Table of contents

A universal variational expansion in the interparticle coordinates is proposed for the accurate calculation of the bound states of an arbitrary three-body system. This fully non-adiabatic variational expansion has been applied successfully to various three-body systems with different masses, including the so-called nearly adiabatic systems (two particle masses are significantly greater than the third mass). Accurate calculations for the bound states in the latter adiabatic systems present a very complicated problem for the usual variational approaches in relative coordinates.

We study the time dependence of the ionization of Stark Rydberg states of hydrogen subject to half-cycle electromagnetic pulses. We find that the quantum ionization probability for the most 'blue-shifted' Stark state exhibits a purely classical oscillating pattern as a function of time whose frequency is associated with the perturbed orbital frequency of the electron. We show that these oscillations could be observed experimentally when a scaled-time analysis of ionization data for hydrogen is employed. Our calculations are compared with experimental data for sodium.

The electron energy-loss spectra of H₂ have been measured in coincidence with the neutral dissociation leading to H(1s)+H(2p) near the first ionization potential at the electron impact energy of 150 eV and the scattering angles of 2 degrees and 10 degrees , and compared with ordinary electron energy-loss spectra. The two spectra, the coincident and the ordinary ones, are similar to each other in shape below the energy level of H₂⁺(X² Sigma _g⁺, upsilon _i=1), while they are different from each other above this energy level. Such a large change around upsilon _i=1 is discussed in terms of the dynamics of superexcited hydrogen molecules, and the availability of this method for such studies is examined.

The long-range asymptotic form of the polarization potential for an electron interacting with a Ca⁺ 'core' is evaluated up to terms of order r^-8. The static dipole polarizability of Ca⁺ is calculated to be 73.8 au, while higher-order polarizabilities are found to be very much larger. Energies of 4snl Rydberg levels for l=3, 4 and 5 are evaluated in first- and second-order perturbation theory and it is suggested that 4s5g ¹G is located at 44874.55+or-0.2 cm^-1. Frequencies for 4snf ¹F⁰-4sng ¹G (n=23-25) microwave transitions are compared with experiment and with a model-potential calculation which corrects for core-penetration effects.

Relative intensities of dielectronic satellite lines determined by radiative transition probabilities and autoionization rates have been calculated for the transitions 1s2l"nl-1s²n'l', n, n'=2, 3 in a wide range of nuclear charge Z. These data together with the wavelengths were computed for all satellite lines. Using these data synthetic spectra could be built for different plasma sources. Perturbation theory method has been applied for the calculation of atomic properties of autoionization states. Correlation, relativistic and radiative effects have been taken into account. Factor intensities Q_d of dielectronic satellite lines are shown as functions of Z for different types of transitions. The Z-dependencies of the Q_d values have been analysed and very pronounced sharp peaks have been discovered for this atomic characteristic indicating that some lines are prominent for specific ionic states and disappear for other ones. A possible explanation for such a behaviour of this atomic characteristic is presented. The resulting theoretical wavelengths for both satellite lines (1s2l"nl-1s²n'l', n, n'=2, 3) and resonance lines (1snp ^1,3P_t-1s₂, n=2, 3 and 1s2s ^1,3S-1s²) are compared with available experimental data along isoelectronic sequences to identify irregularities in the experimental data and to predict wavelengths of ions whose values are unknown or uncertain.

In the calculation of opacities for stellar interiors it is found that very large numbers of spectrum lines of iron ions make important contributions. In the work of the Opacity Project the main computations of atomic data have been made using R-matrix methods, referred to as RMAT. For the important ionization stages of Fe VIII to Fe XIII further atomic data are required. For those stages calculations have been made using the code SUPERSTRUCTURE for a further 2472790 spectral multiplets in LS coupling. These are referred to as PLUS data. Results are compared for cases in which both RMAT and PLUS calculations have been made. For many individual transitions the differences can be quite large, which is a consequence of the larger amount of configuration-interaction included in the RMAT work. For statistical purposes, such as the calculation of Rosseland-mean opacities, the accuracy of the PLUS data should be adequate.

