A transition amplitude for resonant transfer and excitation (RTE) in a crystal channel is derived and compared with observations of Belkacem et al. [Phys. Rev. Lett. 64, 380 (1990)] of titanium ions channeled in a thin gold crystal. Using a one-dimensional tight-binding model of the gold lattice, the RTE cross section is shown to be proportional to the Compton profile of atomic gold multiplied by a crystal-structure factor, analogous to multislit interference of light with diffraction. It is suggested that Belkacem et al. observed a single (principal) maximum with a very narrow width determined by the crystal’s Fermi energy. It is found that the observed shift in the RTE peak, relative to that expected for a quasifree crystal electron, can be accounted for by a change in the Coulomb energy of the transferred electron caused by the highly charged ‘‘impurity’’ ion prior to transfer.

Analytical expressions describing double-scattering intensities of the Compton and Rayleigh effects (Compton-Compton, Compton-Rayleigh, Rayleigh-Compton, and Rayleigh-Rayleigh contributions), are deduced in the framework of the transport theory for an infinitely thick sample irradiated with collimated monochromatic radiation. An orders-of-interaction solution of the integro-differential Boltzmann equation for unpolarized photons is used to separate the multiple-order terms. Interaction kernels for coherent and incoherent scattering include atomic form factors describing the effect of the electronic distribution in multielectron atoms. The total attenuation coefficient of the target takes into account, besides the mentioned scattering processes, the photoelectric effect, important in the x-ray regime. First-order Compton and Rayleigh interactions give monochromatic peaks according to the theoretical model that neglects electron motion. In contrast, Compton-Compton, Compton-Rayleigh, and Rayleigh-Compton contribute asymmetric continuous spectra whose wavelength breadths are the DuMond width and 2${\lambda }_{\mathit{C}}$ (${\lambda }_{\mathit{C}}$ is the Compton wavelength), respectively. Single- and double-scattering intensities of the Rayleigh and Compton effects are computed for pure and composite materials as a function of the excitation energy and the angular orientations of the incident and take-off beams. Since absorption in the target is considered, computations can be straightforwardly compared with experimental data and with realistic Monte Carlo simulations. The agreement is good for low excitation energies because the second-order term remains dominant in multiple scattering and bremsstrahlung emission is weaker. However, for higher excitation energies the probability of higher orders of multiple scattering increases, and they cannot be neglected. Although analytical calculations are performed up to the second order in this work, a Monte Carlo simulation is used to show the importance of higher orders in light elements.

Collisions of trapped atoms occur over time intervals that are often longer than radiative lifetimes. This leads to radiative interruption of excited-state collision dynamics and constrains collisional interactions to atom pairs that are in close proximity at the time of excitation. Using classical dynamics I show that this causes Na associative ionization to be dominated by a four-step process in which the longer-range ${\mathrm{Na}}^{\mathrm{*}}$-Na interaction initiates inward motion that is completed as ${\mathrm{Na}}^{\mathrm{*}}$+${\mathrm{Na}}^{\mathrm{*}}$→${\mathrm{Na}}_2^{\mathrm{°}}$+e. Thus low-temperature associative ionization does not result from ${\mathrm{Na}}^{\mathrm{*}}$+${\mathrm{Na}}^{\mathrm{*}}$ collisions. I predict strong single- and two-wavelength dependences, which have a molecular character due to the close proximity at the time of photoabsorption. These are amenable to experimental observation. I also calculate saturation that results from the depletion of interacting pairs. This produces very-low-power saturation and explains the associative ionization measurements of Gould et al. [Phys. Rev. Lett. 60, 788 (1988)].

We have populated the double circular Ba 4f5g levels using multistep laser excitation from the ground state. The observed autoionization rates of these levels are quite low, which may indicate strong electron correlations. We present a semiclassical picture that explains the long lifetimes of these states without quantum-mechanical correlation effects. In addition to the 4f5g states, we have also observed higher-lying 4fng states. In particular, we have observed the mixing of the bound 4f6g and 4f7g levels with the 6${\mathit{d}}_{\mathit{j}}$nf and 6${\mathit{d}}_{\mathit{j}}$nh series indicating direct orbital angular momentum exchange between the two valence electrons. We observe orbital angular momentum exchange between bound electrons in which the initial and final states of both electrons can be identified.

We have used two-photon excitation of Ba 6snl Rydberg states to create Ba 6djnl autoionizing states. The excitation of the initial Rydberg states is performed using two, three, or four tunable dye lasers for l=2, 3, and 4 respectively. We have used very efficient laser pumping schemes for population of the 6snf and 6sng series. The 6djnl states exhibit a wide range of interseries interaction from large to negligible channel couplings. Furthermore, the quantum defects of all of the series have a pronounced energy dependence even in the absence of any coupling to other 6dj core channels. The perturbing series that is the source of the energy variation of the quantum defects is believed to belong to one or more 4fj′ core configurations. Each 6dnl series has been fit using a multiconfiguration quantum-defect theory model, and the results of the fits are generally in good agreement with the observed spectra.

