The triply differential cross section for Compton scattering from atomic electrons is obtained numerically in a full relativistic second-order $S$-matrix calculation based on the independent particle approximation. We compare our results with the results of more approximate approaches. Special attention is paid to the validity of the impulse approximation (IA), which has often been used for calculating the doubly differential cross section even when the photon momentum transfer K is similar to the average momentum $p_{\mathrm{av}}$ of the bound electron, which is ionized (and IA is found to be fairly accurate even in such circumstances). We here show that, on the contrary, IA calculations of the (less averaged) triply differential cross section are quite inaccurate for $|\mathbf{K}|\sim p_{\mathrm{av}},$ even near the peak in the triply differential cross section (where the free kinematics for scattering from an initial free electron at rest are satisfied and where IA should work the best). We conclude that electron momentum distribution determination through the Compton profile, using the doubly differential cross section, is more accurate at lower energies than direct determination through the measurement of the triply differential cross section at the same energy. In addition, viewing the total cross section for double ionization in Compton scattering as another observable less averaged than the doubly differential cross section in single ionization, we estimate that IA predictions of the total cross section for double ionization in Compton scattering from Helium are adequate above about 50 keV.

Target ionization and projectile charge changing were investigated for $0.5–8-\mathrm{MeV}/q\hspace{0.5em}{\mathrm{Li}}^{q+}+\mathrm{He}$ $\mathrm{}(q=1,2,3\mathrm{})\mathrm{}$ collisions. Cross sections for the single and double ionization of He associated with no projectile charge change (direct ionization), single-electron capture, and single-electron loss were measured using coincidence techniques and compared to existing experimental and theoretical results. Total cross sections for single-electron capture (for ${\mathrm{Li}}^{1,2,3+}$) and single-electron loss (for ${\mathrm{Li}}^{1,2+}$) were also obtained. For a given incoming projectile charge state the double-to-single target ionization ratios $R$ vary strongly with the outgoing reaction channel. For direct ionization, $R$ is nearly independent of the incident projectile charge state and can be described by the semi-empirical scaling rule of Knudsen et al. [J. Phys. B 17, 3545 (1984)]. However, for ionization associated with single-electron loss and single-electron capture by the projectile, $R$ depends quite strongly on the incident charge state of the projectile. These ionization ratios $R$ are interpreted in terms of theoretical formulations involving electron-nucleus interactions and electron-electron interactions.

The present work deals with single electron capture processes from ${\mathrm{H}}_2$ targets by impact of heavy bare ions at intermediate and high collision energies. Within the distorted wave formalism, the one active electron model developed to describe collisions with multielectronic atomic targets is used for the case of ${\mathrm{H}}_2$ targets. Three different distorted wave models are employed: the continuum distorted wave, the continuum distorted wave–eikonal initial state, and the continuum distorted wave–eikonal final state. Using these distorted wave approximations, total cross sections for ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{2+}$ impact are calculated employing two different representations of the initial bound molecular wave function. The variation of the total cross sections due to this change in the description of the initial wave function and due to changes of the distortion factors in the entrance and exit channels is studied. The theoretical results are compared with available experimental data.

Recently, Robicheaux and Pindzola [Phys. Rev. Lett. 79, 2237 (1997)] reported on model calculations of dielectronic recombination (DR) in the presence of crossed electric and magnetic fields. They showed that the enhancement of the DR by an electric field may be increased further when a magnetic field perpendicular to the electric field is present in the collision region. In this paper, we describe the results of distorted-wave calculations of dielectronic recombination in the presence of crossed electric and magnetic fields for Li-like C${}^{3+}$ and Si${}^{11+}$. Two sets of calculations are performed and compared. The first is based on a full intermediate-coupling (IC) calculation, while the second is based on a much simpler configuration-average (CA) approximation. Both sets of calculations predict substantial added enhancement of DR due to the magnetic field. However, the CA approximation overestimates the field-enhanced DR as compared to both the IC calculations and previous measurements of total recombination. Comparisons of our IC results with total recombination measurements are not possible because the IC Hamiltonian matrices are too large for the high values of $n$ included in these experiments; however, in anticipation of possible high-resolution experimental studies of partial DR in fields, we report on an IC calculation of the crossed-fields enhancement of DR as a function of electric-field strength for $n$=24 in Si${}^{11+}$.

