The order-α correction to the decay rate of orthopositronium is obtained using the Fried-Yennie gauge. The result (m${\mathrm{\alpha }}^7$/${\mathrm{\pi }}^2$) [-1.987 84(11)] is consistent with but more accurate than the results of previous evaluations.

A theoretical consideration is presented concerning the atomic core-polarization effect on the E1 transitions of large-l circular orbitals in hadronic helium atoms (${\mathit{e}}^{\mathrm{-}}$${\mathit{X}}^{\mathrm{-}}$${\mathrm{He}}^{2+}$), where the ${\mathit{e}}^{\mathrm{-}}$ occupies the 1s orbital and the negative hadron (${\mathit{X}}^{\mathrm{-}}$) occupies a large-l circular orbital. The first-order mixing of the 2p electron configuration due to the repulsive ${\mathit{e}}^{\mathrm{-}\mathrm{-}}$${\mathit{X}}^{\mathrm{-}}$ interaction gives rise to a substantial suppression (∼1/3) of the E1 transitions, compared with a single-particle estimate, since the slowly moving particle (${\mathit{X}}^{\mathrm{-}}$) polarizes the electron cloud in the opposite direction. Recently discovered metastable states of antiprotonic helium atoms are discussed.

We present lower bounds to the binding energies of the three-body Coulomb system, which were drawn from Rayleigh-Ritz variational upper bounds to the inverse of the Hamiltonian. The method is shown to generalize Temple’s lower bound [Proc. R. Soc. London Ser. A 119, 276 (1928)]. We applied the method to eigenvalues of the muonic molecular ion td${\mathrm{\mu }}^{+}$ where we reached accuracies of ${10}^{\mathrm{-}5}$ natural atomic units or better.

The natural radiative lifetimes of the ${\mathit{ns}}^2$${\mathit{np}}^4$${(}^1$${\mathit{S}}_0$) metastable states of ${\mathrm{Ar}}^{2+}$, ${\mathrm{Kr}}^{2+}$, and ${\mathrm{Xe}}^{2+}$ with n=3, 4, or 5, respectively, have been measured using an ion-trapping technique. In addition, we have measured the natural radiative lifetimes of the 4${\mathit{s}}^2$4${\mathit{p}}^3$${(}^2$${\mathit{P}}_{1/2}$) metastable state of ${\mathrm{Kr}}^{3+}$ and also of the 5${\mathit{s}}^2$5${\mathit{p}}^3$${(}^2$${\mathit{P}}_{1/2}$ and ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$) metastable states of ${\mathrm{Xe}}^{3+}$. The decay rate of each state was determined by counting for equal time intervals the uv photons emitted as the metastable ion population decayed to a lower level of the ground-state configuration. A population of ions was produced inside a cylindrical electrostatic ion trap by electron bombardment of the appropriate parent gas maintained at pressures ranging from 4 to 80×${10}^{\mathrm{-}8}$ Torr. The trap consisted of a 5.0-cm-diam, 7.5-cm-long cylinder with end caps plus a concentric 0.003-cm-diam central cylinder maintained at a negative potential of 150 V. Following electron impact of the parent gas, some of the photons emitted by the decaying metastable ion population were focused onto a narrow-bandwidth interference filter.

Photons transmitted by the filter were detected by a photomultiplier tube as a function of time and generated a decay curve. As dictated by the composition of the photon decay curve, the pressure-dependent decay rate of each metastable state was obtained from a least-squares fit of either a single- or double-component exponential decay to the natural logarithm of its decay curve. The mean decay rate was extrapolated to zero pressure of the parent gas using a straight-line least-squares fit, and the inverse of the zero-pressure intercept yields the natural radiative lifetime of the relevant metastable state. Our measurements for the ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$ metastable states of ${\mathrm{Ar}}^{2+}$, ${\mathrm{Kr}}^{2+}$, and ${\mathrm{Xe}}^{2+}$ yield 133±24, 14.8±0.8, and 4.6±0.3 msec for the natural radiative lifetime of the respective metastable state. Our result for the lifetime of the ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{1/2}$ metastable state of ${\mathrm{Kr}}^{3+}$ is 47±5 msec, while the lifetimes of the ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{1/2}$ and ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$ metastable states of ${\mathrm{Xe}}^{3+}$ are 15.6±0.9 and 5.3±0.5 msec, respectively.

