The time evolution of a particle that switches randomly between two deterministic states, one stationary and the other one mobile, is studied. The methods of J. C. Giddings and H. Eyring [J. Phys. Chem. 59, 416 (1955); J. C. Giddings, J. Chem. Phys. 26, 169 (1957)] and that of G. H. Weiss, J. Masoliver, K. Lindenberg, and B. J. West [Phys. Rev. A 36, 1435 (1987)] are extended to treat arbitrary evolution laws in the mobile phase and shown to lead to identical results. Several applications of interest are presented.

Finite-temperature (0≤T<∞) field (FTF) theory with an effective spectral Lagrangian density formulation is used to study quantum noise in an electrical network. Solutions for the finite second moments that satisfy the uncertainty principle bound are given for a dissipative quantum oscillator. A regularization method, based on the analysis of a semi-infinite low-pass filter, is employed, and it leads to results which differ from those of the Drude model. To illustrate the FTF method, an example is given using an ideal finite-temperature coherent state.

The theory of the Auger effect is extended to include the modifications due to the presence of a strong laser field. The laser makes possible the simultaneous ejection of electrons into many channels, each characterized by the number of exchanged photons. The laser also introduces a new physical axis, which reduces the geometrical symmetry of the process. An estimate is given for the limit of weak fields.

Through a particle-simulation investigation, a new mode-conversion process, through which an incident fast extraordinary mode (fast $X$ mode) is converted into an electron Bernstein mode ($B$ mode) via a slow extraordinary mode (slow $X$ mode), is discovered in plasmas whose maximum density exceeds the cutoff density of the slow $X$ mode. The converted $B$ mode is found to heat the electrons efficiently in an overdense plasma region, when the plasma has the optimum density gradient at the plasma surface.

We present a density-functional-variational theory to investigate the nature of the freezing transition in a system consisting of hard ellipsoid and hard dumbbell molecules. We predict the isotropic-plastic and the isotropic-nematic phase coexistence for hard ellipsoid molecules. The results are in fair agreement with Monte Carlo simulations. For hard dumbbell molecules we find an orientationally disordered fcc lattice phase for bond length $\frac{L}{\sigma }$ in the range $0<\mathrm{}\frac{L}{\sigma \le 0.26}$.

Numerical solutions of the multicentered Thomas-Fermi equation for molecules are obtained by use of an efficient multigrid scheme based on the standard finite-difference discretization of the Laplacian in a modification of this nonlinear equation. It is found that the difference between the molecular potential and the sum of atomic potentials is bounded and varies smoothly through the nuclei, and is thus a good choice for the dependent variable to be discretized. This choice also results in a general program that is applicable to any molecule whose structure is known; the input data are the nuclear charges and their locations. The electron density and the difference between the electron density and the sum of atomic densities are obtained for ${\mathrm{N}}_2$ and compared with the results of molecular-orbital calculations. The method’s applicability to larger molecules is illustrated by a calculation of the electron density of ${\mathrm{C}}_6$${\mathrm{H}}_6$. Its extension to crystalline solids with cubic unit cells is straightforward.

Using the general anharmonic oscillator as a case study, we examine the coupled-cluster method (CCM) in some detail. Emphasis is specially placed on the accuracy of the standard ground-state energy calculation and the excitation spectrum as derived via the Emrich ansatz. We are particularly interested in problems that can arise from large differences between the exact wave function and its model counterpart that forms the starting point of the CCM. To this end we begin with a variational Hartree approximation applied to three particular anharmonic-oscillator Hamiltonians that contain, respectively, pure quartic, equally weighted cubic and quartic, and double-well perturbations. These are chosen to provide increasingly stringent tests for the CCM in the above sense. Considerable attention is paid to the variational description of the double-well case, where the various possible solutions to the Hartree equations are considered and a further calculation on the energy-level splitting is performed. The CCM is then used to improve systematically upon our chosen starting point and is shown, particularly for the ground state, both to produce extremely accurate results and to be rather resilient to even gross deficiencies in the starting wave function. The main problem is the double-well system, where the CCM shows its lack of intrinsic inbuilt tunneling mechanisms via the absence of level splitting. Even here the CCM still produces very accurate average energies for very deep wells. While clearly needing some modification in such extreme cases, the CCM is shown to be quite adaptable and robust with regard to inaccurate starting wave functions.