Triple differential cross sections have been measured for the electron-impact ionization of the $3d$ orbital of krypton. The measurements have been performed in asymmetric coplanar geometry at incident energies around 1 keV, and ejected electron energies of 72 eV (corresponding to bound Bethe ridge conditions) and 50 eV. Both of these energies are significantly below the binding energy of the $3d$ orbital of approximately 94 eV. The measured cross sections are compared with calculations performed in the distorted-wave Born approximation and the distorted-wave impulse approximation. There is quite good agreement in the former case, but no agreement in the latter, despite kinematics which should emphasize the impulsive nature of the collision.

Angle-resolved Auger electron spectra have been measured at the Cl ${2p}_{3/2}$ to ${\sigma }^{*}$ and $4s\sigma$ resonances of the HCl molecule. The ${2p}_{3/2}^{-1}{\sigma }^{*}$ state can dissociate before electronic deexcitation, resulting in Auger decay in the core-excited chlorine atom. The asymmetry parameters ${\alpha }_2$ of these atomic decay lines are similar to those of the normal Auger transitions in isoelectronic Ar. The molecular decay features at both the HCl resonances also show anisotropy which is different in each case despite the states of the same symmetry. Their angular distribution appears to have no correlation with the spectra of argon.

Convoy electron spectra have been measured in coincidence with specularly reflected ions in grazing-incidence collisions of carbon ions with a silicon (100) surface. Absolute yields of convoy electrons have been determined experimentally and compared with the results obtained from a classical trajectory Monte Carlo simulation. By comparing the velocity dependence of the convoy yields with theory, we are able to assess the relative contribution to convoy emission from valence and core electron capture to continuum states of the projectile.

We measured the energy loss of 10-, 16-, and 25-MeV oxygen ions in a thin $\left(7.5\mathrm{}\mu {\mathrm{g}/\mathrm{c}\mathrm{m}}^2\right)$ carbon foil as a function of input charge state $(q=4\mathrm{}–8).$ At this thickness, charge-state equilibrium is nearly attained, but the energy losses accumulated over the ion’s trajectory are a function of its initial charge state. Using a set of pertinent microscopic atomic collision cross sections computed using the classical trajectory Monte Carlo method, we link these parameters to the observed charge-state dependence on energy loss through a classical transport simulation. This simulation also leads to a prediction of charge-state equilibrium fractions.

The present work reports the experimental evidence of anomalies exhibited by the energy loss and energy straggling of channeled protons in silicon in transmission measurements versus the incident angle. Results are presented for 1.0-MeV protons channeled along the 〈100〉 axis for a silicon foil of 3.8 μm thickness. It is shown that the transition from random to a channeling condition is discrete. The energy spectra of transmitted ions show a random peak (lower energy) and a channeled peak (higher energy). The random peak has a fixed energy, while the energy of the channeled peak increases as the target crystal’s axis approaches alignment with the direction of the incident ion beam. The results support a model suggesting that the channeled ions lose energy only to valence electrons and are concentrated in a narrow cone about the direction of incidence when they emerge from the crystal. The energy straggling of channeled particles reaches a minimum in the hyperchanneled condition. Both the energy loss and the energy straggling of channeled protons show a dependence on the local electron density.

The energy-loss spectrum of ions in the presence of charge exchange is analyzed theoretically for large path lengths. The evaluation is based upon steepest-descent integration of the generalized Bothe-Landau formula. General approximation formulas for the spectrum are derived without reference to particular collision cross sections, and their regions of validity are discussed. The approximations are tested against numerical curves for 32-MeV ${}^3$He ions in carbon. General asymptotic expressions for the peak energy loss and half-widths of the spectrum are found. The asymptotic expression for the mean-to-peak interval is specified and analyzed. In particular, it is shown and illustrated on a model system that the sign of the difference between the peak and mean energy loss may be opposite to that in the one-state case.

