Relying on an entirely ab initio quantum-mechanical scheme, we investigate the threshold behavior of a model electron-impact ionization problem in which the target hydrogen atom interacts with the incident electron only by monopole. The total ionization cross section for a singlet is shown to follow the threshold law of an exponential form as proposed by Macek and Ihra [Phys. Rev. A 55, 2024 (1997)], thus supporting the argument based on the local instability of the “ridge” motion despite the reported absence of classical ridge trajectories. Below the classical threshold, quantum diffraction allows the two electrons to have a large probability amplitude in the region inaccessible to the classical trajectories. The energy distribution for singlet in final continuum channels is shown to have a hitherto unexpected V-shaped structure at energies between 0 and 1 a.u. above the ionization threshold. The V structure becomes sharper toward the threshold while it approaches the quadratic form surmised by Bray [Phys. Rev. Lett. 78, 4721 (1997)] at higher energies.

Single- and double-ionization processes in electron scattering from a model helium atom are calculated by a direct solution of the time-dependent Schrödinger equation on a 3D lattice. The Coulomb interaction between the electrons is described by ${v(r}_1{,r}_2{)=1/r}_{>}$ and all angular momenta are set to zero. The initial state is a product of the ground state of the model helium atom on a 2D lattice with a wave packet for the free electron. At times following the collision and for incident electron energies around 200 eV, the probability density associated with double ionization is found to be quite small when compared to other scattering processes. Projections onto 1D continuum states are employed to calculate single- and double-ionization cross sections. Although absolute cross sections for the model cannot be compared to experiments on helium due to the neglect of higher partial waves, the ratio of double to single ionization for the model is found to be 1% or less, in fair agreement with experiment.

We report on the projectile energy dependence of the cross sections for producing K Auger electrons in 2–170-keV ${\mathrm{N}}^{7+}+\mathrm{N}\mathrm{e}$ collisions. The present studies, which extend previous work performed at the impact energy of 35 keV, give evidence for a dielectronic excitation process produced by electron-electron interaction. At impact energies in the range 5–170 keV, the cross sections are found to be larger than ${10}^{-17}{\mathrm{cm}}^2.$ Molecular-orbital energy diagrams were determined to analyze details of the collision. Analytic models were used to evaluate cross sections associated with dielectronic excitation. The results show good agreement with experiment. The dielectronic excitation process is dominant when three electrons from the target are transferred into an excited state of the projectile.

A series of superelastic scattering experiments is described in which electrons are scattered from lithium atoms optically pumped to the $2P$ excited state. The incident electron energies were 5.2, 12.2, and 20 eV and superelastically scattered electrons with energies of 7, 14, and 21.8 eV were detected over the angular range from $0°$ to $140°$. Both differential cross sections and the orientation and alignment parameters were deduced from the same measurements. A complete set of the four spin-averaged parameters, which fully characterize the electron impact excitation of the $2S-2P$ transition in lithium, is presented. The experimental results are compared with those predicted from convergent-close-coupling (CCC) calculations and two distorted-wave calculations. Excellent agreement is found with the CCC calculations.

We have applied the hidden-crossing method to compute S-, P-, and D-wave cross sections for Ps formation in positron-hydrogen collisions in the Ore gap. The hidden-crossing method has provided a physical explanation of why the S-wave cross section is so small and why the D wave is significant. The one-Sturmian theory is used to correct the hidden-crossing theory to take into account the factor $\left(〈\phi |d^2\phi {/dR}^2〉{+(1/4)/R}^2\right).\mathrm{}$ We have considered this correction term in computing the P- and D-wave cross sections. The hidden-crossing results are compared with accurate results from other methods. This comparison helps assess the accuracy of the hidden-crossing method in describing three-body collisions.

Collision-induced ionization and fragmentation of ${\mathrm{C}}_{60}$ molecules are studied using 2-MeV ${\mathrm{Li}}^{q+}$ projectile ions with $q=1\mathrm{}–3.$ Cross sections for the production of fragment ions ${\mathrm{C}}_n^{+}$ $(n=1\mathrm{}–14)$ and ionized parent ions ${\mathrm{C}}_{60}^{r+}$ $(r=1\mathrm{}–4)$ were obtained by time-of-flight measurements in coincidence with outgoing projectiles with charge states k ranging from 0 to 3. The mass-to-charge spectra revealed a wide variety of distribution patterns for different $\overrightarrow{q}k$ collision processes. For all three incident charges, the degree of target ionization and fragmentation was found to increase with increasing number of electrons captured by or lost from the projectile ions in collisions. Analysis of cross section data for small fragment ions provides evidence of the quasiequilibrium charge distribution achieved for ions penetrating the ${\mathrm{C}}_{60}$ molecular cage.