Explicit density-matrix solutions are obtained for several simple open quantum systems without the usual trace being taken over the thermal bath. This is achieved using a representation of the density matrix in terms of the dynamic operators of the system. The results are compared with those from conventional reduced calculations. Different types of irreversible behavior are found, and the implications of this distinction for models of quantum measurement are considered.

Based on Klauder’s continuous representations in Hilbert space, we discuss a fundamental fuzziness of the coordinate and momentum variables for a quantum particle. These fuzzy dynamical variables are intimately related to a very small radical length R(≲${10}^{\mathrm{-}18}$ cm) and a very large cosmological length S (≳${10}^{10}$ light years). The uncertainty Δq for the measurement of a particle’s position alone is assumed to be restricted by S≳Δq≳R. Physical properties of fuzzy base states (p‖ and (q‖ and their general cosmological and very-high-energy implications are discussed. In particular, a new density of states and its possible effects on decay rates and scattering cross sections at large momenta p≳ħ/R∼${10}^4$ GeV/c (c is the velocity of light) are considered. We also discuss the superposition of waves in the presence of R and S.

In the general form of quantum-defect theory, the basis functions of the long-range potential are parametrized in terms of six real functions of energy and angular momentum. The parameters above and below zero energy are shown to be connected through simple relationships upon analytic continuation. These relationships are completely general, valid for any long-range field, and are expressions of the analyticity properties of the underlying Jost functions.

The behavior of the quasistatic wing of the light-induced collisional-energy-transfer (LICET) line shape is analyzed. The reaction can be written as $A^{\mathrm{*}}$+B+ħΩ→A+$B^{\mathrm{*}\mathrm{*}}$. The two-level model is found to be inadequate to describe the falloff of the spectral line shape in the quasistatic wing. The three-level model, on the other hand, leads to the prediction of a cross section that depends on the detuning in various ways, according to the strength of the collisional coupling and the influence of the van der Waals shifts on the levels under consideration. Good agreement with existing experimental results is found. To check the approximations made in the derivation of these results, numerical evaluation of the transition probability has been performed for several cases. A Monte Carlo simulation of the full LICET spectrum is also presented.

We consider the interaction of a positron with a bound electron of a high-Z atom. In particular, we present relativistic calculations of the cross sections for knockout and/or positronium formation in a thorium target, as well as an internal process in a hypothetical Z=164 ‘‘atom.’’ Both plane-wave and distorted-wave Born approximations are discussed. We find peaks in the final sum energy of the electron-positron pairs due to the momentum distribution of the atomic electron. However, these peaks are too low in energy and are too broad compared with peaks claimed in $e^{+}$$e^{\mathrm{-}}$ scattering or with the anomalous positron peaks in heavy-ion experiments.

The contribution to electron detachment in low-energy ${\mathrm{H}}^{\mathrm{-}}$(${\mathrm{D}}^{\mathrm{-}}$)-Na collisions due to a direct transition from the bound ground state of (HNa${)}^{\mathrm{-}}$ to the (HNa+$e^{\mathrm{-}}$) continuum is studied in an effective-range approximation, based on ab initio calculations. Neutralization of ${\mathrm{H}}^{\mathrm{-}}$ in collisions with Na atoms is shown to be dominated by this direct detachment process at low collision energies. At higher collision energies, charge transfer to (H+${\mathrm{Na}}^{\mathrm{-}}$) and detachment via charge transfer (previously studied by R. E. Olson and B. J. Liu [J. Chem. Phys. 73, 2817 (1980)]) become the dominant ${\mathrm{H}}^{\mathrm{-}}$ neutralization mechanisms. These theoretical results are compared with the experimental data of Y. Wang, R. L. Champion, and L. D. Doverspike [Phys. Rev. A 35, 1503 (1987)]. A new calibration of these measurements is proposed, based on a comparison with previous absolute measurements at higher energies by A. M. Howald, L. W. Anderson, and C. C. Lin [Phys. Rev. A 24, 44 (1981)].

Total cross sections for excitation of He singlet levels by highly charged ions of charge states q from 3 to 45, and for projectile velocities from 1.6 to 7.5 a.u., were measured. A scaling of cross section per q with specific energy per q was observed, and is discussed on the basis of classical arguments. In addition to their fundamental aspect, the measurements are important for the prospects of neutral-beam injection heating and beam-driven currents in tokamaks.

