Table of contents

Anomalously large rates for radiative recombination observed in merged beam experiments at very low temperature, T₀<10^-2 eV, have been attributed recently to a large increase in the electron density near the recombining ions, via nonlinear shielding generated by the Maxwell-Boltzmann factor. The starting temperature T₀ for enhancement and rate of rise for a given electron density were determined. This report extends the previous result and shows further that lowering of the ionization threshold at low temperatures introduces a natural cut-off, which in turn leads to a maximum enhancement. Both the ion charge dependence and approximate independence of the density of the enhancement are derived, which are in agreement with the experimental observation.

The Ühling-Uhlenbeck equations and the discrete velocity model are used to study the propagation of plane waves in a dilute system of Bose particles with hard-sphere interactions. The Ühling-Uhlenbeck collision sum, as it is highly nonlinear, is first linearized and then decomposed by using the plane wave assumption. We address the dispersion relations thus obtained by the relevant parameter θ or B which describes the Bose particles for the quantum analogue of the discrete Boltzmann system when B is positive (and θ = 1). The attenuation of plane waves propagating in dilute Bose gases in the low density or temperature regime shows a rather sharp change which is completely different from that in dilute Boltzmann gases.

We have measured the double-to-single photoionization ratio of beryllium (1s²2s²) between 32 and 80 eV and compare our data with the electron-impact ionization cross section of Be⁺. In this energy region only simultaneous but not sequential emission of both 2s electrons can take place. We also compare our data with the double-to-single photoionization ratios of He, H₂ and Li and find surprisingly good agreement of the photon-energy dependence if the ratio is scaled appropriately. The double-to-single photoionization ratio is lower for Be than for He due to a weaker electron-electron interaction between the valence electrons.

Near-threshold electron-impact excitation of the metastable levels (3p⁵4s)³P₀ and (3p⁵4s)³P₂ of Ar is calculated in a 41-state semi-relativistic R-matrix (close-coupling) approach and the negative-ion resonance structure in the cross sections is analysed. For the first time, angle-differential excitation cross sections of the Ar metastable states at low energies are compared with the available experimental data. Drastic changes in the shape of the excitation functions with scattering angle reflect the contributions of different partial-wave symmetries and negative-ion resonances to the cross section.

We have measured the total cross section for photodetachment of the Na^- ion over the photon energy range 30-51 eV. Electron detachment leads predominantly to the production of Na⁺ ions in this energy range. The structures in the measured cross section are associated with correlated processes involving the detachment or excitation of a 2p core electron, processes which are often accompanied by the excitation of one or more valence electrons. The most prominent feature in the cross section is a strong resonance associated with the excitation of a 2p electron from the core and a 3s valence electron. As in previous experiments on double excitation of valence electrons, electron correlation is seen to play an important role in the dynamics of negative ions.

Concentration of the strongest transitions on the high-energy side of some emission, photoexcitation and Auger spectra of atoms is related to the existence of the additional selection rule for the number of vacancy-electron pairs.

The role of hyperfine structure in electromagnetically induced transparency (EIT) is investigated by studying the 5S_1/2-5P_3/2-5D_3/2,5/2 cascade system in ⁸⁵Rb and ⁸⁷Rb. We show that if the hyperfine splitting Δ_hfs is larger than the Rabi frequency of the coupling beam, Ω_c, then the observed EIT spectra can be modelled by adding terms corresponding to each hyperfine state. However, in the strong-coupling limit, Ω_c>>Δ_hfs, a complete multilevel description is required.

Positronium formation through electron capture from ground-state closed-shell sodium clusters is studied theoretically at intermediate impact energies. The charge transfer process is described in the independent electron model by a continuum distorted-wave approximation. In this approximation, distortions in the final state are introduced by using two Coulomb wavefunctions associated with the positron- and electron-residual target interactions. The cluster is described within the framework of the spherical background jellium model and the Kohn-Sham formalism with a local-density approximation that includes exchange, correlation, and a self-interaction correction. Using a partial-wave technique, differential and total cross sections for the isoelectronic neutral and charged closed-shell sodium clusters Na₂₀, Na₁₉^- and Na₂₁⁺ are computed and compared with the predictions of the Coulomb-Born type approximation which neglects the distortions in the final state.