High-resolution Br $\mathrm{MVV}$ Auger spectra of HBr and DBr have been recorded using monochromatized synchrotron radiation. We present a detailed analysis and reassignment of the spectra in the ${3d}^{-1}\to 4p{\pi }^{-2}$ region. This part of the spectra exhibits the spin-orbit and ligand-field splitting of the ${3d}^{-1}$ core-hole states and a vibrational structure of the final states influenced by vibrational lifetime interference. The equilibrium distances and the vibrational energies of the final states ${}^1{\Sigma }^{+},$ ${}^1\Delta$, and ${}^3{\Sigma }^{-}$ are derived from the intensity distribution of the vibrational substates. From the shape of the peaks identified as transitions to the ${}^3{\Sigma }^{-}$ state a splitting of $\cong 50$ meV between the components ${}^3{\Sigma }_0^{-}$ and ${}^3{\Sigma }_1^{-}$ is concluded. Agreement between the experimental spectra and fitted results for both HBr and DBr is very satisfactory.

We present a parameter-free theoretical study of the negative-ion production from fluorine projectiles during low-energy (0.5–4 keV), small incidence angle (3.5°) collisions with a MgO(100) surface. Our calculations are based on the binary encounter model developed to describe the electron capture by a projectile upon collision with an insulating ionic crystal surface [see A. G. Borisov and V. Sidis, Phys. Rev. B 56, 10 628 (1997)]. For the low-velocity behavior of the negative-ion yield, good agreement is obtained between our theoretical results and the available experimental data by S. Ustaze et al. [Phys. Rev. Lett. 79, 3526 (1997)].

Direct determination of average charge states have been performed with an electrostatic analysis method for carbon ions and carbon cluster $({\mathrm{C}}_n,$ $n=3\mathrm{}$–10) constituents exiting thin carbon foils. The velocity of the mono and polyatomic projectiles was identical, ranging from 1–4 MeV per atom. The average charge state of carbon cluster constituents is significantly reduced as compared to that of a single carbon projectile at the same velocity, the larger the size of the cluster, the smaller the charge state per cluster atom. This lowering effect decreases when the foil thickness increases.

We describe the dynamics of electrons during and after the collision at large impact parameters between a highly charged ion and a cluster of from a few ten up to several hundred alkali-metal atoms. The theoretical model gives the time evolution of the electronic one-body phase-space density as the solution of the self-consistent Vlasov equation. It allows us to predict the ionization state and the excitation energy of the cluster after collision. We demonstrate that with sufficiently charged ions it is possible to ionize metal clusters close to their Rayleigh charge limit without significant heating, thus providing a unique method to investigate charge instabilities in clusters.

It has been observed that under MeV-ion bombardment of a polymer, such as polycarbonate (PC) or polyvinylidene fluoride (PVDF), large quantities of carbon clusters $({\mathrm{C}}_n^{-}$ and ${\mathrm{C}}_n{\mathrm{H}}^{\mathrm{-}})$ are generated. However, when PC or PVDF is bombarded with keV atomic ions, very few carbon-cluster ions are produced. This different behavior was attributed to the different sputtering/desorption mechanisms for keV- and MeV-ion impacts. Low-energy keV ions deposit their energy into a solid through nuclear stopping, while MeV ions deposit their energy mainly through electronic stopping. The formation of carbon clusters is thought to be facilitated by the high-temperatures and high-energy densities produced in the region nearest the point of MeV-ion impact, the infratrack region. We have observed extensive carbon-cluster formation from PC and PVDF under keV-cluster-ion bombardment. Despite the vastly different velocities of the high- and low-energy projectiles, identical carbon-cluster trends are produced from MeV ${}^{252}\mathrm{Cf}$ fission fragments and ${20-\mathrm{k}\mathrm{e}\mathrm{V}\mathrm{}\mathrm{C}}_{60}^{+}$ projectile impacts on the same target. This leads us to the conclusion that a polyatomic ion impact, which deposits its kinetic energy near the surface, may create a region of high-temperature and high-energy density that is similar to the infratrack of a MeV-ion impact.