Kinetic energy distributions of electrons emitted in the interaction of slow (10q-keV) multicharged ions (N,O,Ne,Ar) with metal surfaces (Cu,Au) are presented. We observe secondary-electron emission, quasielastic-scattering processes, and, superimposed upon this, one-center Auger decay of both projectile and target vacancies. Two classes of target Auger-decay features are observed. Long-lived target vacancies decay, leading to discrete Auger line features, similar to those arising from electron-impact excitation. Short-lived target vacancies appear to decay in the field of the projectile, leading to broadened Auger line features. Observation of the decay of target vacancies, and their correlation with the presence of particular projectile vacancies, shows that a measurable fraction of projectile vacancies survive until small-impact-parameter collisions with surface or subsurface atoms. The data imply that projectile neutralization proceeds via two channels, viz., capture of valence-band electrons to projectile excited states, followed by one-center Auger decay, and at pseudocrossings of molecular orbitals correlating with discrete inner-shell levels of the target and projectile.

The theory for a fully relativistic, two-center, coupled-channel treatment of atomic collisions between high-Z ions has been developed and applied to ${\mathrm{U}}^{92+}$+${\mathrm{U}}^{91+}$ collisions at 500 MeV/amu and to ${\mathrm{Xe}}^{54+}$+Ag and ${\mathrm{Xe}}^{54+}$+Au collisions at 82, 140, and 197 MeV/amu. In the former case, a set of 36, and in the latter a set of up to 29 atomic Dirac basis states has been used. Detailed state-to-state cross sections for excitation and charge transfer are presented. For charge transfer in ${\mathrm{Xe}}^{54+}$+Ag and ${\mathrm{Xe}}^{54+}$+Au collisions, the calculated results are generally in good agreement with experimental data. Relativistically induced magnetic couplings of unusually long range are identified and classified with respect to their selection rules and range dependences.

Dressed states of an atom in a classical laser field of frequency close to an atomic transition frequency are obtained, which are a considerable improvement over the usual two-level rotating-wave approximation. These wave functions are used in a calculation of the cross section for elastic electron-atom scattering in a laser field. Numerical calculations for electron-hydrogen scattering demonstrate that scattering involving absorption or emission of photons can be strongly influenced by coupling to the nonresonant states, regardless of the field strength. Inclusion of the counter-rotating terms in the calculation is shown to be essential for extending the applicability of the theory to scattering in nonresonant fields, where a first-order perturbation-theoretic treatment of the atom-field interaction would be expected to be quite justified.

The photon emission spectrum from He between 35 and 55 eV has been studied by means of a 3-m normal-incidence grating spectrometer. The radiation was generated by a new vuv-microwave source employing cyclotron resonance obtained by a static magnetic field. A large number of lines have been observed in the spectrum. Many Rydberg series have been identified not only for singly excited atoms but also for doubly excited atoms. The series involve both singlet and triplet states. The energy of many doubly excited states have been determined more accurately than in previous studies and in addition some new energies have been measured. The lines observed correspond to both one-electron and two-electron transitions, indicating that configuration interaction is important.

Dielectronic recombination rate coefficients for the three initial states of the F-like ions have been calculated for six ions with Z=26, 35, 42, 47, 54, and 63. The calculations were carried out in the isolated resonance approximation for temperatures in the range 0.01≤T≤9 keV. The Auger and radiation rates for each autoionizing state were computed explicitly by use of the multiconfiguration Dirac-Fock model in intermediate coupling with configuration interaction. Comparison is made with previous calculations. The effect of relativity on the Coster-Kronig-type transitions is also discussed.

We have studied the single and double nonresonant multiphoton ionization of calcium atoms using both 532-nm and 1.06-μm radiation. These studies were conducted in the intensity regime of ${10}^{10}$–${10}^{13}$ W/${\mathrm{cm}}^2$. We use mass and angularly resolved electron-energy analysis in an attempt to gain insight into the dominant mechanisms responsible for ionization. An analysis is presented for intensity- and frequency-dependent results, and electron angular distributions are reduced in terms of atomic parameters derivable from lowest-order perturbation theory.