We report the results of a theoretical study of low-energy electron-${\mathrm{NH}}_3$ scattering in which target polarization effects are included by using an ab initio optical potential. The calculations employ the complex Kohn variational technique and are undertaken at both the static-exchange and polarized-self-consistent-field levels. Particular attention is paid to the complications attending electron scattering from target molecules that possess a permanent dipole moment. We describe the steps necessary to extract meaningful differential cross sections from fixed-nuclei calculations that ignore the rotational motion of the target. Good agreement is found between our results and recent experimental measurements.

Two-center integrals involving excited bound states on one center and Coulomb waves on the other are evaluated for inelastic scattering following the method of Deb and Crothers [J. Phys. B 22, 3695 (1989)] who have earlier considered such integrals but only for the cases where there was a ground state on one center and a Coulomb wave on the other, generally applicable to elastic scattering.

A variational principle is applied to the study of the relativistic scattering of a charged particle from a center of force in the presence of a strong, slowly varying external electromagnetic field. The analysis is first given in terms of a spin-zero wave equation and then modified to describe the scattering of a Dirac particle. In contrast to previous treatments of problems of this nature, the field is not assumed to be of the plane-wave type. More realistically, it is modeled as a superposition of modes, each with a different frequency and direction of propagation. Exact solutions of the wave equation describing the asymptotic motion of the particle in the presence of the field are not available for fields of this type. Nevertheless, the variational principle allows for a formulation of the scattering problem and, by a suitable choice of trial functions, generates a gauge-invariant low-frequency approximation for the transition amplitude. The error introduced by the inaccuracy in the asymptotic wave function is compensated for by the inclusion of a variational correction term which properly accounts for the effect of stimulated Compton scattering in initial and final states. It is verified that previously derived low-frequency approximations for one- and two-photon spontaneous bremsstrahlung are reproduced by taking the weak-field limit of the approximate stimulated-bremsstrahlung amplitude.

Electron-degradation spectra, ionization yields, and Fano factors for Ar-${\mathrm{H}}_2$ gaseous mixtures are rigorously studied using the Spencer-Fano theory along with a realistic set of electron collision cross sections. The present study elucidates the relation between the Fano factor and ionization cross sections and indicates principles underlying statistical fluctuations of the ionization yield. Our results show a smooth composition dependence of the Fano factor for a mixture.

We report calculations on the total (elastic plus inelastic) positron-scattering cross sections from several diatomic and polyatomic molecules (${\mathrm{H}}_2$, ${\mathrm{H}}_2$O, ${\mathrm{NH}}_3$, ${\mathrm{CH}}_4$, ${\mathrm{N}}_2$, CO, ${\mathrm{C}}_2$${\mathrm{H}}_2$, ${\mathrm{O}}_2$, ${\mathrm{SiH}}_4$, ${\mathrm{CO}}_2$, ${\mathrm{N}}_2$O, and ${\mathrm{CF}}_4$) where experimental data are available. The impact energy (E) range is 10–5000 eV. A local spherical complex optical potential (SCOP) is calculated for each positron-molecule system from the target charge density [ρ(r)], which in turn is determined from the corresponding molecular wave function at the Hartree-Fock level. The real part of the SCOP is composed of the repulsive static and attractive positron-correlation-polarization potential of Jain [Phys. Rev. A 39, 2437 (1990)]. The imaginary component of the SCOP, the so-called absorption potential, is derived semiempirically as a function of ρ(r), E, and the mean excitation energy. The resulting complex optical potential is treated exactly in a variable-phase approach to yield complex phase shift and the total-cross-section quantities. In this intermediate- and high-energy region, the small contribution due to the nonspherical nature of the target is neglected. In addition, we fit the total-cross-section values to a simple analytic formula. For molecules possessing a permanent dipole or quadrupole moment, the present results are reliable only roughly above 50 eV.

