The modified Faddeev equations in configuration space are applied to solve numerically the problem of the zero-energy elastic scattering of an electron by positronium at zero angular momentum. The singlet and triplet scattering lengths are calculated and zero-energy limits of elastic and ortho-para conversion cross sections are established. The results confirm and improve previous variational estimations of these parameters.

Optical spectroscopy has been used to study electron capture into ${\mathrm{Ar}}^{9+}$ and ${\mathrm{Ar}}^{8+}$ ions from cesium at several impact energies of between 8 and 180 keV. Many Rydberg lines were observed. At high spectral resolution the quasi-l-degeneracy of final Rydberg states was removed and the fine-structure components of doubly excited Rydberg transitions were resolved. Cross sections for state-selective electron capture in collision of ${\mathrm{Ar}}^{8+}$ with cesium were determined and then compared with theoretical data calculated by means of a three-body classical-trajectory Monte Carlo method and with predictions of a classical barrier model.

Single- (SC) and double-electron capture (DC) have been studied for collisions of ${\mathrm{Ar}}^{8+}$ with He using x-ray and vacuum ultraviolet (vuv) spectroscopy at a collision energy of 80 keV as well as translational-energy spectroscopy (TES) at lower collision energies (8 keV). It is shown that both processes involving the ground-state ion terminate in vuv photon emission, i.e., nonautoionizing levels are populated. The SC, by the long-lived metastable species ${\mathrm{Ar}}^{8+\mathrm{*}}$ (2${\mathit{p}}^5$3s${)}^3$${\mathit{P}}_{0,2}$, populates mostly the Na-like core-excited levels with n=4: ${\mathrm{Ar}}^{7+\mathrm{*}}$ (2${\mathit{p}}^5$3s ${}_{}{}^3{}_{}{}^{}$P 4l${)}^{2,4}$${\mathit{L}}_{\mathit{J}}$. With the use of calculated decay and autoionization rates, the measured vuv spectra are analyzed and compared with the TES results and the Auger spectra measured by other authors. It is shown that the stabilization of these core-excited states is both radiative and autoionizing. The DC by the metastable projectile is not yet fully understood.

We report the results of an ab initio calculation of the electron-impact dissociation of ${\mathrm{H}}_3^{+}$ via excitation of the two lowest (${\mathrm{E}}^3$′,${\mathrm{E}}^1$′) excited electronic states, for electron energies between 15 eV, the threshold for excitation of the first electronic state, ${\mathrm{E}}^3$′, and 20.7 eV, corresponding to the excitation energy of the third excited electronic state, ${\mathrm{A}}_2^{1\mathrm{″}}$. The calculations, which were carried out using the complex Kohn variational method, employed extensive correlated target wave functions that accurately reproduced the energy levels and transition dipoles of the ground and excited states of the ion. All open channels were included explicitly in the scattering calculation. Closed-channel effects were included via an ab initio optical potential. The fixed-nuclei excitation cross sections were found to be dominated by a series of sharp resonance structures. However, when the cross sections are averaged over the Franck-Condon envelope of the ground vibrational state, these sharp features are no longer seen.

We have measured photoelectron angular distributions for the single-photon ionization of cesium at various wavelengths in the range from 313.5 to 266 nm. The photoelectron-angular-distribution asymmetry parameter β shows strong variations with energy, in contrast to nonrelativistic prediction of β=2. This variation is due to the spin-orbit interaction of the continuum electron. This effect is particularly dramatic near the Cooper minimum. From these results we determine the Fano parameter, providing confirmation of the semiempirical results of Norcross [Phys. Rev. A 7, 606 (1973)] for this range of energies.

Shake-up transitions during the Auger decay of the Kr 3${\mathit{d}}^{\mathrm{-}1}$np, n=5,6 resonance states have been studied with high photon and electron resolution (<0.1 eV) using the Aladdin undulator beamline in Stoughton, Wisconsin. This resolution enables us to resolve the complex spectra and study the distribution of intensity between the multiplets of the 4${\mathit{p}}^{\mathrm{-}2}$np final states. The intensity distribution between these multiplets has been found to depend strongly on the principal quantum number n, and theoretical intensity calculations are in good agreement with experiment. Correspondingly, shake fractions differ for each parent and also depend strongly on whether the electron is excited to the first or second Rydberg orbital.

