The triple-differential cross section (TDCS) for electron-impact ionization of the krypton $4p$ orbital has been measured at an incident energy of 919.4 eV, scattered electron energy of 880 eV, and ejected electron energy of 25 eV. The measurements were performed at scattering angles ranging from 3° to 20°. The cross sections have been placed on an absolute scale using a technique that relies on a relative normalization against a reliable theoretical TDCS for helium $1s$ ionization. The experimental data are compared with a distorted-wave Born-approximation calculation. There is good shape agreement between the theoretical results and the experimental data, and satisfactory agreement in magnitude (within the experimental uncertainty of 23%) across all scattering angles.

We derive a rigorous optical potential for electron-molecule scattering including the effects of nuclear dynamics by extending the common many-body Green’s function approach to optical potentials beyond the fixed-nuclei limit for molecular targets. Our formalism treats the projectile electron and the nuclear motion of the target molecule on the same footing whereby the dynamical optical potential rigorously accounts for the complex many-body nature of the scattering target. One central result of the present work is that the common fixed-nuclei optical potential is a valid adiabatic approximation to the dynamical optical potential even when projectile and nuclear motion are (nonadiabatically) coupled as long as the scattering energy is well below the electronic excitation thresholds of the target. For extremely low projectile velocities, however, when the cross sections are most sensitive to the scattering potential, we expect the influences of the nuclear dynamics on the optical potential to become relevant. For these cases, a systematic way to improve the adiabatic approximation to the dynamical optical potential is presented that yields nonlocal operators with respect to the nuclear coordinates.

Using the parallel-plate-condenser method, we have measured gross ionization cross sections in fully stripped–ion (charge state $q=2\mathrm{}–18)$ impact on rare gases and simple molecules $({\mathrm{H}}_2,{\mathrm{N}}_2,{\mathrm{O}}_2,$ and ${\mathrm{CO}}_2)$ at a fixed collision energy E of 6 MeV/amu. The observed cross sections have been found to depend weakly on the charge state q compared with the $q^2$ dependence, especially for highly charged projectiles and/or heavy target elements. Therefore, the Bethe-Born calculations tend to overestimate the gross ionization cross sections in multiply charged ion impact at the present collision energy. For a particular target, the gross ionization cross sections have been found to gather on a common curve irrespective of the projectile energy and charge state when the cross section divided by the charge state is plotted against the projectile energy per nucleon divided by the charge state, giving some credence to a scaling law based upon the classical trajectory Monte Carlo (CTMC) method. However, CTMC calculations tend to underestimate the ionization cross sections especially at the high-energy region of $E/q>\mathrm{}0.3\mathrm{}\mathrm{}(\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{a}\mathrm{m}\mathrm{u})\mathrm{}.$ A scaling for the gross ionization cross sections of rare gases in fully stripped–ion impact is proposed.

The $3p$ photoelectron spectrum and the corresponding $M_{2,3}$ Auger spectrum of atomic Ni have been measured with the use of monochromatized synchrotron radiation and atomic beam technique. The thresholds of the direct $3p$ photoionization have been determined. In order to interpret the complex ${3p}^{-1}$ multiplet structure calculations were performed taking into account the initial states of both configurations ${3d}^8{4s}^2$ and ${3d}^{9}4s$ and the $\mathrm{LS}$-term dependence of the Ni $3p$ core-hole lifetimes. The strong influence of both the $3p-3d$ exchange interaction and the term dependent lifetime broadening by super-Coster-Kronig decays on the shape of the $3p$ photoelectron spectrum is demonstrated.

The fivefold differential cross sections of the simple ionization of hydrogen, deuterium, and tritium diatomic molecules are determined by the use of one-center Coulomb continuum wave functions describing the slow ejected electron. Vertical transitions from the lowest vibrational state of the fundamental ${}^1{\Sigma }_g$ state of the target to the vibrational levels of the fundamental ${}^2{\Sigma }_g$ state of ${\mathrm{H}}_2^{+},$ ${\mathrm{D}}_2^{+},$ or ${\mathrm{T}}_2^{+}$ are considered. The results obtained for two different energy resolutions (1 and 3 eV) show that the difference between these three species should increase with a decrease of the energy resolution.

