Impact ionization by singly charged ions from Rydberg $\mathrm{Na}\mathrm{}\left(24d\right)\mathrm{}$ and $\mathrm{Na}\mathrm{}\left(25s\right)\mathrm{}$ states has been investigated over the reduced velocity region $\tilde{v}=0.5\mathrm{}–\mathrm{2.2.}$ The ions of ${\mathrm{Na}}^{+},$ ${\mathrm{K}}^{+},$ and ${\mathrm{Li}}^{+}$ were accelerated to energies from 360 to 4400 eV and used to bombard the laser-excited Rydberg atoms. Electrons released in the ionization of Rydberg atoms were collected from the collision volume and registered by a channel electron multiplier. Measured relative ionization cross sections from $\mathrm{Na}\mathrm{}\left(24d\right)\mathrm{}$ and $\mathrm{Na}\mathrm{}\left(25s\right)\mathrm{}$ states show maxima at $\tilde{v}=1.6\mathrm{}–1.7$ and agree moderately well with classical-trajectory Monte Carlo calculations.

The retardation effect causes a small but finite change in the two-body potential, which in turn results in a large reduction in the scattering length A when the strength is near its critical value for forming a zero-energy bound state. This coupling to the vacuum fluctuation field thus affects the two-body dynamics such that the Efimov phenomenon is destroyed. Enhanced response of the scattering length $\Delta A$ for a small change in the two-body potential may be employed to gain efficiency and accuracy in experiments; for example, in testing body bound states in a cold atomic gas. The retardation effect can remove several Efimov bound states, depending on how large the change in scattering length is. Manipulation of the two-body potential by external fields is suggested to modify or eliminate some Efimov states. The optimal choice suggested from this analysis is to manipulate A such that $A/\Delta A\ll 1.$ Also included is a discussion on the additivity correction to the two-body polarization potential due to the Casimir effect.

Electric-field effects on the doubly excited resonant structures in He ground-state photoionization are investigated theoretically using the complex-rotation method with a B-spline-based configuration interaction basis. Angular-momentum states up to $L_{\max }=3\mathrm{}$ are coupled together by the external electric field for the final states. The variations of the structure profiles for the $M_L=0\mathrm{}$ components of He $\mathrm{}(2,5a)\mathrm{}$ and $\mathrm{}(2,6a)\mathrm{}$ ${}^1{P}^o$ and ${}^1{D}^e$ resonances for selected dc electric-field strengths are examined. The changes of the resonant energies and widths are also presented.

We report ab initio results for the integral cross section of the process $\mathrm{H}+\mathrm{N}\mathrm{a}\mathrm{}\left(3s\right)\mathrm{}\to \mathrm{H}+\mathrm{N}\mathrm{a}\mathrm{}\left(3p\right)\mathrm{}$ for collision energies from the threshold (2.1 eV) to 600 eV. We achieve a reasonable agreement with the experimental data, which are available for energies above 10 eV. The main contributions to the cross section come from a rotational coupling mechanism in the NaH triplet molecular system and from a curve-crossing mechanism in the singlet system. At very low energy (2.1–2.4 eV), the process is governed by a centrifugal barrier in the exit channel leading to orbital resonances. The Landau-Zener model provides a reasonable qualitative description of the radial coupling mechanism at high energies, but fails below 10 eV.

We apply our recently developed explicitly time-dependent theory for one-step resonant excitation-deexcitation processes of core hole states in diatomic molecules to the case of purely dissociative intermediate states and exemplify it by application to $\mathrm{Cl}{2p}_{3/2}$ excitation in HCl. A main goal is the conceptual understanding of the influence of detuning of the exciting radiation away from the excitation resonances. The calculated spectra for decay to different final states (binding as well dissociative ones, as well as to hypothetical final states selected to illustrate certain points of the behavior), are very rich; their main features agree with experimental observations and previous calculations using other approaches. The spectra are dominated by a sharp asymmetric peak which does not shift with photon energy. While its previous designation as “atomic decay peak” is warranted to first order, we show that it contains contributions from throughout the evolution in the intermediate state, i.e., also from times and internuclear distances for which the molecule has not yet separated. Its width is given by the core hole lifetime. The broad “molecular decay” contribution to the spectra can be very complicated, depending on the final state potential and the exciting radiation tuning conditions. Its width is determined mainly by the Franck-Condon width of the intermediate state wave packet. For strong detuning it approaches the direct photoemission spectral shape. When these two features overlap, deep spectral holes develop which we explain by interference of different paths (atomic and molecular decays) to the same final state. These and other findings are discussed in a conceptual way.