The angular distribution of the transferred spin polarization of Kr M_4,5N₁N_2,3 ¹P₁ Auger electrons after primary ionization with circularly polarized light has been measured and the intrinsic parameters δ₁ and ξ₁ have been determined from experimental results. The explicit equation interconnecting the intrinsic parameters for Auger decay from a (d_5/2)^-1 core hole has been derived and is used for a detailed analysis of the transitions in terms of Auger amplitude ratios and phase shift differences. The results indicate that the final ionic state is well described by a ¹P symmetry. Additional constraints imposed on the intrinsic parameters for singlet states prevent unambiguous determination of Auger amplitudes from experiment even in the limiting case of pure LS coupling.

Superelastic electron scattering involving the collisional de-excitation of laser-excited ¹³⁸Ba(...6s6p ¹P₁) atoms to the (...6s^2 1S₀) ground state has been used to measure electron impact coherence parameters for the related (...6s^2 1S₀) to (...6s6p ¹P₁) inelastic process. Measurements of the orbital angular momentum transfer parameter, L_perp⁺, were made for excitation at impact energies of 7, 8.5, 11 and 16 eV. Experimental data are compared with available theoretical results.

The application of Brillouin-Wigner perturbation theory to a hydrogenic model problem is described using both a relativistic and a nonrelativistic formalism. The nonrelativistic study is carried out by employing a basis set of Coulomb Sturmian functions whereas calculations within the relativistic formalism use basis sets of L-spinors. A two-state hydrogenic model problem provides a dramatic example of the dependence of the convergence behaviour of both the Brillouin-Wigner and the Rayleigh-Schrödinger perturbation expansions on the choice of basis set. Two types of Brillouin-Wigner perturbation expansion are compared in both the relativistic and the nonrelativistic formulations, the first using the exact energy in the denominator factors and the second employing total energies determined by a self-consistent procedure applied at finite order. Extrapolation procedures are also investigated. For the relativistic formulation the second-order energy is divided into two components, one arising from a sum over positive-energy states and the other corresponding to a sum over the negative-energy states. The application of correction terms, based on the known relation between the denominator factors occuring in Brillouin-Wigner perturbation theory and those in the Rayleigh-Schrödinger formulation, which restore linear scaling with particle number in many-body formulations, is investigated for the hydrogenic model problem.

The scattering lengths that characterize the low-energy elastic scattering of ²³Na atoms by ³⁹K, ⁴⁰K and ⁴¹K atoms, when they interact via the X ¹Σ⁺ and a ³Σ⁺ states of the NaK dimer, are calculated. The systematic variation of the short- and long-range parts of the interaction potentials reveals that the calculated scattering lengths are sensitive to uncertainties in the potentials. It is concluded that precise spectroscopic measurements of the least-bound ro-vibrational levels supported by the ground electronic state, which would reduce the uncertainties in both the molecular potentials, are needed for an accurate determination of the scattering lengths.

We tested wave-particle duality with the ensemble-averaged spin states of ¹³C nuclei in ¹³CHCl₃ using the spin states of ¹H nuclei as the path marker. It turns out that wave-particle duality holds when one merely measures the probability density of quantum states, and that wave- and particle-like behaviour is simultaneously observed when measuring populations and coherences in a single nuclear magnetic resonance experiment. The effects of path-marking schemes and the causes of the appearance and disappearance of wave behaviour are analysed.

A path integral approach has been generalized for the non-relativistic electron charge transfer processes. The charge transfer - the capture of an electron by an ion passing another atom, or more generally the problem of rearrangement collisions - is formulated in terms of influence functionals. It has been shown that the electron charge transfer process can be treated either as an electron transition problem or as ion and atom elastic scattering in the effective potential field. The first-order Born approximation for the electron charge transfer reaction cross section has been reproduced to prove the adequacy of the path integral approach for this problem.

