We use closed-orbit theory to study the absorption spectrum of an atom in an electric field. In previous work we examined absorption spectra above the zero-field ionization threshold. Only one closed orbit exists there, and it is unstable. Now we examine the situation below threshold. Here, the orbit parallel to the electric field is stable and, as the energy decreases, many other closed orbits bifurcate out of it. These closed orbits have simple patterns, and the associated recurrences are most clear if the absorption spectrum is measured using a scaled-variables method. The relation between the semiclassical Einstein-Brillouin-Keller-Marcus (EBKM) theory and periodic-orbit or closed-orbit theory is examined: they are complementary methods in the same sense that energy and time are complementary variables in quantum mechanics. Our numerical calculations show that sinusoidal fluctuations contributed by the closed orbits combine into peaks, and these peaks are in the locations predicted by EBKM theory.

The velocity dependence of methane ionization and fragmentation have been studied using a coincidence time-of-flight technique. The relative yields of single-ion, ion-pair, and ion-triple channels have been determined for 1- to 12-MeV proton impact. The multiple-ionization cross sections have been determined and are in accord with our model semiclassical Coulomb approximation calculations. The single-ion channels resulting from the breakup of ${\mathrm{CH}}_4^{+\mathrm{*}}$ were found to be independent of the collision velocity. Good agreement with previous proton-impact measurements and with electron-impact measurements is observed. On the other hand, the ion-pair breakup channels of ${\mathrm{CH}}_4^{2+\mathrm{*}}$ show a surprising dependence on the collision velocity. The ion-pair data are in reasonable agreement with fast-electron-impact measurements at 10 keV while differing significantly with a lower, 1-keV, electron-impact measurement.

Elastic-scattering cross sections of photons for momentum transfer in the range 0.1≤x≤2.2 A${\mathrm{̊}}^{\mathrm{-}1}$ with ${\mathit{E}}_{\gamma }$=59.5 and 316 keV were measured in order to test the limits of momentum transfer within which the scattering by real samples can be considered as being due to free-atom Rayleigh scattering. Polycrystalline aggregate, liquid, perfect-single-crystal, and crystal powder samples were used as scatterers. The measurements for ${\mathit{E}}_{\gamma }$=316 and 59.5 keV were carried out in the angular ranges θ=0.5°–10° and 2°–23° with a geometrical resolution of 0.1° and 1°, respectively. At the present time, most of the experimental data on elastic photon scattering have been measured using solid molecular structures as scatterers. The present results allow the conclusion that highly precise data (Δdσ/dΩ≤2%) on elastic scattering of photons can be compared with free-atom Rayleigh-scattering theories only for x>2 A${\mathrm{̊}}^{\mathrm{-}1}$.

A very recent experimental observation of the K-shell Auger spectrum in atomic oxygen has been verified theoretically by a calculation using the multiconfiguration Hartree-Fock method. Results obtained for the energies of each of the observed transitions are in excellent agreement with the experimental measurements. In addition, energies have been determined for the excited-state resonances of the type 1s→2p and the subsequent excited states to which these excitations decay. Relative intensities calculated for each of these transitions are found to be in excellent agreement with experiment.

A relativistic approach is employed to compute the differential and integrated cross sections, spin-polarization P, and the spin-polarization parameters T and U for the scattering of electrons from zinc and lead atoms in the energy range 2.0–200 eV. The projectile-target interaction is represented both by real and complex optical potentials in the solution of the Dirac equation for scattered electrons. We compare our results for spin-polarization P with recent experiment and available theoretical calculations.

We have observed resonant-energy-transfer collisions between K Rydberg atoms. We have tuned the ${\mathrm{ns}}_{1/2}$+(n-2)${\mathrm{d}}_{\mathrm{j}}$→${\mathrm{np}}_{\mathrm{j}\prime }$+(n-1)${\mathrm{p}}_{\mathrm{j}\mathrm{″}}$ collisional process into resonance with a static electric field. The measured resonance field positions from n=24 to 29 agree well with the calculated positions. We show that the dipole-dipole selection rule, Δ${\mathrm{m}}_{\mathrm{j}}$=0,±1, is valid for these resonances but that ΔJ=0,±1 is not, due to the presence of the electric field. In addition, the polarization of the lasers used to excite the Rydberg states can suppress many of the resonances.

The Beliaev-Galitskii relation between the two-body t matrix and the scattering amplitude is shown to be slightly erroneous in hard-core scattering because of an incorrect interchange of limits. The correction term is computed exactly at zero momentum both in three dimensions [to relative order O(${\mathit{Ea}}^2$)] and in two dimensions [to relative order O(${\mathit{Ea}}^2$ln${\mathit{Ea}}^2$)].