We consider the transition probability for two-level quantum-mechanical systems in the adiabatic limit when the Hamiltonian is analytic. We give a general formula for the leading term of the transition probability when it is governed by N complex eigenvalue crossings. This leading term is equal to a decreasing exponential times an oscillating function of the adiabaticity parameter. The oscillating function comes from an interference phenomenon between the contributions from each complex eigenvalue crossing, and when N=1, it reduces to the geometric prefactor recently studied.

The complex Kohn method is used to study electron-methane scattering with polarized trial functions at incident energies ranging from 0.2 to 10 eV. A perturbative method, which only requires the construction of Fock operators, is used to generate a set of polarized virtual orbitals. These polarized orbitals are then used to construct a compact ab initio scattering function that produces cross sections that are in excellent agreement with experiments in the region of the Ramsauer-Townsend minimum and at higher energies, where a broad maximum is present in the integral cross section. This is the first ab initio study to accurately characterize low-energy electron-methane scattering.

Angular distributions of fragment ions produced in the dissociative double photoionization of OCS have been studied in the photon energy region 37–100 eV with the use of synchrotron radiation and photoion-photoion coincidence. The asymmetry parameter β ranges from 0.01 to 0.48 for the three major dissociation channels ${\mathrm{OC}}^{+}$+${\mathrm{S}}^{+}$, ${\mathrm{O}}^{+}$+${\mathrm{S}}^{+}$+C, and ${\mathrm{C}}^{+}$+${\mathrm{S}}^{+}$+O of ${\mathrm{OCS}}^{2+}$. The observation of an anisotropy in the direct double photoionization from valence orbitals is discussed in relation to the symmetry consideration involved in the transition and the dynamics of the dissociation.

Laser-induced-fluorescence spectroscopy has been applied to neon and helium-neon discharge plasmas in magnetic fields lower than 10 T. Disalignment-rate coefficients of the neon 2${\mathit{p}}_2$ and 2${\mathit{p}}_7$ atoms (2${\mathit{p}}^5$3p configuration, Paschen notation) have been determined for neon and helium collisions in a field strength that is lower than the critical field at which the collision time is equal to the inverse Larmor frequency. The 2${\mathit{p}}_7$ atoms have rate coefficients independent of the magnetic-field strength, whereas the 2${\mathit{p}}_2$ atoms have a field-dependent rate coefficient for neon collisions.

A relativistic model for differential electron-impact-ionization cross sections that allows the energy spectrum of secondary electrons to be calculated over a wide range of primary-electron energies is presented. The semiempirical method requires only experimental total ionization cross sections and optical oscillator strengths for the target species of interest, but other information, if available, can be incorporated to make the formulation still more accurate. Results for ionization of helium indicate that the lower limit on primary-electron energy for application of the model is about 100 eV. The simple analytic form of the model facilitates investigation of the regions of the secondary-electron energy spectrum where relativistic effects are important.

We report the results of an ab initio variational treatment of electron-impact cross sections for the process ${\mathit{e}}^{\mathrm{-}}$+${\mathrm{HeH}}^{+}$→${\mathit{e}}^{\mathrm{-}}$+${\mathrm{He}}^{+}$+H for electron energies between 21 eV, the threshold for excitation of the dissociative 1 ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ state, and 26 eV, corresponding to the excitation energy of the first ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ excited state. The calculations, which were carried out using a recent modification of the complex Kohn method, employed accurate correlated target wave functions, as well as ab initio optical potentials to incorporate the effect of closed channels. The fixed-nuclei excitation cross sections were found to be dominated by a series of sharp resonance structures. However, when the cross sections are averaged over the Franck-Condon envelope of the ground vibrational state, these sharp features are no longer seen. We conclude that the recent experiments that report structure in the ${\mathrm{HeH}}^{+}$ dissociation cross section cannot be explained within the context of traditional adiabatic-nuclei theory.

The effects of excitation-autoionization have been studied for nine sodiumlike ions with 26≤Z≤92. Relativistic-distorted-wave methods were used to calculate cross sections for direct electron-impact ionization of these ions. Relativistic-distorted-wave cross sections were computed for electron-impact excitation of n=2 electrons to n=3 levels of the ions, and detailed relativistic Auger rates and radiative rates were calculated in order to obtain the excitation-autoionization cross sections. We investigated the variation of these cross sections along the Na-like isoelectronic sequence. We found that configuration interaction significantly affects some cross sections in ions where level crossings occur for some of the doubly excited intermediate states.

We consider paths to the observation of the ultraheavy, negatively charged particle, a possible remnant from the early Universe. In particular, we study the reactions in a deuterium environment.