It is known that the energy loss in ion-atom collisions is dominated by elastic scattering of one collision partner and excitation or ionization of the second. This is true for both structured and structureless projectiles. Here, I show that this fact leads to a relatively simple extension of the Bethe stopping-power formula for structureless projectiles to structured projectiles. The formula replaces the Bethe mean excitation energy with two other mean energies which reflect the elastic scattering factor of the otherwise structureless projectile. The expression is symmetrical in the center-of-mass system. The formula is in excellent agreement with numerical calculations, when the numerical calculations are asymptotic. To obtain the extended Bethe stopping formula, it is necessary to evaluate integrals over energy and momentum transfer involving generalized oscillator strengths, properly treating the energy-dependent lower limit in the momentum transfer integral. I do this by a constructive procedure using two cutoffs (instead of the traditional one) in momentum transfer. Peek had earlier developed a procedure to address the question of whether the Bethe mean excitation energy calculated with optical oscillator strengths is the same as the Bethe mean excitation energy appearing in the stopping power formula. I show that the result of the constructive procedure agrees with the result of Peek in the asymptotic limit, and I develop a criterion to determine the asymptotic limit.

We discuss general criteria differentiating incoherent excitation processes from coherent processes allowing the production of resonant coherent excitation (RCE) of channeled ions in crystals. We give special consideration to the ionization of C${}^{5+}$ ions traveling with energies between 15 and 25 MeV along the $〈100〉$ direction of a gold crystal. We show that incoherent ionization of the $n=2\mathrm{}$ state by the valence electrons partially destroys the coherence producing resonant coherent excitation of the $n=\overrightarrow{1}n=2\mathrm{}$ transition by the target static potential. It thus causes a considerable broadening of the resonance signal, as observed in the experiments. Finally we show that far from resonance, coherence quenches projectile excitation by the target static potential (anti-Okorokov effect).

The feasibility of observing two-electron interference is considered. The detection of antibunching in an intensity cross-correlation function using coincidence-delay measurement would be the most feasible experiment. The data acquisition time to observe electron antibunching in a coherent field-emission beam is estimated by taking into account the transversal coherence with a possible experimental configuration. With the available measurement system in an optimized condition, the data acquisition time is found to be $\sim {10}^2$ h, being considered to be the permissible duration of measurement time.

The time-resolved decay of silver clusters ${\mathrm{Ag}}_n^{+}$ $(n=8\mathrm{}-21)$ has been observed after excitation by photons with energies 1.5–4 eV. Clusters were found to decay by emission of neutral atoms or dimers with lifetimes in the range 100 $\mu$s to 15 ms. Separation energies were calculated from the lifetimes assuming a statistical unimolecular decay. As a function of cluster size, the resulting values increase towards the bulk cohesive energy of silver. They show a pronounced odd-even alternation and an indication of a shell closure at $n=9\mathrm{}$. The separation energies for $n=8,9\mathrm{}$ are in good agreement with configuration-interaction ab initio calculations.

When intense light interacts with a molecule it induces a force proportional to the gradient of the Stark shift. We use this nonresonant force to deflect ${\mathrm{I}}_2$ and ${\mathrm{CS}}_2$ molecules. We trace the direction of molecules in a molecular beam, showing that the molecules that pass near the center of a 1.06-μm or 10.6-μm laser beam will focus. We predict that Stark shifts on the order of 50 meV can be obtained for all small molecules and atoms while maintaining ionization rates below ${10}^6{\mathrm{s}}^{\mathrm{-}1}.$ Among the devices that can be based on the nonresonant Stark shift are molecular accelerators and molecular quantum wires.