Asymmetric $\mathrm{}(e,2e)\mathrm{}$ experiments have been carried out on ${\mathrm{N}}_2$ for incident energy 100 eV, incident electron scattering angle 4°, secondary-electron energy 5 eV, and secondary-electron ejection angles from -32° to 100°. The experiments have sufficient energy resolution (∼0.13 eV) to resolve the vibrational structure in the coincidence energy-loss spectrum. Ionization transitions to the $X{}^2{\Sigma }_g^{+}$ and $A{}^2{\Pi }_u^{+}$ states were observed. When the final state of the product ion was $X{}^2{\Sigma }_g^{+},$ no vibrational excitation was observed at secondary electron ejection angles of 100° (recoil) or -80° (binary). However, at -32°, near the peak of the binary lobe of the cross section, a strong increase in the production of the $v^{\prime }=1\mathrm{}$ state was observed. In contrast, production of the $A{}^2{\Pi }_u^{+}$ state was characterized by the expected Frank-Condon vibrational intensities at 100° and 80° on the recoil side but a decrease in the relative intensities of the $v^{\prime }=1,$ 2, and 3 vibrational states at -32°. The A state deviations occur where the cross section is small, so we conclude that 100-eV electron impact does not produce significant vibrational excitation in the $A{}^2{\Pi }_u^{+}$ state. On the other hand, the $X{}^2{\Sigma }_g^{+}$ state is produced with significant vibrational excitation in the binary lobe where the cross section is large. This excitation is important for both a theoretical understanding of the ionization process as well as plasma and atmospheric phenomena.

We develop an alternative close-coupling scheme based on an eikonal-distorted-wave expansion. The convergence of the basis expansion is improved remarkably at intermediate and high energies in comparison with the ordinary expansion in terms of the unperturbed atomic orbitals. The present electron-capture cross sections of a proton from atomic hydrogen agree well with those of elaborate large-scale close-coupling calculations above 15 keV, even within a two-state approximation. A question is raised as to the effect of the Coulomb boundary conditions to the Oppenheimer-Brinkman-Kramers approximation.

We report the results of a theoretical study of electron scattering by ${\mathrm{CO}}_2$ at incident electron energies ranging from 0.25 to 10 eV using the complex Kohn variational method. These are the first fully ab initio calculations to accurately reproduce the two dominant features observed in experiment, namely, the dramatic rise in the integral cross sections below 2.0 eV and the resonance enhancement near 3.8 eV. Both of these effects are sensitive to the inclusion of electronic correlation effects involving long-range target polarization and short-range distortion. We have also carried out a preliminary study of effects of target vibrational motion in ${}^2{\Pi }_u$ symmetry with an adiabatic nuclei treatment of the symmetric stretch mode. We find that this has a substantial effect on the width of the 3.8 eV resonance feature and gives results for both the integral elastic and total cross sections in excellent agreement with experiment. Our calculated differential elastic cross sections are also in good accord with recent experimental results.

Electron point projection microscopes, such as the Fresnel projection microscope (FPM), provide a convenient tool to observe large molecules, such as carbon nanotubes, over a long period of time, without risking destroying them. In this paper, a Green’s function technique is used to simulate FPM images of $\mathrm{}(n,n)\mathrm{}$ carbon nanotubes, with realistic distances, incident electron energies, and molecular potential. We show that, indeed, the resolution of the three-dimensional (3D) Schrödinger equation gives similar results to the Fresnel theory of diffraction by a 2D mask. Furthermore, the criterium, given by Fresnel theory, for observing the shape of the objects in the images is well verified. Finally our study of the influence of the widths and the lengths of the nanotubes on the images shows that for long nanotubes, the central fringe appears darker as the diameter of the tube is increased, in agreement with experiment.

We present a theoretical study of dissociative photoionization of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ via ${}^1{\Pi }_u$ states, in the photon energy range 30–37 eV where several ionization and dissociation channels are open. The theory includes coherently competition between the different dissociation and ionization channels, and makes use of B-spline functions to represent both the electronic and nuclear motions. Our results agree fairly well with recent experiments, and show that the various peaks observed in the spectra are due to the lowest $Q_2$ ${}^1{\Pi }_u$ doubly excited state, which decays following both a direct and a multistep mechanism. The differences between ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ results can be explained in terms of calculated Franck-Condon factors and autoionization probabilities.

