We study the angular distribution of electrons, emitted in ion-helium collisions, with low velocities relative to the target. We use an expansion of the ionization doubly differential cross section, in Legendre polynomials, with electron velocity-dependent weight coefficients that define the angular shape of the distribution. We determine these coefficients from available experimental data for ${\mathrm{H}}^{+}$, ${\mathrm{He}}^{2+}$, ${\mathrm{C}}^{6+}$, and ${\mathrm{O}}^{8+}$ projectiles at 1- and 1.84-MeV impact energies, and we evaluate them by using the first Born and the continuum-distorted-wave–eikonal-initial-state (CDW-EIS) approximations. The experimental results and CDW-EIS calculations confirm the existence of forward-backward asymmetry in the soft-electron emission peak. We discuss the dependence of theoretical and experimental parameters on the impact energy and the charge of the projectile.

Spin polarizations (SP’s) of elastic electron scattering from alkaline-earth-metal atoms in Ramsauer-Townsend (RT) and low-lying shape resonance (SR) regions are calculated using a relativistic method. The detailed SP distributions both with scattering angle and with electron energy are presented via the energy- and angle-dependent surfaces of SP parameters. It is shown that the SP effects of the collisions of electrons with Ca, Sr, and Ba atoms in the RT region are significant in a considerable area on the energy-angle plane and that the spin-orbit interaction is well increased around the low-lying p-wave SR states of Be and Mg and the d-wave SR states of Ca, Sr, and Ba.

We have observed the photodissociation of ${\mathrm{Mg}}^{2+}$ via the repulsive 1 ${\mathrm{\Sigma }}_{\mathrm{g}}^{2+}$ state in a reflection time-of-flight mass spectrometer. The photodissociation spectrum consists of a broad structureless continuum ranging from ≈580 to ≈690 nm and peaking near 640 nm. A spectral simulation based on theoretical ab initio potential-energy curves agrees well with the experimental profile. We have observed a triple-vector (E⁁-μ⁁-v⁁) correlation in the photofragment time-of-flight spectrum. Analysis of the correlation allows a determination of the anisotropy parameter β and the recoil velocity as a function of the laser wavelength. In the long- and short-wavelength limits of the photodissociation spectrum, the anisotropy parameter attains its maximum value, β=2, characterizing the process as a parallel (1 ${\mathrm{\Sigma }}_{\mathrm{u}}^{2+}$→1 ${\mathrm{\Sigma }}_{\mathrm{g}}^{2+}$) transition followed by a rapid, recoil-like fragmentation. However, excitation in the middle of the spectrum (≈620–660 nm) results in an anisotropy parameter that varies smoothly and significantly from the maximum value, indicating a second dissociation pathway for excitation in this region. Several alternatives for the competing channel are discussed. The measured recoil energy varies from E≈4100 to 7100 ${\mathrm{cm}}^{\mathrm{-}1}$ as the dissociating wavelength is tuned from λ=680 to 590 nm. This allows us to obtain an estimate for the ground-state binding energy ${\mathrm{D}}_0^{\mathrm{″}}$≈10 200±300 ${\mathrm{cm}}^{\mathrm{-}1}$, a result in very good agreement with recent theoretical predictions [G. Durand, J.-P. Daudey, and J.-P. Malrieu, J. Phys. (Paris) 47, 1335 (1986); M. Sodupe, C. W. Bauschlicher, Jr., and H. Partridge, Chem. Phys. Lett. 192, 185 (1992)].

The production of (${\mathit{e}}^{+}$,${\mathit{e}}^{\mathrm{-}}$) pairs is calculated in a nonperturbative approach for the model situation of a one-dimensional square well with a depth varying with time. Detailed energy distributions for the average particle number of electrons and positrons are obtained. In contrast to other (almost) exactly solvable models for pair-creation bound-state effects can be included. Situations with both undercritical and overcritical potentials are considered.