Low-energy collision studies have shown that the scattering cross section for electrons by benzene rises sharply at electron impact energies below ~ 200 meV. This was attributed in earlier work to electron attachment and formation of benzene anions with lifetimes τ⩾1 µs. A detailed study of electron attachment to benzene is reported here that makes use of Rydberg atom techniques. The data provide no evidence for the formation of long-lived benzene anions, although a small collisional ionization signal is observed which is consistent with the formation of short-lived excited anions (τ⩽3 ps).

An alternative explanation for the rapid rise in cross section at low energy is presented, based on ab initio scattering calculations. These calculations indicate that virtual state scattering takes place at low energies. Possible implications of this are briefly discussed.

We consider the influence of a weak low-frequency electromagnetic field on the binary-encounter electron emission in fast ion-atom collisions. It is shown that the electromagnetic field, which would have no impact on the collision-free system, can be very effectively coupled to the projectile-target system in collisions where an electron experiences strong acceleration during the process of emission. In such a case a large amount of energy is exchanged between the system and the field, which can result in profound modifications of the energy and angular distributions of the emitted electron. It is also found that at a certain collision geometry a large energy exchange with the field has only a very weak impact on the electron spectra. This might suggest that, in field-assisted ion-atom collisions, a heavy particle, whose direct interaction with the electromagnetic field is negligible, can be rather effectively coupled to the field in an indirect way by `using' an electron as a mediator.

We present the results of a 61-term, 138-level intermediate-coupling frame-transformation R-matrix close-coupling calculation of the electron-impact excitation of fluorine-like Ne⁺. All levels of the 2s²2p⁵, 2s2p⁶, 2s²2p⁴3ℓ and 2s²2p⁴4ℓ configurations that lie below the ionization limit are included in the close-coupling expansion. With the exception of several R-matrix calculations of excitation between the fine structure levels of 2s²2p⁵²P, this represents the first close-coupling calculation for this ion. Here we describe this calculation and present radiative rates and effective collision strengths for a selected number of the 9453 transitions resulting from this work. The full set of data is available at the Oak Ridge National Laboratory Controlled Fusion Atomic Data Center Web site.

High-resolution electron spectroscopy studies of the NO molecule in the regions of the N 1s→2π core excitations have been performed. By selecting electrons within certain binding energy ranges, either the Auger electron yield - a good approximation for the x-ray absorption spectrum - or the electrons emitted after decay to a particular ionic final state (constant ionic state (CIS)) were detected. By selecting the X ¹Σ⁺ (2π⁰) final state, the superposition of several intermediate states can be disentangled by exploiting a selection rule which permits only two of the three dipole-allowed intermediate states to decay to this specific final state. This makes it possible to obtain more detailed information on the potential energy curves of the intermediate states than is available from regular absorption measurements. We have also obtained CIS spectra for individual vibrational sublevels within this state. The role of lifetime vibrational interference on the appearance of these spectra is discussed.

The velocity selection of the V-type three-level-atom mazer with atomic coherence is studied. It is shown that the atomic coherence can strongly affect transmission probability, and in turn, the velocity selection of atoms through the micromaser cavity. When the incident atoms are initially prepared in the lower state of the two upper ones (i.e., no injected atomic coherence), velocity selectivity is at a maximum. This shows that atomic coherence does not have a favourable effect on velocity selectivity in this scheme. The effects of other parameters are also studied.

Muon spin dynamics of anisotropic Mu undergoing fast electron spin flip collisions with paramagnetic species has been investigated theoretically, where the tensor nature of anisotropic hyperfine interaction is fully taken into account. In the presence of fast electron spin exchange, the muon spin in Mu behaves as if the muon were a free positive muon. It is crucial to understand the true nature of the muon environment to interpret experimental data correctly. It is shown that the muon spin relaxation rate and the precession frequency in such a muonium system mimicking diamagnetic species can be expressed entirely in terms of the quantities which appear in the Hamiltonian. The orientation and field dependence of the relaxation rate and of the precession frequency can be used to distinguish a neutral muonium species undergoing rapid electron spin flip from a free positive muon. It has been shown that even at zero field one can observe a finite single precession frequency which depends sensitively on Mu orientation.