We observe the production of ${\mathrm{Na}}^{+}$ ions in a single effusive sodium atomic beam via intrabeam dissociating collisions of laser-excited Na(3p) atoms with ${\mathrm{Na}}_2^{+}$ ions created by associative ionization. The rather large cross section for this collisional dissociation process has been experimentally determined to be 3(±1)×${10}^{\mathrm{-}13}$ ${\mathrm{cm}}^2$ at a source temperature of ≊530 K. A classical Langevin model gives an estimate based on long-range forces of about twice this value.

Measurements of the optical emission cross sections for the ${\mathrm{N}}_2^{+}$ ${\mathit{D}}^2$ ${\mathrm{\Pi }}_{\mathit{g}}$→A ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathit{u}}^{}{}_{}{}^{}$ emission bands produced by electron impact on ${\mathrm{N}}_2$ are reported. From these optical data, estimates of the apparent cross sections for the electron-impact production of ${\mathrm{N}}_2^{+}$ (D ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathit{g}}^{}{}_{}{}^{}$) in the vibrational states v=4, 7, and 9 are obtained. Combining these data with similar cross sections for the ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathrm{u}}^{}{}_{}{}^{}$→X ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{u}}^{+}{}_{}{}^{}$→X ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$ bands and with ionization cross sections of ${\mathrm{N}}_2$ enables us to analyze the systematics of the cross sections for producing ${\mathrm{N}}_2^{+}$ in the X ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$, A ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathrm{u}}^{}{}_{}{}^{}$, ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{u}}^{+}{}_{}{}^{}$, and D ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathrm{g}}^{}{}_{}{}^{}$ states. Two different kinds of excitation behaviors are found and are discussed in reference to the electronic structure of the ${\mathrm{N}}_2^{+}$ ion.

Theoretical and experimental results are presented for the electron-impact ionization of ${\mathrm{Sc}}^{2+}$ in the near-threshold energy region where strong resonance features due to dielectronic capture processes are found. The indirect ionization contributions are calculated in the close-coupling approximation. Both absolute and scan measurements for the ionization cross section are obtained in a crossed-beams experimental geometry. The overall agreement between theory and experiment is good, once a sufficient number of singly excited states are included in the close-coupling expansion. The additional states allow a proper theoretical determination of the decay pathways available to the resonances formed following dielectronic capture of the incident electron.

Total and partial photoionization cross sections are computed for CO at the fixed equilibrium internuclear separation. Finite basis sets were used, with no explicit recourse to continuum orbitals, and there is no need for input of empirical data such as the ionization threshold. By the present method the first step is to compute the (real) polarizability α(iη) on the imaginary frequency axis. Many-body perturbation theory is used for that purpose, and the expansion is complete to second order in the Coulomb interaction. In the next step the cross sections are obtained from α(iη) by inverting a simple integral equation of the Fredholm first kind (dispersion relation). For the partial cross sections, extra approximations may be necessary, as some diagrams to first and second order make contributions to more than one specific partial cross section. The computed total cross section is in very good agreement with experiment, and good agreement is also obtained for the 1π partial cross section. For the strongly interacting σ shells, the comparison with experiment is less certain. The present results, which include comprehensive correlation corrections, are also compared with existing theoretical results that are on the independent-particle level of accuracy.

We have determined the ${\mathit{L}}_{\mathrm{\perp }}$ coherence parameter for electron-impact excitation of the (...6s6p 6 ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}_1$) state in ${}_{}{}^{138}{}_{}{}^{}\mathrm{Ba}$ by measuring the superelastic scattering of electrons from laser-excited ${}_{}{}^{138}{}_{}{}^{}\mathrm{Ba}$. Three electron-impact energies were studied and the scattering-angle coverage in each case was sufficient to allow the observation of pronounced structure in the angular behavior of this parameter. We also present a detailed comparison of our results with available theory.

We calculated 2p inner-shell photoionization cross sections of atomic sodium in the ground state in the photon-energy region close to the 2s threshold using the R-matrix method. We included five residual ${\mathrm{Na}}^{+}$ (2${\mathit{p}}^6$ ${}_{}{}^1{}_{}{}^{}$S, 2${\mathit{p}}^5$3s ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$, ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$, 2s2${\mathit{p}}^6$3s ${}_{}{}^3{}_{}{}^{}$S${,}^1$S) states in the R-matrix expansion. The initial Na ${(}^2$S) bound state, the final ${\mathrm{Na}}^{+}$ plus photoelectron ${(}^2$${\mathit{P}}^{\mathit{o}}$) state, and the residual ionic states are represented by fairly extensive configuration-interaction wave functions. The prominent features in the cross sections are two autoionization Rydberg series converging to ${\mathrm{Na}}^{+}$ (2s2${\mathit{p}}^6$3s ${}_{}{}^3{}_{}{}^{}$S${,}^1$S) excited-state thresholds. The present results are compared with the available experiments.