An analytic expression for a potential with the same phase shifts as a given potential but an arbitrarily different bound spectrum is derived with supersymmetric factorizations. The wave functions of the phase-equivalent potential can be calculated from the wave functions of the original potential. From their expressions, a general form of the integral kernel appearing in the Abraham-Moses method for deriving phase-equivalent potentials is established, with which the equivalance between the two approaches is proved. Different particular cases are considered: removing (or adding) one of several bound states at arbitrary negative energies and modifying the energy of a bound state. The similarity between the expressions obtained for removing or adding an excited state with those obtained in earlier works for the ground state leads to an extension of supersymmetric factorizations where potentials which are singular at finite distance are employed in intermediate steps of the calculation.

The influence of relaxation and final-state channel interaction on 3p and 3s photoionization of Ar, ${\mathrm{K}}^{+}$, and ${\mathrm{Ca}}^{2+}$ has been studied by multiconfiguration multichannel Dirac-Fock method. The total relaxation effect was separated into reduction of the ionization energy and into an energy-dependent modification of the photoionization cross section. For 3p subshell the relaxation reduced the cross section at their maxima, near threshold, by 10–20 % and vanished about 100–140 eV above threshold. The relaxation decreased the Ar 3s cross section whereas the ${\mathrm{Ca}}^{2+}$ 3s cross section is enhanced slightly even at large photon energies. The relaxation effect decreases rather slowly with the increasing ionic charge. Our results indicate that the relaxation is nonadditive and of comparable importance with other leading many-electron effects. This emphasizes the necessity of a unified treatment of static and dynamic correlation effects.

We report a measurement of the energy dependence of the branching ratios for autoionization of the (6${\mathit{p}}_{1/2}$${\mathit{nd}}_{5/2}$${)}_{\mathit{J}=\mathit{M}\mathit{J}}$=3 states of barium into the 5${\mathit{d}}_{3/2}$εl continuum channels by monitoring the population in each ${\mathit{m}}_{\mathit{j}}$ state of the 5${\mathit{d}}_{3/2}$ residual ion. We use polarization-sensitive laser-induced fluorescence to detect the ion populations. Near the energy of the (6${\mathit{p}}_{1/2}$${\mathit{ng}}_{7/2}$${)}_{\mathit{J}=3\mathrm{}}$ perturbing resonances, nearly 25% of the doubly excited barium atoms produce the 5${\mathit{d}}_{3/2}$ε${\mathit{h}}_{9/2}$ channel, requiring an exchange of at least three units of orbital angular momentum between the excited electrons.

A direct numerical approach to the solution of the Schrödinger equation in configuration space for electron-hydrogen scattering was developed. For a given total angular momentum and spin, we obtained a set of coupled partial differential equations in two radial coordinates which we solved by a propagation method subject to boundary conditions imposed by symmetry. The full interaction potential was considered and terms up to l=3 in the individual-particle angular momenta were included. The scattering information was extracted by matching the propagating solution to functions of the required asymptotic form, which may be of either S-matrix or K-matrix type. The effectiveness and accuracy of this approach were demonstrated through calculations of electron-hydrogen scattering for total angular momentum L=0 and incident energies below the n=3 threshold. Comparisons with close-coupling methods and other direct numerical methods in this energy range are given.

The plural- and multiple-scattering contribution to the thick-target Rutherford backscattering (RBS) yield was calculated as a part of the single-scattering yield. The method is based on an approximation of the finite total scattering cross section and employs Poisson statistics for the number of collisions along the ionic trajectory. Within a linear approximation, trajectories with different numbers of collisions contribute to the total scattering yield in a way similar to the double-scattering process. The resulting total scattering yield is expressed as a multiple of the single-scattering one, and is suitable for applications in RBS spectroscopy. The validity of the model is limited to systems of light projectiles and heavy targets, to backscattering angles close to 180° and to the high-energy part of the spectra.