In photon-atom scattering the customary partitioning of the elastic scattering amplitude into Rayleigh and Delbrück amplitudes (as well as nuclear amplitudes) in the single-electron formalism involves summations over complete sets of intermediate electron states in the atomic potential. This leads to the Rayleigh amplitude which is usually compared with experiment. However, another consequence of the partition is that the total cross section for bound-electron pair-production into all bound states, regardless of occupation, is included in the optical theorem for the imaginary part of the forward Delbrück amplitude. The corresponding real part of the forward Delbrück amplitude can be obtained through the use of a dispersion relation. We show that for $Z=92\mathrm{}$ the inclusion of the bound-electron pair-production cross section leads to a contribution to the real part of forward Delbrück amplitude, which can be as much as $\approx 12\%$ of the Born approximation result, for photon energies near the pair-production threshold. This bound-electron pair-production contribution is therefore comparable in magnitude to the corrections due to Coulomb and screening effects in the ordinary pair-production cross section (electron in continuum). The net correction to Born approximation is small well below threshold, significant well above threshold.

Charge-transfer cross sections into different subshells of ${\mathrm{C}}^{q+},$ ${\mathrm{N}}^{q+},$ and ${\mathrm{O}}^{q+}$ $(q=1\mathrm{}–5)$ from atomic hydrogen in the ground state have been calculated in the framework of the boundary corrected continuum intermediate-state approximation in the energy range of 40–200 keV/amu. Interactions of the active electron with the projectile ion have been approximated by a model potential containing both a long-range Coulomb potential part and a short-range part. Total cross-section results for all the ions are in reasonable agreement with available experimental results. However, oscillatory structure of charge-state dependence of total cross sections has not been found in our calculation.

Within a semiclassical model, we investigate the dynamic neutralization and relaxation of slow ${(E}_{\mathrm{kin}}<100\mathrm{eV})$ multiply charged ions which are reflected on metal surfaces. Special emphasis is devoted to near-surface interaction mechanisms. Our model includes a Monte Carlo sampling over projectile parameters and detailed ionic structure calculations of projectile energy levels. In a full trajectory simulation, our results simultaneously comply with measured trends in projectile kinetic energy gains and final charge-state distributions of the reflected ions as well as total electron yields and spectra. Recently discovered characteristic features in the electron spectra can be uniquely assigned to distinct above-surface regions of the projectile trajectory.

Electronic stopping cross sections for atoms and molecules in the gas phase have been evaluated within the independent-particle model for the range of velocities where the first Born approximation is valid for targets with $Z_2<~18.$ It is shown that in this approximation, the electronic stopping power is expressed as a Bragg sum rule. In the case that the target electron is considered as harmonically bound, the stopping cross section is obtained in an analytical form depending solely on the frequency of the electron. Using the virial theorem, a relation is found between the harmonic oscillator frequency and the electronic properties of the target, such that the stopping power is described in a self-contained way. The results are compared with other theoretical treatments and with available experimental data. For the case of projectiles with structure, I use the results for the effective charge (screening effect) of Cabrera-Trujillo, Cruz, Oddershede, and Sabin [Phys. Rev. A 55, 2864 (1997)]. From the analytical expression for $S_e,$ one obtains the shell contributions to the electronic stopping cross section, finding that in this model, shell corrections also come from considering the whole set of allowed excitations in the target with a dependence on the orbital mean excitation energy and the projectile mass.