A molecular-confinement model is proposed for the calculation of density effects on the electronic stopping cross section ${(S}_e)$ in a condensed medium. In this model, the collective intermolecular interactions in the medium are represented by a mean field in which a particular molecule is embedded including the spatial constrictions imposed by the surrounding molecules. A molecule is thus viewed as a caged-in system within a spherical boundary with finite potential barrier height $V_B.$ Changes in the molecular electronic properties and molecular conformation as a function of medium density are self-consistently treated. As a first example of a general treatment for more complicated target structures, the model is explicitly applied to the case of proton stopping in dense molecular hydrogen. The lowest barrier height ${(V}_B=0)$ was selected for the stopping calculations since it provides a more realistic pressure-density relation at $T=0\mathrm{}\mathrm{K}$ than higher barrier values. Our results for dense molecular hydrogen predict a very small to moderate reduction in $S_e$ relative to the gas phase in going from atmospheric pressure (0.036 ${\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{c}\mathrm{m}}^3$, $\Delta S_e\approx 0.5\%$) up to 136 GPa (0.380 ${\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{c}\mathrm{m}}^3$, $\Delta S_e\approx 24\%$) for either the liquid or solid phase as determined by the phase diagram for this medium.

The high-order multielectron ionization of ${\mathrm{Xe}}_{531},$ ${\mathrm{Xe}}_{1061},$ and ${\mathrm{Xe}}_{2097}$ clusters in a strong laser field is studied using classical dynamics. The simulations are performed in the frozen geometry approximation for fixed ionic charges, so that only the evolution of the electrons, which are unbound to the ions, is examined. The dependence of the ionization efficiency on the laser frequency and cluster radius is manifested by pronounced maxima. The presence of these maxima favors the quasiresonance ionization mechanism, which is related to the motion of the unbound electrons in the entire cluster. The high level of ionization $(q\sim 8–18$ per atom) is realized in the expanded cluster, due to the Coulomb explosion.

Fission of doubly charged silver clusters ${\mathrm{Ag}}_n^{2+}$ is studied in the size range of $12<~n<~22.$ It is assumed that two fragment clusters are in contact at the transition state. The shape of each fragment is an energetically optimized spheroid. The energy barrier for various fission channels is calculated by using the shell correction method. This simple model closely reproduces the fragment size distribution measured by Katakuse et al. It is confirmed that the shell effects govern the distribution of fission fragments, while the liquid droplet model cannot explain the distribution. In particular, the even-odd oscillations observed in the distribution are found to be due to deformations of fragment clusters. Competition between fission and evaporation is discussed.

We have analyzed by perturbation theory technique two-photon polarization-dependent fractional optical collisions. In its general form the cross section of the collision is expressed as the overlap integral of its spectral profile with the spectrum of the second order correlation function of the electromagnetic field. Based on the semiclassical expansion of radial wave functions and of a retarded Green function we have expressed the spectral profile of the cross section in terms of well understood semiclassical characteristics of the process such as the rotational transformations of the transition dipole moments and the transition amplitudes describing the adiabatic or nonadiabatic dynamics of the electronic subsystem along classical trajectories. As a practical example, we have calculated the Mg-He ${(3s}^2{}^1{S}_0\to 3p{}^1{P}_1\to 5s{}^1{S}_0,4d{}^1{D}_2)$ fractional optical collision cross section. The partial wave analysis for excitation up to the $5s{}^1{S}_0$ state has shown the nonvalidity of quasistatic approximation based on successive single-photon transitions for some frequency detunings. For example, for the positive detuning from two-photon atomic resonance we have obtained the dominant contribution coming from the direct two-photon Franck-Condon transitions. Such a contribution gives the magnitude of the polarization ratio, characterizing the dependence of the cross section on mutual laser polarizations at the first and at the second steps of the photoexcitation, different than for successive single-photon Franck-Condon transitions. In the more complicated case of excitation up to the $4d{}^1{D}_2$ state of magnesium the role of the interference between different photoexcitation channels becomes important. The polarization ratio has a stable value close to $\frac{1}{7}$ in a broad spectral range in the blue wing of the second frequency detuning. There is a strong dependence of the polarization ratio in the red wing of the second detuning. Such spectral behavior can be explained by the partial selection in the red wing of the atomic resonance line, perturbed by the collisions of those Zeeman transitions in which contributions in the polarization ratio have different orders and different signs.