We have theoretically investigated the population transfer in a four-level ${\mathrm{H}}_2$ system by stimulated Raman transition from the ground $X{}^1{\Sigma }_g^{+}({\nu }_g{=0,J}_g=0)$ level to higher rovibrational levels $\left({\nu }_f{,J}_f\right)$ of the $X{}^1{\Sigma }_g^{+}$ state via the excited intermediate $B{}^1{\Sigma }_u^{+}({\nu }_i{=14,J}_i=1)$ and $C{}^1{\Pi }_u^{+}({\nu }_i{=3,J}_i=1)$ levels coupled with each other by nonadiabatic interaction, using time-dependent overlapping pump and Stokes laser fields. The density-matrix treatment, which permits the convenient inclusion of the spontaneous emissions from the intermediate levels, has been employed to describe the dynamics of the two-photon Raman resonance process. The present study performs the calculations of final populations (after both the pulses are over) of the ground and terminal levels for Q-branch ${(J}_f=0)$ fundamental $({\nu }_f=1)$ and first overtone $({\nu }_f=2)$ transitions and the S-branch ${(J}_f=2)$ fundamental $({\nu }_f=1)$ transition as a function of time delay between the two pulses for the cases of on-resonance as well as off-resonance excitations in a wide range $(2\mathrm{}\times {10}^5–2\times {10}^7{\mathrm{W}/\mathrm{c}\mathrm{m}}^2)$ of peak intensities $I_P^0$ ${(I}_S^0)$ of the pump (Stokes) fields. Both fields are assumed to have the same temporal shape, duration, peak intensities, and linear parallel polarizations. The accurate values of spontaneous radiative relaxation rates of the intermediate levels to the initial and final levels, taking into account their J and M dependence, are explicitly included in our calculations. The pulse width (full width at half maximum) ${\tau }_p$ is taken as 170 ns so that total spontaneous decay can occur during the pulse duration. The transfer efficiency is found to be very sensitive to the peak intensities of the laser pulses in each case of transition considered. Special attention is paid to the effects of the nonadiabatic (NA) interaction between $B(14,1)\mathrm{}$ and $C(3,1)\mathrm{}$ levels on population transfer efficiency. Calculations are also done in some particular cases using the adiabatic Born-Oppenheimer (ABO) approximation. The results with ABO approximation are found to differ remarkably from those obtained including NA interaction. Our calculations for the four-level ${\mathrm{H}}_2$ system reveal that almost complete population is transferred in counterintuitive pulse order for both on-resonance and off-resonance excitations with intermediate and high values of $I_P^0$ ${(I}_S^0).\mathrm{}$ For intuitive pulse sequence also a large population transfer is achieved for on-resonance excitation at intermediate values of $I_P^0$ ${(I}_S^0)$ and for off-resonance excitation at intermediate and high values of $I_P^0$ ${(I}_S^0).\mathrm{}$

Using a semiclassical density matrix formalism we have calculated the behavior of multilevel atoms interacting with a standing wave field, and show how complex nonlinear phenomena, including multiphoton effects, combine to produce saturation spectra as observed in experiments. We consider both 20-level sodium and 24-level rubidium models, contrasting these with a simple 2-level case. The influence of parameters such as atomic trajectory and the time the atom remains in the beam are shown to have a critical effect on the line shape of these resonances and the emission/absorption processes. Stable oscillations in the excited state populations for both the two-level and multilevel cases are shown to be limit cycles. These limit cycles undergo period doubling as the system evolves into chaos. Finally, using a Monte Carlo treatment, these processes average to produce saturated absorption spectra complete with power and Doppler broadening effects consistent with experiment.

Adiabatic passage induced by chirped laser pulses in three-level $\Lambda$ systems is investigated. Two different strategies for efficient population transfer (equal and opposite sign of the chirp) are analyzed. The role of detuning, crucial for the attempt to reduce the population of the intermediate state, is demonstrated. The scheme of adiabatic passage by laser pulses with chirps of different sign is not symmetric with respect to the sign of the one-photon detuning. Blue detuning results in a smaller transient population of the intermediate state than does red detuning. The results are explained on the basis of dressed states. Numerical results for a fully off-resonant adiabatic passage scheme, based on two pulses with the same sign of the chirps, are also considered. The robustness of the schemes in terms of the Rabi frequency, chirp rate, detuning, and time delay of the pulses is explored.

A time-independent close-coupling calculation of multiphoton dissociation of ${\mathrm{HD}}^{+}$ from the ground vibrational level in two fields, each of intensity ${10}^{12}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2,$ and frequencies below the dissociation threshold, is presented. The lower frequency ${\omega }_1$ is varied around the difference between the energies of the Stark shifted $v=6\mathrm{}$ and $v=0\mathrm{}$ levels, while the higher frequency ${\omega }_2$ is varied by about 2000 ${\mathrm{cm}}^{\mathrm{-}1}$ up to the dissociation threshold of the $v=0\mathrm{}$ level. The dissociation linewidth of the initial $v=0\mathrm{}$ level as a function of ${\omega }_1$ is a Lorentzian (as expected for a simple resonant two photon dissociation process) whose height, position, and width all depend upon the value of ${\omega }_2.$ With respect to ${\omega }_2$ this linewidth of the initial $v=0\mathrm{}$ level exhibits a series of asymmetric peaks. The position of each of these peaks shifts considerably when ${\omega }_1$ is changed. The energy of the intermediate resonance in the closed channel corresponding to the absorption of a single ${\omega }_1$ photon strongly depends on the second frequency, ${\omega }_2.$ The peaks against ${\omega }_2$ occur whenever this resonance, with a dominant $v=6\mathrm{}$ character, coincides in energy with the Stark shifted $v=0\mathrm{}$ level in the initial closed channel.