We have investigated the structural stability and energetics for small copper clusters, ${\mathrm{Cu}}_n(n=3,\dots ,55)$ by using a Monte Carlo technique at room temperature $(T=300\mathrm{}\mathrm{K}).\mathrm{}$ In the simulation we have adopted two approaches; one of them was optimizing the cluster from a random configuration as a starting point, and the other was optimizing the cluster by adding one atom randomly to an optimized geometry. The empirical potential-energy function proposed by Erkoç has been used, which contains two-body atomic interactions. It has been found that the fivefold symmetry appears in all the clusters with the number of atoms $n>~7,$ and the icosahedral structure dominates in the clusters with the number of atoms $n>~13.$

The experimentally interesting ${\mathrm{Nb}}_3$ and ${\mathrm{Nb}}_4$ clusters and their cations have been studied in great detail using density-functional methodology in conjunction with relativistic effective core potentials. Close attention has been paid to full optimization along the flat potential energy surfaces and numerous minima and several transition states have been characterized. The ${\mathrm{Nb}}_3$ cation is predicted to have a ${}^3{A}_1^{\prime }$ ground state with an equilibrium geometry of $D_{3h}$ symmetry. The ground state of the neutral is predicted to be a ${}^2{B}_1$ state of $C_{2v}$ symmetry with two shorter bonds and one longer. The transition state for pseudorotation or “peak” atom interchange, however, lies only 0.01 kcal/mol higher in energy, implying a fluxional structure for the neutral species. The global minimum of the ${\mathrm{Nb}}_4$ cationic cluster is a $C_{2v}$ structure, Jahn-Teller distorted from the $T_d$ global minimum of the singlet neutral. Numerous other energetically low-lying stationary points are characterized for each species. We discuss the bonding features of these minima and relate our predictions to the existing experimental data.

We study low-energy $(E\sim 0–5\mathrm{eV})$ electron collisions with free neutral ${\mathrm{Na}}_n$ clusters $\mathrm{}(n=20,40,57,58,70\mathrm{}).$ The measured inelastic cross sections range from several hundred ${\AA }^2$ at $E>\mathrm{}1\mathrm{}\mathrm{eV}$ up to well over a thousand ${\AA }^2$ for $\overrightarrow{E}0,$ signifying the action of strong long-range polarization forces. For collision energies below 1 eV, the cross sections rise strongly with decreasing energy, and are in good agreement with values predicted by the mechanism of electron capture by the polarization field of the cluster. At higher energies, the cross sections become approximately independent of the collision energy, implying efficient energy transfer at large impact parameters. This may proceed via excitation of collective and single-particle resonances of the cluster electron cloud by the incoming electron, followed by fragmentation.

We present measurements of ion energies from the interaction of intense, femtosecond laser pulses with large mixed-species clusters. Multi-keV protons and ∼100-keV iodine ions are observed from the explosion of HI clusters produced in a gas jet operated at room temperature. Clusters formed from molecular gases such as HI are thus seen to extend the advantages of the laser-cluster interaction to elements that do not readily form single-species clusters. In the light of recently reported nuclear fusion in laser-heated clusters, we also examine the possibility of boosting the explosion energies of low-Z ions through the use of mixed species clusters.

We study different mechanisms of adiabatic population transfer in N-level systems by means of optimal control algorithms. Using two-dimensional topographic maps of the yield of population transfer as a function of time delay and intensity of the pulses we analyze the global properties of the schemes and the conditions that lead to optimization. For three-level systems it is shown that the optimal pulse sequence is the well-known STIRAP (stimulated Raman adiabatic passage) scheme. For five-level systems a family of solutions ranging from the alternating STIRAP scheme to the new straddling STIRAP (S-STIRAP) scheme is obtained and the behavior of the solutions is compared. For four-level systems we obtain as optimal a S-STIRAP type sequence that behaves as an effective two-level system. For both odd and even numbers of N-level systems, the crucial role of the straddling pulse in reducing the population of all intermediate levels is demonstrated.

We describe the physical effects emerging in three-level atoms as a result of a drastic modification of the relaxation processes under the action of a strong coherent field when one of two dynamic Stark levels crosses a nearby atomic state.