Resonant transfer and excitation (RTE) is investigated for ${\mathrm{Fe}}^{\mathit{q}+}$ ions (q=23, 24, and 25) colliding with ${\mathrm{H}}_2$. For each charge state, cross sections for RTE were obtained from measurements of K x rays, emitted from the doubly excited intermediate state, coincident with single-electron capture by the incident ion. Additionally, for ${\mathrm{Fe}}^{25+}$ cross sections were obtained from measurements of coincidences between the two K x rays emitted from the intermediate state. These latter measurements provide information on the lifetimes of intermediate metastable states formed in the RTE process. In all cases, measured cross sections are in good agreement with calculations based on theoretical cross sections for dielectronic recombination (DR). Since RTE closely approximates DR, the results indicate that dielectronic-recombination cross sections involving K-shell excitation can be accurately predicted for highly charged iron ions. The results for ${\mathrm{Fe}}^{25+}$ show that metastable states are sufficiently short lived to be observable in the RTE (or DR) process for these hydrogenlike ions.

A general approach calculating reduction of the dissociation energy of diatomic molecules immersed in a dense (n<${10}^{22}$ ${\mathrm{cm}}^{\mathrm{-}3}$) gas of molecules and atoms is presented. The dissociation energy of a molecule in a dense gas differs from that of the molecule in vacuum because the intermolecular forces change the intramolecular dynamics of the molecule, and, consequently, the energy of the molecular bond.

K-shell-ionization cross sections for Na, Mg, Al, Si, Cl, K, Ca, and Ti by protons in the energy range 0.5–2.5 MeV have been measured using thin targets. Measurements have also been performed for thin targets of Fe, Ni, and Cu at a few energies. The energy range of protons for these targets corresponds to the reduced velocity (${\mathit{v}}_1$/${\mathit{v}}_{2\mathit{K}}$) range 0.2–1.1, in which the cross sections are very sensitive to the increased binding energy and the Coulomb-deflection effects. The measured ionization cross sections are compared with the predictions of the theory based on the perturbed-stationary-state approach including the Coulomb-deflection, energy-loss, and relativistic corrections. The data have been scaled according to various scaling laws to test the validity of the universal nature of the various Coulomb ionization theories based on the plane-wave Born approximation, the binary-encounter approximation, and the simplified semiclassical approximation model as given by Lægsgaard, Andersen, and Lund [in Proceedings of the Tenth International Conference on the Physics of Electronic and Atomic Collisions, edited by G. Watel (North-Holland, Amsterdam, 1978), p. 353]. The measured data have also been compared with the calculations of Montenegro and Siguad [J. Phys. B 18, 299 (1985)] based on the theory of 1sσ molecular-orbital ionization.

Measurements and calculations of the rate coefficient 〈vσ〉 for dielectronic recombination associated with He-like ions of nitrogen, fluorine, and silicon are reported for electron energies between 0 and 27 eV. In this energy region, no resonances are associated with excitation of the 1${\mathit{s}}^2$ ground state, however, the metastable states 1s2s${(}^1$S) and 1s2s${(}^3$S) contribute with very large rate coefficients. The population of the 1s2s${(}^3$S) metastable state was determined by an independent x-ray measurement. We performed calculations in the LS-coupling and intermediate-coupling approximations for dielectronic recombination from the initial ${}_{}{}^3{}_{}{}^{}$S and ${}_{}{}^1{}_{}{}^{}$S states. For the ${}_{}{}^3{}_{}{}^{}$S initial state, a comparison on an absolute scale between theory and experiment is presented. We find good agreement with LS-coupling calculations for ${\mathrm{N}}^{5+}$${(}^3$S), one exception being a structure observed below the ${}_{}{}^1{}_{}{}^{}$S and ${}_{}{}^3{}_{}{}^{}$P threshold. For ${\mathrm{Si}}^{12+}$${(}^3$S), the intermediate-coupling calculation reproduces the data fairly well. In the case of ${\mathrm{F}}^{7+}$${(}^3$S), we find discrepancies with both the LS- and intermediate-coupling calculations at low energy and just below the ${}_{}{}^1{}_{}{}^{}$S and ${}_{}{}^3{}_{}{}^{}$P thresholds. The contribution from the initial 1s2s${(}^1$S) state is important only for ${\mathrm{N}}^{5+}$. Here, the experimental data are in excellent agreement with the theory if the ${}_{}{}^1{}_{}{}^{}$S metastable fraction is assumed to be about 1%.