We present the results of an 83-term, 180-level intermediate-coupling frame-transformation (ICFT) R-matrix close-coupling calculation of the electron-impact excitation of boron-like Ne⁵⁺. All levels of the 2s²2p, 2s2p², 2p³, 2s²3ℓ, 2s²4ℓ, 2s2p3ℓ, 2p²3ℓ, 2s2p4s, 2s2p4p, and 2s2p4d configurations are included in the close-coupling expansion. This sizable calculation was performed using our new ICFT R-matrix codes designed for distributed-memory parallel computers. We compare our results from this calculation with those from our 11-term 20-level ICFT calculation and an earlier eight-term 15-level R-matrix calculation. Here we describe the nature of these calculations and present radiative rates and effective collision strengths for a selected number of the 16 110 transitions resulting from this work. The full set of data is available at the Oak Ridge National Laboratory Controlled Fusion Atomic Data Center Web site.

The indium spectrum emitted from a hollow cathode discharge has been recorded using a Fourier transform spectrometer. Accurate wavelengths have been determined for 54 In II lines, and improved In II energy levels have been derived. The hyperfine structure of 42 lines has been analysed, yielding hyperfine constants A and B for 23 In II levels. The spin-forbidden In I transition 5s²5p ²P-5s5p²⁴P has also been analysed and absolute wavelengths, hyperfine constants A and B and improved energy level values are reported.

We develop an approximate formalism suitable for performing simulations of the thermal dynamics of interacting Bose gases. The method is based on the observation that when the lowest-energy modes of the Bose field operator are highly occupied, they may be treated classically to a good approximation. We derive a finite-temperature Gross-Pitaevskii equation (FTGPE) for these modes which is coupled to an effective reservoir described by quantum kinetic theory. We discuss each of the terms that arise in this GPE, and their relevance to experimental systems. We then describe a simpler projected GPE that may be useful in simulating thermal Bose condensates. This classical method could be applied to other Bose fields.

The correspondence between the ionization of light atoms by charged particle impact (virtual photons) and by photoabsorption (real photons) is analysed. It is shown that the `genuine' photo-like mechanism in the impact ionization of light targets can already be observed at collision energies corresponding to the Lorentz factor γ≳10, but that it starts to dominate electron emission only at γ≳1000. It is demonstrated that the non-relativistic Bethe-Born and Weizsäcker-Williams approximations represent extremely different limits of the impact ionization but that `by chance' their results are formally close for electron emission in non-relativistic fast collisions.

Effective collision strengths for electron impact excitation of all fine-structure levels arising from the 1s²2s²2p⁴ ground state configuration of oxygen-like iron ions, Fe¹⁸⁺ (Fe XIX), have been calculated over a range of temperatures applicable to many laboratory and astrophysical plasmas. The R-matrix method, within the semi-relativistic Breit-Pauli approximation, was employed. Twenty fine-structure levels arising from twelve 1s²2s²2p⁴, 1s²2s2p⁵, 1s²2p⁶ and 1s²2s²2p³3s LSπ target terms were included in the electron scattering calculation. Comparisons of the present results, which exhibit strong resonance structure in the cross sections, with previous results obtained by distorted wave methods show significant differences.

This work reports atomic transition probabilities of 33 spectral lines belonging to 3s-3p, 3p-3d and 3d-4f multiplets of O II, all of them measured in the 405-465 nm spectral region in an emission experiment. Relative intensity measurements have been made on a pulsed discharge lamp and the absolute A_ki-values have been obtained taking the NIST database as a reference in temperature diagnosis. The results of this work confirm the other recent available data measured by Veres and Wiese (Veres G and Wiese W L 1996 Phys. Rev. A 54 1999) with a different source and the calculations of Bell et al (Bell K L, Hibbert A, Stafford R P and McLaughlin B M 1994 Phys. Scr.50 343) with a very satisfactory agreement (usually within 10%).

One of us (DB) is indebted to M Cornille for pointing out to him that the above paper and a previous article published in 2000 [1] could be misunderstood to imply that the spontaneous emission rate and the collisional de-excitation rate have been calculated in this body of work. The authors would like to take the opportunity to replace the first sentence of the first paragraph in section 4 of the above article, and the second paragraph of section 5 in [1], by: We have used the spontaneous emission rate and the collisional de-excitation rate calculated in previous works [2, 3].