Highly charged molecular ions are generated in Coulomb explosion experiments involving multielectron dissociative ionization, but little is known about the precise mechanisms involved in their formation. To help improve the understanding of such experiments, potential energy curves are calculated in this paper for diatomic chlorine $\left({\mathrm{Cl}}_2\right)$ and its ions ${\mathrm{Cl}}_2^{n+},$ where $n=1,2,3,4,6,8,10\mathrm{}$. Bound vibrational states are obtained in three low-lying electronic states for ${\mathrm{Cl}}_2^{2+}$ and one state for ${\mathrm{Cl}}_2^{3+}.$ Vertical excitation energies are given for stepwise excitations up to ${\mathrm{Cl}}_2^{10+}.$ For all the ions examined there is a significant energy defect $\left(\Delta \right)$ from the corresponding Coulomb potential, in one case reaching magnitudes of over 20 eV. We analyze the origin of these energy defects in terms of residual chemical bonding, and discuss the contribution of strongly bonding configurations at short internuclear distance. Finally, we present a simple physical model which describes the qualitative behavior of $\Delta \mathrm{}(R,Q)\mathrm{}$.

Free-electron–photon interaction, if it occurs inside an intense laser field, will be dominated by nonlinear Compton scattering (NLCS), which is quite different from the ordinary Compton scattering. The difference can be, for example, in the number of photons involved in one interaction. In this paper, by both analytical and numerical methods, we will explore in detail their characteristics from the viewpoint of energy exchange in the interaction. Based upon the well-displayed results, we are able to, furthermore, reveal some phenomena underlying NLCS and investigate the corresponding physical implications, such as the threshold effects in NLCS. That is, when ${Q=eE/(m}_e\omega c)>\mathrm{}0.1,$ the low-order NLCS will be depressed while the high orders will be excited. And when Q>10, the high-order NLCS process will become dominant in the electron-photon interaction. As for Q>100, the extra-high-order NLCS will be so strongly excited that the electron dynamics will enter a new regime. Also, we can make use of the obtained results to explain qualitatively the NLCS experiment by Bula et al. [Phys. Rev. Lett. 76, 3116 (1996)].

We present a phase-space picture of laser cooling of the radial motion of ions in a Penning trap. This picture enables a particularly simple derivation of the condition for simultaneous cooling of all degrees of freedom of a single ion to be obtained. It also allows a physically intuitive approach to be taken to the cooling process. Using this approach, we discuss different aspects of the cooling of a single ion in a Penning trap, including the formation of “trapped” states where the steady-state motion amplitude is nonzero for either the magnetron or modified cyclotron motion. We use an analytical approach where the approximation of small-amplitude motions can be made, and we use numerical calculations for larger-amplitude motions. The best procedure to use for effective laser cooling is derived. A similar approach is then used to treat the laser cooling of two ions in the Penning trap. We show that the use of the phase-space approach allows new insights into many aspects of the laser-cooling process to be gained, and we indicate how these insights may be applied in experimental investigations.

We calculate the Zeeman frequency shifts due to interactions with the light in a far-off-resonance optical dipole trap. These shifts are important for potential use of such a trap to search for an atomic permanent electric-dipole moment (EDM). We present numerical results for Cs and Hg, as examples of paramagnetic and diamagnetic atoms. The vector and tensor light shifts are calculated for a large range of trap optical frequencies, for both red-detuned and blue-detuned traps. We also consider frequency shifts resulting from magnetic dipole and electric quadrupole transitions mixed in by a static electric field. These shifts are particularly important for EDM experiments since they are linear in the electric field. The Zeeman frequency shifts represent a substantial problem for EDM experiments in a dipole trap and must be controlled with care to achieve theoretical sensitivity.

Scaled-energy spectroscopy is used to measure the $m=0\mathrm{}$ Stark recurrence spectrum of the two spin forms of helium. This case is characterized by much stronger orbit-core interaction than the $m=1\mathrm{}$ case studied previously. Viewed globally as a function of scaled energy $\varepsilon$ and scaled action $Ŝ$ clusters of core-scattered recurrences are revealed, emphasizing their importance in the Stark spectrum. Data are interpreted using closed-orbit theory. Integrating recurrence peaks over scaled energy allows comparison between singlet and triplet core scattering, highlighting the link with $e+{\mathrm{He}}^{+}$ scattering.