Two-photon resonant parametric four-wave mixing and a newly developed variant called seeded parametric four-wave mixing are used to detect trace quantities of sodium in a flame. Both techniques are simple, requiring only a single laser to generate a signal beam at a different wavelength which propagates collinearly with the pump beam, allowing efficient signal recovery. A comparison of the two techniques reveals that seeded parametric four-wave mixing is more than two orders of magnitude more sensitive than parametric four-wave mixing, with an estimated detection sensitivity of $5\times {10}^9$ atoms/${\mathrm{cm}}^3$. Seeded parametric four-wave mixing is achieved by cascading two parametric four-wave mixing media such that one of the parametric fields generated in the first high-density medium is then used to “seed” the same four-wave mixing process in a second medium in order to increase the four-wave mixing gain. The behavior of this seeded parametric four-wave mixing is described using semiclassical perturbation theory. A simplified small-signal theory is found to model most of the data satisfactorily. However, an anomalous saturationlike behavior is observed in the large signal regime. The full perturbation treatment, which includes the competition between two different four-wave mixing processes coupled via the signal field, accounts for this apparently anomalous behavior.

The hydrogen atom interacting with a circularly polarized microwave field and a magnetic field perpendicular to the plane of polarization is, in general, a nonintegrable system that may exhibit strongly chaotic motion. However, two integrable limits are known, one of which is separable in elliptical coordinates. A detailed investigation of the classical and quantum mechanics of the separable limit is presented.

Ab initio theoretical investigation of two- and three-photon ionization of the isoelectronic species H, He${}^{+}$, and Li${}^{2+}$ in the vacuum ultraviolet and x-ray ultraviolet frequency regions, is carried out using the fully nonperturbative Floquet-coupled-channel method. We present the results for the total ionization rates in the wavelength region between approximately 260 to 10 nm, and in the intensity domain between about ${10}^{10}$ and ${10}^{17}$ W/cm${}^2$, depending on the member of the isoelectronic species investigated. Comparison with the perturbation theoretical results at moderate intensities is made, a corresponding scaling law is tested, and the finite values of the rates of ionization at the resonance frequencies are obtained. The dependence of the rates on the intensity of the field shows the presence of the so-called “stability windows,” for all three species investigated here. The role of polarization on the ionization rates shows a general dominance of the linear polarization over the circular polarization in the intensity and frequency domains investigated, for all the three species concerned.

Angular-resolved fragment kinetic-energy spectra resulting from the photo-dissociation of odd-charged homonuclear ions induced by short, intense laser pulses are calculated using an efficient three-dimensional Fourier-transform method in spherical coordinates. The peculiarity of these ions is their linearly increasing transition dipole moment with the internuclear separation, leading to asymptotically divergent radiative couplings. The method deals with such couplings by splitting the wave function into two regions, an internal and an asymptotic one. The latter is analyzed analytically by a three-dimensional generalization of the Volkov-type solutions of Keller [Phys. Rev. A 52, 1450 (1995)], while the numerical propagation on the former is performed by combining a contact transformation with the radial and angular grid method of Dateo and Metiu [J. Chem. Phys. 95, 7392 (1991)]. The procedure basically offers the advantage of a reduced grid extension by avoiding some numerical instabilities and inaccuracies related to the cosine transform of previous algorithms. Unitarity and energy conservation are successfully tested and an application is provided to ${\mathrm{H}}_2^{+}$ photodissociation using the Nd:YAG laser (where YAG denotes yttrium aluminum garnet) second harmonic $(\lambda =532\mathrm{}$ $\mathrm{nm})$ at an intensity of 10 ${\mathrm{T}\mathrm{W}/\mathrm{c}\mathrm{m}}^2$ for which angular-resolved spectra are experimentally available.

Following the paper by Combescure [Ann. Phys. (N.Y.) 204, 113 (1990)], we apply the quantum singular time-dependent oscillator model to describe the relative one-dimensional motion of two ions in a trap. We argue that the model can be justified for low-energy excited states with the quantum numbers $n\ll n_{\max }\sim 100$, provided the dimensionless constant characterizing the strength of the repulsive potential is large enough $g_{*}\sim {10}^5$. Time-dependent Gaussian-like wave packets generalizing odd coherent states of the harmonic oscillator and excitation number eigenstates are constructed. We show that the relative motion of the ions, in contradistinction to its center-of-mass counterpart, is extremely sensitive to the time dependence of the binding harmonic potential since the large value of $g_{*}$ results in a significant amplification of the transition probabilities between energy eigenstate even for slow time variations of the frequency.