The molecular states that control the charge transfer of protons with chlorine atoms and of chlorine ions with hydrogen atoms are identified, and estimates are made of the strengths of the spin-orbit interactions that drive the processes. The cross sections are calculated by solving numerically the coupled equations of scattering theory and the resulting rate coefficients are presented. The rate coefficient for the charge transfer of protons with chlorine atoms at 70 K is 1.2×${10}^{\mathrm{-}12}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$ and at 300 K is 4.3×${10}^{\mathrm{-}11}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$.

The first five coefficients in the small-angle expansion of the differential cross section for elastic scattering of fast electrons by ${\mathrm{H}}_2$, HD, and ${\mathrm{D}}_2$ molecules are calculated in the first Born approximation. Highly accurate electronic wave functions containing an explicit dependence on the interelectronic coordinate are used to describe the target. Numerical wave functions are used to describe the vibrational motions. The calculated small-angle expansion coefficients for the ground state of ${\mathrm{H}}_2$ are in good agreement with experiment.

The multichannel-quantum-defect theory (MQDT) of the Stark effect is applied to the simulation of transitions in molecular hydrogen to the autoionizing Rydberg states in the region between the ${\mathit{v}}^{+}$=1 and ${\mathit{v}}^{+}$=2 thresholds, with principal quantum numbers 13–19, for applied electric fields in the range 100–1300 V/cm. The vibrational coupling between Rydberg states belonging to different series is included for the first time in a MQDT Stark-effect calculation. The results of the calculations are compared with previously reported experimental results [Chem. Phys. Lett. 185, 199 (1991)]. The calculations not only advance the understanding of the experimental Stark spectra, but they also suggest that additional insight into zero-field interactions can be gained from a detailed understanding of the Stark effect in the hydrogen Rydberg states.

Knowledge of the long-range interaction between atoms and molecules is of fundamental importance for low-energy and low-temperature collisions. The electronic interaction between the charge distributions of two ground-state alkali-metal atoms can be expanded in inverse powers of R, the internuclear distance. The coefficients ${\mathit{C}}_6$, ${\mathit{C}}_8$, and ${\mathit{C}}_{10}$ of, respectively, the ${\mathit{R}}^{\mathrm{-}6}$, ${\mathit{R}}^{\mathrm{-}8}$, and ${\mathit{R}}^{\mathrm{-}10}$ terms are calculated by integrating the products of the dynamic electric multipole polarizabilities of the individual atoms at imaginary frequencies, which are in turn obtained by solving two coupled inhomogeneous differential equations. Precise one-electron model potentials are developed to represent the motion of the valence electron in the field of the closed alkali-metal positive-ion core. The numerical results for the static multipole polarizabilities for the alkali-metal atoms and the coefficients ${\mathit{C}}_6$, ${\mathit{C}}_8$, and ${\mathit{C}}_{10}$ for homonuclear and heteronuclear alkali-metal diatoms are compared with other calculations.

A distorted-wave Born exchange approximation was used to calculate the cross section for electron-impact ionization of the hydrogen atoms. Both the integral and energy-differential cross section were calculated. The results were compared with the latest experimental data and other theoretical calculations. Comparison shows that the calculations agree with differential cross-section measurements in general. For integral cross sections the calculation shows a better agreement with an earlier measurement [M.B. Shah, D. S. Elliott, and H. B. Gilbody, J. Phys. B 20, 3501 (1987)] in which the cross sections are normalized to the first Born approximation.

The nonlinear response to external disturbances of a system of interest scrA (the ‘‘atom’’) in contact with a thermal bath is studied. The central issue is the relationship between the nonlinear susceptibility and the equilibrium relaxation spectra of observables of scrA. Paralleling the results of the linear-response theory, the nth-order nonlinear susceptibility is factorized by quantum-mechanical arguments into two terms pertaining to the disturbances and the spin-bath system, respectively. The role of slow atom states, i.e., states relaxing on times T longer than the bath correlation time ${\mathrm{\tau }}_{\mathit{c}}$, is pointed out, by recurring to recent findings on their general features [Phys. Rev. A 46, 6222 (1992)]. In the frame of a stochastic picture of the bath and a multilevel scheme of the system scrA, the conditions under which the nonlinear susceptibility provides the spectrum of the slow atom states are clarified. Illustrations are drawn from magnetic resonance to demonstrate that techniques based on the nonlinear response of a spin system to multiple continuous waves compete favorably with pulsed techniques to provide information on longitudinal spin relaxation.

The detection of cavity radiation with the detector placed outside the cavity is studied. Each leaked photon has a certain probability of propagating away without being detected. It is viewed as a continuous quantum measurement in which the density matrix is continuously revised according to the readout of the detector. By analyzing in terms of the more elementary no-leak and one-leak-undetected processes, the superoperator for the no-count process is derived, which leads to a pseudo-master-equation governing the time evolution of unnormalized photon-number distribution.