${\mathrm{N}}_2$ gas rototranslational scattering is studied at high frequency up to 700 ${\mathrm{cm}}^{\mathrm{-}1}$ theoretically and experimentally for several temperatures and from low densities up to 170 amagat. Our theoretical calculations take into account multipole contributions from the mean value and anisotropy of the dipole-dipole polarizability tensor a→ and two independent components of the dipole-octopole polarizability tensor E→. The E→ tensor is found to give the most important contribution at high frequency. The comparison of the experimental and theoretical spectra allows us to estimate at ‖Ẽ ${}_0{}^{\mathrm{}\left(4\right)\mathrm{}}{}_{}{}^{}\mathrm{\Vert }$=4.6 Å ${}_{}{}^5{}_{}{}^{}$ the absolute value of the experimental (λ=514.5 nm) dynamic irreducible spherical component of the dipole-octopole polarizability tensor of ${\mathrm{N}}_2$. When using this value, good agreement is obtained between the theoretical and experimental spectra at room temperature (294 K) and low temperature (150 K).

The multichannel multiconfiguration Dirac-Fock method has been used to study the Ne KLL and Ar ${\mathit{L}}_2$${\mathit{M}}_{2,3}$${\mathit{M}}_{2,3}$ Auger-electron transitions. Final-state correlation effects are accounted for by simultaneously including several ionic configurations and ionization channels in the calculation of the final-state many-electron wave functions. Our computational approach is based on scattering theory, which properly accounts for the incoming wave boundary condition. The line intensities from our test calculation for Ne KLL transitions are in good agreement with earlier theoretical results and with experiment. The interchannel coupling is stronger for the final states of the Ar ${\mathit{L}}_2$${\mathit{M}}_{2,3}$${\mathit{M}}_{2,3}$ transitions, but the net effect on the line intensities is somewhat smaller than that for Ne. Even the most extensive multichannel calculations are not fully able to reproduce the measured total rate and the ratio of ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_{0,1,2}$ to ${}_{}{}^1{}_{}{}^{}$${\mathit{D}}_2$ transition rates. The remaining discrepancy in the branching ratios of Auger lines is attributed to the omission of initial-state configuration interaction, whereas the discrepancy between the measured and calculated total rates is suggested to be mainly due to the neglect of relaxation. Our results indicate that, in analogy to outer-shell photoionization, the final-state interchannel interaction does not in general affect the strongest Auger lines very much when the kinetic energy of the Auger electrons exceeds about 200 eV.

K-shell ionization induced by fast ions was simulated by the excitation of a harmonic oscillator perturbed by time-dependent coupling of a parallel spring. Permanent coupling of the spring corresponds to the atom where both nuclei are united. In this adiabatic limit the two systems have an increased energy and reduced length scale. The ratio of the united-atom cross sections to the experimental ones was compared to the ratio of the oscillator excitation probabilities calculated in the adiabatic limit and by the coupled-channel method. Strong similarities between the behavior of both types of ratios were observed, and it was pointed out that the adiabatic approximation overestimates the transition probabilities even at moderate collision velocities. The effective nuclear charges were deduced from the model and used for the cross-section calculation with hydrogenic Dirac wave functions. For the proton data, calculated and experimental cross sections agree within 10%. Due to the strong coupling of states in the adiabatic region, the effective nuclear charges exceed the united-atom limit by 20%.

Cross sections for 1s ionization of the 1${\mathit{s}}^2$2${\mathit{s}}^2$ and 1${\mathit{s}}^2$2s2${\mathit{p}}^3$P (metastable) states of ${\mathrm{C}}^{2+}$ and ${\mathrm{O}}^{4+}$ ions in 0.5–1.8 MeV/u collisions with ${\mathrm{H}}_2$ and He targets were measured using projectile Auger electron spectroscopy at 0°. Calculations of the K-shell ionization cross section ${\mathrm{\sigma }}_{\mathit{K}}$ were performed including contributions from projectile-electron–target-nucleus interactions (${\mathrm{\sigma }}_{\mathit{e}\mathit{n}\mathit{I}}$) within a plane-wave Born approximation (PWBA), and contributions from projectile-electron–target-electron interactions (${\mathrm{\sigma }}_{\mathit{e}\mathit{e}\mathit{I}}$) within an impulse approximation (IA). The theoretical total cross section ${\mathrm{\sigma }}_{\mathit{K}}$=${\mathrm{\sigma }}_{\mathit{e}\mathit{n}\mathit{I}}$+${\mathrm{\sigma }}_{\mathit{e}\mathit{e}\mathit{I}}$ was found to be in overall agreement with the K-shell ionization measurements indicating a contribution of up to ∼30% due to electron-electron interactions. Separate screening-antiscreening calculations of ${\mathrm{\sigma }}_{\mathit{K}}$ were also performed and found to be in overall agreement with our data. Additionally, our PWBA-IA ionization calculation was also tested for H-like projectiles and found to be in agreement with recently published data. These results suggest that the IA calculation of ${\mathrm{\sigma }}_{\mathit{e}\mathit{e}\mathit{I}}$ when combined with a PWBA calculation of ${\mathrm{\sigma }}_{\mathit{e}\mathit{n}\mathit{I}}$ is quite adequate for describing K-shell ionization.