Using a high-resolution difference-frequency spectrometer, we have studied the shape of four infrared lines of CO, highly diluted in ${\mathrm{N}}_2$. We have approached the analysis from two perspectives. In the first, the spectral profile of an isolated line is empirically represented by a variation from the ‘‘standard’’ model in which the mass-diffusion constant is replaced with a pressure- and line-dependent ‘‘optical’’ diffusion constant. In the second, we abandon the concept of an ‘‘optical’’ diffusion constant and replace it with a physically more tenable hypothesis that the departures from the standard model are due to a slight deviation from the usual exponential decay used to describe the collision broadening of spectral lines. This shifts the focus of the discussion from an analysis of the translational dynamics to an analysis of the dynamics of the internal degrees of freedom.

In this paper we show the existence of circular dichroism in both integrated as well as differential Auger currents produced in the decay of a vacancy created by the absorption of a circularly polarized photon in a gaseous linear molecule. It arises due to a rotational orientation of the photoexcited molecule or excited photoion. Although Auger electrons in such experiments should necessarily be observed along with their spins, neither the spin-orbit nor spin-rotation interactions, however, need to be taken into account. Circumstances when dichroic effects in such experiments are independent of photoabsorption and/or Auger-emission dynamics have also been clearly brought out by us. We have calculated the magnitude of the circular dichroism for certain realistic cases and have shown it to be of the order of magnitude of the total Auger intensity. It is proposed that circular dichroism in Auger spectroscopy provides a direct and simple method to probe rotational orientation produced by the absorption of circularly polarized light in gaseous linear molecules and to calibrate the degree of circular polarization of electromagnetic radiations over a wide range of photon energy.

X-ray-absorption measurements at the ${\mathit{L}}_3$, ${\mathit{L}}_2$, and ${\mathit{L}}_1$ edges of Hg vapor have been performed using synchroton radiation. The onset of multielectron excitation channels involving 2p (or 2s) and 4f electrons has been revealed. The experimental onset energies are compared with self-consistent-field Dirac-Fock (SCF-DF) calculations and Z+1 estimates, which, in this specific case, were inaccuracte. The importance of these features in x-ray-absorption fine-structure structural data analysis for sixth-period elements is addressed, and the SCF-DF onset energies, covering the elements Hf–Rn, are presented.

A cesium atomic beam having a high degree of polarization is prepared by optical pumping with two circularly polarized diode lasers, and a technique for state-sensitive atomic detection involving laser-induced fluorescence in a magnetic field (LIFB) is introduced. An atomic-beam polarization of 93% is measured using the LIFB technique, and the combined use of optical-state preparation and LIFB detection in an atomic-recoil experiment to study spin-changing electron-atom collisions is demonstrated.

A theoretical study of the coherent interaction of multilevel atoms with a magneto-optical potential is presented. The potential is formed by counterpropagating linearly polarized laser beams whose polarization vectors intersect at an angle cphi and a static magnetic field applied parallel to the laser propagation direction. For a particular ratio of the light and magnetic field amplitudes, the light shift at positions of purely circularly polarized light is equal to the Zeeman splitting. In this case, for a three-level atom, one of the eigenvalues has a triangular spatial form. The diffraction of atoms from this triangular phase grating is an efficient beam splitter. The splitting is symmetric for cphi=90° and asymmetric for cphi<90°. In addition we show that at well-defined positions in the light field, the atom undergoes nonadiabatic transitions and thus by using state-selective detection, one could observe an interference pattern produced by an array of double slits.

Measurements have been made of the absorption spectrum of Ba from the 6s6p ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}_1$ level to states near the ionization threshold in parallel electric and magnetic fields. The absorption spectrum shows oscillations superposed on a smooth background. Each oscillation is correlated with a closed orbit of the electron. At strong electric fields, trajectories are regular, and closed orbits form orderly patterns. For weak electric fields, trajectories are chaotic, and many more closed orbits are present. Many of these are produced by bifurcations as the electric field is reduced.