Electron-capture processes in collisions of ${\mathrm{C}}^{5+}$ ions with H atoms are investigated theoretically by using quantum-mechanical and semiclassical molecular-orbital methods in the collision-energy range from meV/amu to keV/amu. Atomic-type electron-translation factors are appropriately incorporated to ensure correct scattering boundary conditions. Total capture cross sections have a broad minimum at 0.4 keV/amu that is caused by the change of major contributors. The dominant capture channels in the triplet manifold are ${\mathrm{C}}^{4+}$(1s4s) below 0.1 keV/amu and ${\mathrm{C}}^{4+}$(1s4p), ${\mathrm{C}}^{4+}$(1s4d), and ${\mathrm{C}}^{4+}$(1s3p) above that energy. In the singlet manifold, below 0.5 keV/amu, the dominant channel is ${\mathrm{C}}^{4+}$(1s4s), as in the triplet manifold. However, above that energy the ${\mathrm{C}}^{4+}$(1s4f), ${\mathrm{C}}^{4+}$(1s4d), ${\mathrm{C}}^{4+}$(1s3p), and ${\mathrm{C}}^{4+}$(1s3d) channels become important. The present results for the total capture cross section is in excellent accord with the measurements by Phaneuf et al. [Phys. Rev. A 26, 1892 (1982)] and Crandall, Phaneuf, and Meyer [Phys. Rev. A 19, 504 (1979)] over the entire energy region studied. The effect of core electrons on the capture process is examined by using data from the present (${\mathrm{C}}^{5+}$+H) system and the previously studied (${\mathrm{N}}^{5+}$+H) and (${\mathrm{B}}^{5+}$+H) systems, and some remarks on the role of core electrons are made on the basis of an argument concerning the overlap of electronic charge distribution.

Surface light-induced drift caused by backscattering from rough surfaces can be strongly affected by surface-enhanced state-specific chemical reactions. Expressions for the drift fluxes of the incident resonant-gas component and of the reaction product are presented. A resulting asymmetry in the spatial distributions of the components in a ssT-shaped enclosure is discussed, as a possible sensitive tool for studying the state specificity of the scattering and reaction processes.

Close-coupling calculations using the R-matrix method have been carried out for the photoionization cross sections of carbonlike ions: ${\mathrm{F}}^{3+}$, ${\mathrm{Ne}}^{4+}$, ${\mathrm{Na}}^{5+}$, ${\mathrm{Mg}}^{6+}$, ${\mathrm{Al}}^{7+}$, ${\mathrm{Si}}^{8+}$, ${\mathrm{S}}^{10+}$, ${\mathrm{Ar}}^{12+}$, ${\mathrm{Ca}}^{14+}$, and ${\mathrm{Fe}}^{20+}$, which are of interest in the calculation of plasma opacities. This is an extension of previous work for radiative data for the C sequence. Detailed results including extensive structures due to autoionizing resonances are presented for the ground states; however, the calculations have also been carried out for a large number of excited bound states of each ion and their features are discussed. The partial photoionization cross sections of the ground state into excited states of the residual ion are obtained. The effect of resonances on the branching ratios of partial cross sections at excited thresholds is discussed. Interesting features related to the Z dependence of the ground-state photoionization cross sections of all ions in the C sequence are discussed.