We derive the summed and averaged differential cross section for vibrational excitation of molecules in electron scattering in the framework of the algebraic-eikonal approach. The additional ingredient is that the T-matrix elements are derived in the mean-field approximation using intrinsic wave functions for the vibrational excitations.

The cross section for the collisional vibrational transition of a molecule from any state $v$ to any state $v^{\prime \prime }$ is shown to be expressible as a linear combination of the cross sections for the transitions $0\to v^{\prime }$ with $v^{\prime }\le 3Z$, 1 → 1, and 1 → 2 in the adiabatic-vibration approximation, provided that the molecule is regarded as a harmonic oscillator and that the transitions $0\to v^{\prime }$ with $v^{\prime }\ge 4$ hardly occur. Sum rules for vibrational cross sections are also derived.

By means of geometric quantization, we discuss the quantum-deformed-symmetric-top model. We show that the quantum-deformed symmetric top and the quantum-standard symmetric top obey the same Heisenberg equations of motion. The quantum-deformed-symmetric-top model possesses quantum group symmetry; thus the quantum group can be dynamical symmetries of the symmetric-top system. The energy levels and wave functions of stationary states of the quantum-deformed-symmetric-top model are obtained exactly by a quantum-group-theoretic approach. Using the exactly solved model, we investigate the rotational spectra of symmetric-top molecules and diatomic molecules when the moment of inertia about the line joining the nuclei is not exactly zero.

We show how interferometry can be carried out within a molecule. The method can be used to test semiclassical quantum mechanics and to map out Born-Oppenheimer energy surfaces. The interference occurs between molecular wave packets that travel through different molecular states. Packet splitting and recombination is carried out at laser-induced level crossings. We have compared the exact quantum-mechanical behavior (calculated numerically on a Cray computer) with a pseudoclassical method.

Squeezing the vacuum fluctuations of the electromagnetic field modifies the quantum fluctuations of the optical forces exerted on laser-cooled two-level atoms. Under certain conditions, this modification when combined with the enhanced average forces can lead to equilibrium temperatures below those attained under normal-vacuum fluctuations.

It is shown that, as a result of the atomic recoil associated with the absorption and emission of radiation, resonances can appear in nonlinear spectroscopy. The theory is illustrated by calculating the line shapes associated with nearly degenerate four-wave mixing and pump-probe spectroscopy when laser fields interact with an ensemble of two-level atoms. The recoil-induced resonances should be observable for atoms cooled below the Doppler limit of laser cooling, and the line shapes may provide a means for measuring the velocity distribution of these cooled atoms.

We study the production of total entropy in the situation where light has kinetic effects on atoms or molecules. The radiation entropy has the standard form of boson entropy, and the matter entropy is the sum of Boltzmann entropies for excited and nonexcited particles. The matter entropy density is separated into three parts, corresponding to the total velocity distribution, the distribution over internal states, and the correlation between the two. Each of these parts can be changed by the interaction with light. We calculate explicitly the production of both matter and photon entropy in several cases where light is known to induce macroscopic flows in gases. The production rate of matter entropy is expressed in macroscopic quantities. Some other examples, such as laser cooling, are considered briefly. In all cases, the increase of photon entropy is found to be several orders of magnitude larger than the decrease of matter entropy.

Using a simple analytic formula from closed-orbit theory, we calculate photoabsorption cross sections of hydrogen and sodium in a strong electric field. The theoretical spectra show good agreement with experimental results. A scaled variable measurement is also suggested.

A formula for photoabsorption cross sections of hydrogen and alkali-metal atoms in a static electric field is derived, based on the closed-orbit theory previously used to study hydrogen in a magnetic field. Electric fields are simpler than magnetic fields, because the classical motion is regular and closed orbits can be enumerated. In alkali metals the core modifies the relevant dipole matrix elements, and it produces additional phase shifts.

An enhancement of the index of refraction accompanied by vanishing absorption is shown to be possible in an ensemble of phase-coherent atoms (‘‘phaseonium’’). A survey of various possible schemes in which coherence is established by certain coherent or incoherent methods is given, and the main results are compared and contrasted. In particular, the influence of processes such as Doppler broadening that degrade coherence is discussed.