A method for calculating the laser-assisted, electron-potential scattering differential cross section that is based on the Floquet-Lippmann-Schwinger equation is presented. The method is applied to the laser-assisted, low-energy electron-helium scattering in a ${\mathrm{CO}}_2$ laser field, in the static no-exchange approximation, for the scattering geometry in which the polarization vector of the laser field is parallel to the direction of the incident electron. We compare our results with the low-frequency approximation of Kroll and Watson [Phys. Rev. A 8, 804 (1973)] and find, overall, reasonable agreement. An alternative derivation of the Kroll and Watson low-frequency approximation, starting from the Floquet-Lippmann-Schwinger equation, is given in an appendix.

A field-theoretic study of the simultaneous electron-photon excitation (SEPE) and direct electron excitation (DEE) is done for the He $1{}^1\overrightarrow{S}2{}^{3}S$ transition by spin polarized electrons in the field of soft photons. The ratio of SEPE to DEE is found to lie around $3\times {10}^{-3}$ for the 0.117-eV photon and incident electron of energy $E_i$ between 19.817 and 20 eV. The present result is compared with the existing experimental data.

We studied the diamagnetic helium atom under conditions of constant scaled energy over an extended range of Rydberg levels $(n=77\mathrm{}-160).$ Experimental spectra are compared with extensive quantum calculations. In the studied range the effective Planck constant ${ħ}_e$ varies a factor of 2 (from 0.01 to 0.005). When comparing Fourier-transformed spectra in different ${ħ}_e$ regimes, the resulting action spectra show a large similarity, in accordance with the invariance of the classical dynamics. Prominent variations in the recurrence strength of peaks in the action spectra with ${ħ}_e$ could be attributed to interferences between orbits with nearly the same scaled action. In particular, the predicted difference in ${ħ}_e$ scaling of the orbit parallel to the magnetic field when compared with the off-axis orbits is confirmed experimentally.

We have developed a one-dimensional time-dependent Thomas-Fermi model of atoms, molecules, and small clusters exposed to an intense subpicosecond laser field. In this model, the dynamics of an electronic cloud is governed by the hydrodynamic equations of motion, whereas the nuclei move in accordance with the Newton equations. The quantum mechanics enters in this approach only through the constitutive relation between the pressure and the density, which is derived from the application of Fermi-Dirac statistics to a noninteracting $T=0\mathrm{}$ temperature electron gas. The time-independent version of the model, formulated in terms of the integral equation for the electric potential, is also discussed. We present numerical results for diatomic molecules and small clusters irradiated by the strong laser pulses. In the case of molecules, we observe the multielectron ionization accompanied by dissociation. The kinetic energy defect, understood as a reduction of the energy of resulting ions in comparison with a simple Coulomb explosion picture, is explained in terms of a screening effect of escaping fragments by the ejected electrons. For small clusters we find that the explosion of the cluster has a stepwise character; the consecutive layers of atoms are stripped off one by one. We observe the highly energetic (as compared with diatomic systems) atomic fragments even for relatively low laser intensities and a few atoms clusters. Also, the model predicts the nonuniform energy distribution among the same charge state ions and supports the idea of hot electrons generated in the cluster via a mechanism of inverse bremsstrahlung.

We show that both the laser-assisted and field-free cross sections for electron scattering by an excited state of an atom, or molecule, can be found by analyzing the resonant laser-assisted scattering process, provided that the initial low-lying state is brought into resonance with the excited state. The method relies on the adiabatic evolution of the laser field throughout the resonance, and its subsequent adiabatic switchoff out of the resonant region.

We study numerically stabilization against ionization of a fully correlated two-electron model atom in an intense laser pulse. We concentrate on two frequency regimes: very high frequency, where the photon energy exceeds both the ionization potential of the outer and the inner electron, and an intermediate frequency where, from a “single active electron” point of view, the outer electron is expected to stabilize but the inner one is not. Our results reveal that correlation reduces stabilization when compared to results from single active electron calculations. However, despite this destabilizing effect of electron correlation we still observe a decreasing ionization probability within a certain intensity domain in the high-frequency case. We compare our results from the fully correlated simulations with those from simpler, approximate models. This is useful for future work on “real” more-than-one electron atoms, not yet accessible to numerical ab initio methods.