A fully quantum treatment explicitly presents the high harmonic generation as a three-step process: (i) above threshold ionization (ATI) is followed by (ii) electron propagation in a laser-dressed continuum. Subsequently (iii) stimulated (or laser-assisted) recombination brings the electron back into the initial state with emission of a high-energy photon. Contributions of all ATI channels add up coherently. All three stages of the process are described by simple, mostly analytical expressions that allow a detailed physical interpretation. A very good quantitative agreement with the previous calculations on the harmonic generation by the ${\mathrm{H}}^{-}$ ion is demonstrated, thus supplementing the conceptual significance of the theory with its practical efficiency. The virtue of the present scheme is further supported by a good accord between the calculations in length and velocity gauges for the high-energy photon.

An implicit algorithm for integration of the three-dimensional (3D) time-dependent Schrödinger equation of an atomic system interacting with intense laser pulses is developed. It is based on a matrix iteration of the Crank-Nicholson approximant to the short-time propagator using the total Hamiltonian (unsplit) of the system directly. To test the method, 3D Schrödinger wave-packet propagation is carried out, and so-called above-threshold ionization and high-harmonic generation spectra for atomic hydrogen irradiated by intense laser pulses are obtained. They are also compared with that obtained using the popular split-operator method. The present algorithm is shown to provide an alternative to the the split-operator method, and proves to be more efficient in all the cases studied here. A procedure for optimizing the maximum grid size is also given, and its usefulness is illustrated.

Studies of the helium atom in uniform intermediate magnetic fields are performed with B-spline-type functions. All the matrix elements in our calculations are formulated as products of simple, independent one-dimensional numerical integrals and analytic ones. As a result, our energies obtained with the B-spline basis set agree with the best calculations to within 0.13% of a fractional error.

The spatial scrambling of upper-Stark-state ${(KM}_J<0)$-selected beams of ${\mathrm{CH}}_3\mathrm{F},$ ${\mathrm{CH}}_3\mathrm{Cl},$ ${\mathrm{CH}}_3\mathrm{Br},$ and ${\mathrm{CH}}_3\mathrm{I}$ in field-free space has been investigated. It has been proposed that the mechanism for spatial deorientation as the electric-field strength is reduced to zero is the change in precessional frequency and loss of spatial direction as the total angular momentum $\mathbf{J}$ decouples from the collapsing electric field and couples with the nuclear spin. Supersonic beams were quantum-state selected in a hexapole inhomogeneous electric field and directed between a pair of parallel field plates before being focused through a second hexapole field to a quadrupole mass spectrometer detector. Exposure of the beam to zero field in the parallel-plate region leads to an attenuation of the beam signal relative to the non-zero-field case due to defocusing of newly formed lower Stark states and $KM=0\mathrm{}$ states in the second hexapole field. This phenomenon can be used to determine the effect of field strength on the orientation of upper Stark states within the beam. The beam signal at the detector was shown to remain constant for uniform field strengths greater than approximately $3{\mathrm{V}\mathrm{\hspace{0.5em}}\mathrm{cm}}^{-1},$ with a signal attenuation of around 40% relative to this level at zero field. Attempts were made to measure the mean lifetime for spatial scrambling by pulsing the uniform field to ground potential for increasing intervals and observing the beam attenuation. However, these measurements were complicated by the effect of the beam velocity distribution on the signal and it was found that reproducible values could not be obtained, though the results of these experiments are consistent with lifetimes lying in the expected range from 100 to $300\mu \mathrm{s}.$

We show that, in contrast to multiphoton absorption in atoms, the remote region from a nucleus is dominant in the process of a half-cycle pulse interaction with a Rydberg atom, and that this interaction produces a very large change in the angular momentum of the electron. We have found good agreement between classical trajectory Monte Carlo, semiclassical, and quantum-mechanical calculations for the energy distribution of the electron.