of projectile screening hinges on a parameter $s$ that depends on projectile and target parameters and varies substantially over the electron shells of the target. Calculated fixed-charge stopping cross sections agree well with measured values in absolute magnitude and in their variation with charge state. $q_1^2$ scaling frequently has been assumed in the literature. A continuous transition is predicted between such scaling and a stopping power that is almost independent of the projectile charge.

Dynamic properties of hollow Ne atoms moving in shallow layers of an Al(111) solid were studied using the method of Auger spectroscopy. The $K$ and $L$ Auger electrons of the projectiles were measured for various impact energies in the range of 0.1–22.5 keV. Structures in the Auger spectra are reanalyzed by means of theoretical $\mathrm{KLL}$ Auger energies including solid-state effects. For projectile energies below 1 keV the structure of the $\mathrm{KLL}$ maximum changes significantly with the impact energy due to the onset of the charge transfer cross sections between target and projectile $L$ shell. The data were interpreted by an extension of our previously developed cascade model providing specific information about the filling state of the hollow atom during the $K$ Auger transition. In addition $L$ Auger intensities measured in a wide projectile energy range are theoretically reproduced. The separation of the different processes which occur during the electron emission allows us to verify the importance of the capture process and the $L$ Auger process. Finally, the depth of the $L$ and $K$ Auger emission is determined in the projectile range studied.

In analogy to the well-known diffraction of waves at spatial structures, one can also define diffraction in time when a wave is temporarily modulated. In the present paper we investigate this phenomenon starting from a Green’s-function approach. Diffraction in time appears for Schrödinger waves and not for light in vacuum. Specific cases of diffraction in time investigated here are diffraction at an “edge in time” and a Fresnel lens in time. We then investigate in detail the simultaneous diffraction both in space and in time representing a general solution. Then we analyze in detail diffraction at an edge, both in space and in time, of a single slit in space and in time, a double slit in space, and a single slit in time, a single slit in space and a double slit in time, and, finally, a double slit both in space and in time. In all cases we analyze the possibilities of various approximations which can be made, and show the limits and validity of the Fraunhofer approximation both in space and in time. We give explicit results for a gedanken experiment with very cold atoms.

A simple model for atom optical elements for Bose-Einstein condensate of trapped, dilute alkali-metal atoms is proposed, and numerical simulations are presented to illustrate its characteristics. We demonstrate ways of focusing and splitting the condensate by modifying experimentally adjustable parameters. We show that there are at least two ways of implementing atom optical elements: one may modulate the interatomic scattering length in space or, alternatively, use a sinusoidal, externally applied potential.

We consider extensions of the twin-trap Bose-Einstein condensate system of Javanainen and Yoo [Phys. Rev. Lett. 76, 161 (1996)] to include pumping from thermal atom baths as a simple example of an inexhaustible source of coherent atoms. We study this system for two forms of thermal pumping, both with and without the influence of interatomic collisions. In the presence of pumping, the detection process is still able to stabilize the relative phase of the condensates against the diffusion induced by the collisions. We also examine the effects of pumping on the phenomenon of collapses and revivals of the relative phase between the condensates.

The density of states of a Bose-condensed gas confined in a harmonic trap is investigated. The predictions of Bogoliubov theory are compared with those of Hartree-Fock theory and of the hydrodynamic model. We show that the Hartree-Fock scheme provides an excellent description of the excitation spectrum in a wide range of energy, revealing a major role played by single-particle excitations in these confined systems. The crossover from the hydrodynamic regime, holding at low energies, to the independent-particle regime is explicitly explored by studying the frequency of the surface mode as a function of their angular momentum. The applicability of the semiclassical approximation for the excited states is also discussed. We show that the semiclassical approach provides simple and accurate formulas for the density of states and the quantum depletion of the condensate.

Qualitative and quantitative results of a complete relativistic calculation of the $S$-matrix transition amplitudes and first Born cross sections for the Mott scattering of an electron in the presence of an ultraintense single-mode laser field are compared in detail to a spinless and a nonrelativistic treatment. The role of the fermion character of the electron leading to spin-orbit and spin-laser interactions is discussed depending on the laser intensity and on the incoming electron kinetic energy. The differences between radiative transfer spectra of the electron energy in the relativistic and nonrelativistic regimes are addressed.

Ultrafast state-selective vibrational excitation and dissociation controlled by shaped subpicosecond infrared laser pulses is investigated within the reduced density matrix formalism beyond a Markov-type approximation for diatomic molecules, which are coupled to an unobserved quasiresonant environment. Dissipative quantum dynamics in a classical electric field is simulated for discrete vibrational bound states and for dissociative continuum states of a one-dimensional dissociative Morse oscillator, tailored to the local OH bond of the ${\mathrm{H}}_2\mathrm{O}$ and HOD molecules in the electronic ground state. Flexible laser control schemes are developed and demonstrated on a picosecond time scale, which enable one either to localize the population at prescribed high-lying discrete vibrational levels of OH, up to those close to the dissociation threshold, with the probability up to 70–80 % without substantial dissociation or, alternatively, achieve the dissociation yield of about 75%, while the strength of the quasiresonant molecule-environment coupling results in subpicosecond lifetimes of the vibrational bound states. The optimal laser control schemes may include the superposition of up to four subpicosecond laser pulses.