A longitudinal laser cooling of ion beams at about 5% of the velocity of light has been performed at the Heidelberg Test Storage Ring with various cooling schemes employing the spontaneous force. For a 7.29-MeV ${}_{}{}^9{}_{}{}^{}\mathrm{Be}_{}^{+}{}_{}{}^{}$ beam with an initial longitudinal temperature of 2700 K, the main characteristics of laser cooling in a storage ring are discussed. When undamped, the transverse betatron oscillations of the coasting ions limit the longitudinal temperature after laser cooling to typically 1 K. After damping the transverse motion by precooling the ions with an electron cooler, longitudinal temperatures of below 30 mK have been obtained in the subsequent laser cooling. In this case, the longitudinal ion-beam temperature can be understood as an equilibrium of the laser cooling rate with the heating rate due to intrabeam scattering. Moreover, single binary Coulomb collisions between the (still transversely hot) ions can cause such longitudinal velocity changes that ions are lost out of the critical capture range of the laser cooling force. In these two ways, intrabeam scattering imposes a substantial limit on the temperature or number of laser cooled ions in a storage ring. In our experiments, this process presently limits the ratio between the density-dependent Coulomb energy and the longitudinal thermal energy spread to a value on the order of 1, where liquid rather than gaseous behavior of the ion beam is expected to set in.

Photodissociation probabilities of the ${\mathrm{H}}_2^{+}$ molecule calculated from numerical solutions of the time-dependent Schrödinger equation exhibit minima for high-intensity (${10}^{12}$<I<${10}^{14}$ W/${\mathrm{cm}}^2$) subpicosecond (100 fs) laser-pulse excitation. These minima are shown to correspond to stabilization of the molecular ion and hence to suppression of the photodissociation by the creation of stable laser-induced resonances by a laser-induced avoided-crossing mechanism between the field-molecule potentials. Rotational excitations are shown to destroy such stabilization at intensities above ${10}^{13}$ W/${\mathrm{cm}}^2$. Angular distributions of the photofragments demonstrate gradual molecular alignment with increasing intensity. Bimodal distributions can also occur and they are the signature of the stabilization mechanism.

The polarization dependence of resonant two-photon dissociation characteristics has been investigated for ${\mathrm{HD}}^{+}$ excited from the ground state by two fields of different frequencies. The change in dissociation probability with field intensity and the dissociation line shape have been shown in the case of a single intermediate resonance for all possible combinations of polarizations. The photofragment energy spectra and the branching ratio between two possible continuum-energy states have been investigated for these polarization states when two intermediate resonances are present.

The transfer of energy from the translational to the internal degrees of freedom or vice versa is analyzed for radiatively assisted inelastic collisions among atoms in a gas. For a specific atomic-level configuration, it is shown that the absorption of a laser photon may induce a transition between two levels that are displaced by the collisional interaction (a dipole-dipole potential in this treatment) if a fraction of the kinetic energy of the atomic relative motion is simultaneously absorbed. The process thus results in a cooling of the atomic kinetic temperature. In other level configurations, energy goes in the opposite direction, with a heating of the kinetic temperature. This mechanism is described by means of the Wigner-function formalism, which allows a passing to the quasiclassical limit for the atomic motion in a simple way.

The spectrum of resonance fluorescence of a single trapped and laser-cooled ion is studied theoretically. The quantum motion of the trapped particle manifests itself in the form of narrow motional sidebands in the fluorescence spectrum. For our calculations it is assumed that the ion is confined to dimensions much smaller than the optical wavelength (Lamb-Dicke limit) and the approach is valid for multilevel systems, general trapping potentials, and for both traveling-wave and standing-wave configurations. The motional sidebands in the spectrum have asymmetric amplitudes and this asymmetry is shown to depend on the ion energy, the detector position, and the choice of standing- or traveling-wave laser excitation.