Cross sections have been calculated for electron transfer into all states of hydrogen and for ionization in collisions between protons and helium atoms at proton energies from 50 to 200 keV. Cross sections for electron transfer into individual states up to 3${\mathit{d}}_2$, as well as the z component of the average electric dipole moment and of the cross product between the angular momentum and the Runge-Lenz vector for the n=2 and 3 levels, have also been determined. A coupled-state approach is taken using bases of about 50 Sturmian functions, extending earlier work on one-electron systems. The full two-electron interaction is included, neglecting, however, interatomic exchange. Detailed tests of convergence with respect to the size of basis have been carried out. It is found that the results for capture into all states and the individual states 1s, 2s, and 3s, as well as the results for ionization, are converged to at least about 10%. There is excellent agreement with experimental results at least at the highest energy, where interatomic exchange is unimportant. Quantities involving p and d levels, on the other hand, are much more sensitive to the size of basis, and the agreement of the present and other coupled-state results with experimental results is less satisfactory.

A crossed-electron-ion-beam experiment was used to measure the total ionization cross section for ${\mathrm{Ar}}^{8+}$ in the energy range from 50 to 600 eV. The present results are roughly in agreement with the previous measurements of Defrance et al. [Nucl. Instrum. Methods Phys. Res. B 23, 256 (1987)] above 450 eV. However, the present measurements show a significant contribution below the threshold for direct ionization. The present measurements agree with the distorted-wave calculations of Pindzola et al. [Phys. Rev. A 41, 1375 (1990)] when 3% of the incident ion beam is assumed to be in metastable states and excitation autoionization from metastable states is included.

L-shell x-ray production cross sections by protons are reported for elements with x-ray energy between 704 and 1188 eV. The x rays were measured with a windowless Si(Li) detector. The efficiency of this detector, which is one of the major experimental problems for very-low-energy x-ray measurements, was determined by comparing spectra of atomic-field bremsstrahlung induced by 66.5-keV electrons on aluminum, silver, and gold with the theoretically known distributions. The x-ray production cross sections are compared with the first-order Born and perturbed-stationary-state theory with energy-loss, Coulomb deflection, and relativistic corrections of inner-shell ionization. Except for systematic but slight overpredictions of the data at the lowest proton velocities, both theories fit the measured data well.

The differential cross sections for the emission of Ll, L${\mathrm{\alpha }}_2$, Lα, Lβ, and Lγ groups of L x-ray lines induced in Au by 59.57-keV photons have been measured at different angles varying from 40° to 120° at intervals of 10°. The L${\mathrm{\alpha }}_2$ line has been isolated by using preferential absorption of the Lα line in a Zn absorber of suitable thickness. The L x rays represented by the Ll, L${\mathrm{\alpha }}_2$, and Lα peaks are found to be anisotropic in the spatial distribution while those in the Lβ and Lγ peaks are isotropic. The Ll peak has greater anisotropy than the Lα peak. The anisotropy of L${\mathrm{\alpha }}_2$ line is opposite to that of L${\mathrm{\alpha }}_1$ line. The present results contradict the predictions of Cooper and Zare [Atomic Collision Processes (Gordon and Breach, New York, 1969)], that the vacancy state produced after photoionization is not aligned irrespective of its J value, but confirm those of Flugge, Mehlhorn, and Schmidt [Phys. Rev. Lett. 29, 7 (1972)] that the vacancy state with J>1/2 is aligned.

We write a transport equation for the electrons produced when atomic or ionic beams traverse a foil. We consider that the electrons are produced inside the solid by the electron loss and capture-to-continuum mechanisms. They lose energy by inelastic collisions with the target atoms and are deflected by elastic scattering. We study the electron distribution for ${\mathrm{H}}^0$ and ${\mathrm{H}}^{+}$ projectiles on carbon foils for different thicknesses. We obtain a description of the electron-velocity spectra in the forward direction that agrees with the experiments. Studying the ratio between the intrinsic convoy-electron height and the nonconvoy-electron or ‘‘background’’ intensity, at convoy-electron velocity, we obtain a constant behavior for different target thicknesses.

The Feynman diagram method for atomic collisions developed by us has been applied to reactive collisions between alkali-metal atoms and halogen molecules. The velocity-averaged differential and total cross sections for the production of alkali halide molecules have been calculated and compared with available experimental results.

We have measured the continuum electron yield in coincidence with the charge state of the recoiling target ion for the collision systems ${\mathrm{He}}^{+}$→Ne,Ar (170≤${\mathit{E}}_{\mathit{p}}$≤330 keV/u). The target ions were detected with a standard time-of-flight spectrometer employing a pulsed electric field to avoid distorting the trajectory of the continuum electron. Comparing our results with cross sections for single- and multiple-target ionization given by DuBois [Phys. Rev. A 39, 4440 (1989)] we find a similarity in the relative proportions of the recoiling charge states for argon at lower energies, but not for neon. Additionally, the yield of continuum electrons associated with each particular charge state of the recoil target ions falls off more rapidly with increasing projectile energy than might be expected.