Radiative recombination of bare ions with electrons is discussed as a time-reversed photoionization process in the nonrelativistic dipole approximation. Analytical expressions for the radiative-recombination cross section and the angular distribution of the emitted photons are derived for an arbitrary (n,l) state in the low-energy limit. In this approximation, the anisotropy of the emitted photons is described by an energy-independent parameter ${\mathrm{\beta }}_{\mathit{n}\mathit{l}}$. The radiative-recombination rate coefficients, both differential and angle integrated, for any (n,l) state can, in this limit, be expressed in analytical form. This result holds also for an anisotropic electron-velocity distribution characterized by a longitudinal ${\mathit{kT}}_{\mathrm{\parallel }}$ and transverse ${\mathit{kT}}_{\mathrm{\perp }}$ electron-beam temperatures. The results are discussed in context of the state-selective radiative-recombination experiments planned to be studied in electron coolers of heavy-ion storage rings.

The intermolecular contribution to spin relaxation by translational-rotational diffusion dynamics in molecular liquids is calculated for arbitrary relative time scales of the two stochastic processes. The resulting relaxation times are functions of variables that are proportional to ratios between rotational and translational diffusion constants, in substantial generalization of earlier theoretical approaches. In the first part the general treatment for symmetric-top molecules is given with a reduction to quadratures and a simpler version for the spherical case. In order to avoid numerical integrations an essential second part is then devoted to developing accurate analytical approximations to unsolvable integrals. For reasons of efficient and stable numerical evaluations, strong emphasis is given to an algorithmically advantageous representation of all results. A final numerical test yields excellent agreement of the results from numerical integrations with those obtained from the approximate analytical formulas over a wide range of ratios between rotational and translational diffusion constants.

Double-electron excitation process of He atoms from the ground state to the manifolds of the doubly excited 2l3l’ and 3l3l’ states by proton and antiproton impact have been theoretically investigated using a close-coupling method at the 1.5-MeV/u energy regime. The semiclassical impact-parameter method with a straight-line-trajectory approximation is employed to describe collision processes. Hyperspherical wave functions are adopted to take full account of strongly correlated motion of two atomic electrons in He. The difference between the cross section by proton impact and that by antiproton impact is small except for the process to the 3p3p ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}^{\mathit{e}}$ state. The reason for the small difference between σ(p) and σ(p¯) is discussed. The ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}^0$ states are found to play an important role as intermediate ones. For the process to the doubly excited 2l3l’ states, the (2s3p+3s2p${)}^1$${\mathit{P}}^0$ state plays a crucial role. For the excitation process to the 3l3l’ states, the ratio of the second-order process to the first-order one is larger than that in the case of the process to the 2l2l’ and 2l3l’ states, suggesting that the difference between the mechanism of the excitation to the (2l,2l’) and (2l,3l’) manifolds and that to the (3l,3l’) manifold arises from the smaller overlap of the (3l,3l’) wave functions with that of the ground state. It is seen that the physical interpretation of the results based on the rovibrator model of the doubly excited states gives a deeper physical insight into the mechanism of the double-electron excitation.

The generalized-oscillator-strength (GOS) distribution for the Lyman-Birge-Hopfield (LBH) and the K-shell preionization lines in the nitrogen molecule, obtained with electron impact energies of 25–28 keV, are reported for the momentum-transfer range of 0.5–3.1 a.u. The results for the LBH were found to be in agreement with previously published low-energy experiments and also the most recent theoretical predictions. Possible reasons for problems encountered in previous high-energy electron spectroscopic experiments are discussed. The results for the preionization line confirm the measurements of Camilloni et al. [J. Phys. B 20, 1839 (1987)] and agree with the theory presented in the following paper [Bielschowsky, Nascimento, and Hollauer, Phys. Rev. A 45, 7942 (1992)]. The reliability of the results presented here can in part be judged by the quality of the optical-oscillator-strength distribution obtained by extrapolating the GOS to zero momentum transfer.