The effects of magnetic optical activity in the sodium D1 line are investigated in detail both theoretically and experimentally. Particular consideration is given to the ultra-narrow Hanle-type features that are produced by the crossing of ground state Zeeman sub-levels and which although strongly exhibited by the F_g=2 to F_e=1, 2 doublet, are apparently absent from the F_g=1 to F_e=1,2 doublet. We demonstrate, quantitatively for the first time, that these features are principally a manifestation of Delta M=2 (quadrupole) coherences and are therefore not evidence for the existence of Delta M=4 (hexadecapole) coherences as originally proposed by Gawlik and co-workers (1987). Indeed the latter is shown to have little influence on these features even at light field intensities for which significant power broadening is observed. We also show explicitly that the width of these narrow crossing features is, for the typical vapour pressures used to give an optically thin sample, a function of the average time of transit through the light field. Using a simple phenomenological model we predict numerically both the absolute width of these coherence features and the rate at which they power broaden. Finally a brief consideration of the same coherence effects within the D2 line is presented.

The absolute triple differential cross section (TDCS) for double photoionization has been determined experimentally for the first time. The investigation was carried out for helium at a photon energy of 99 eV, for equal energy of the emitted electrons and in a plane perpendicular to the photon beam. Electron, ion and combined ion-electron spectrometry measurements were performed to put the data on an absolute scale. The result is compared with two recent calculations, and a parametrization is given which extends the TDCS obtained at one specific geometry to the whole space.

Two-photon detachment of H^- with simultaneous excitation of the residual H atom is studied using an L² close-coupling method with B-spline basis. Total and partial 1s, 2s and 2p cross sections have been calculated in the vicinity of 2lnl' and 3lnl' doubly excited states of H^-. The corresponding resonance structures are analysed in detail.

We report new data for the even-parity, J=0, 1 and 2 levels of neutral lead using a two-step laser excitation technique in conjunction with a thermionic diode detector. The Rydberg series 6pnp( 1/2 , 3/2)₂, (10<or=n<or=56); 6pnp( 1/2 , 3/2)₁, (10<or=n<or=25); 6pnp( 1/2 , 1/2 )₀, (10<or=n<or=36) and 6pnf( 1/2 , 5/2)₂, (7<or=n<or=29) as well as four perturbing levels, 6p7p(3/2, 1/2 )_1,2 and 6p7p(3/2, 3/2)_1,2 have been identified. The 6pnp( 1/2 , 1/2 )₀ series possesses a constant quantum defect implying its unperturbed behaviour. For the parametrization of the perturbed Rydberg series, the multichannel quantum-defect theory analysis has been performed using three channels for the J=1 levels and four channels for the J=2 levels.

The technique of quantum beats in forward scattering remains a useful tool for atomic and molecular spectroscopy. The detected signal is strongly dependent on experimental conditions such as the dichroism and birefringence of external optical elements. We present a theoretical model and its experimental verification to give a physical insight into these effects.

The rotation-vibration Hamiltonian for a neutral diatomic molecule in an external magnetic field is established by appropriately transforming the pseudoseparated Hamiltonian to the molecular fixed frame. The partially screened Born-Oppenheimer approximation is subsequently performed. By explicitly investigating the dissociation limit of the molecule into neutral atoms we are able to identify the motional screening operator which is responsible for the screening of the nuclei by the electrons against the magnetic field. A connection between the screening operator and the magnetic flux enclosed by the motion of the nuclei is established.

Rotationally selective vibrational autoionization in molecular hydrogen has been investigated using two-dimensional photoelectron spectroscopy. Measurements of electron yield as a function of both electron and photon energy have been carried out using tuneable synchrotron VUV radiation between the upsilon =0 and upsilon =1 vibrational thresholds of H₂⁺X² Sigma _g⁺, 15.45-15.70 eV. Constant rotational transition energy spectra have been extracted revealing the decay routes of autoionizing states. No large changes in the rotational quantum number of the ion core during autoionization were observed.

We have implemented a multistate close-coupling scattering approach to the calculation of the positions and partial and total widths of the rovibrational levels of the ²Sigma⁺ states of the ⁴HeH excimer molecule. Good agreement with available experimental data is obtained while a number of predictions are made. The results of the present multistate approach (MSA) are compared to those of the two-state approach (TSA), previously applied by a number of methods. Only for the lowest bound excited state, the A ²Sigma⁺, is there agreement, signifying the dominance of the A-X ²Sigma⁺ interaction. For the higher lying C and D ²Sigma⁺ states, the MSA produces significantly different results. The same should apply to similar situations in other excimer spectra.

The rate coefficients, beta , for the attachment reactions of electrons with CCl₄, CHCl₃, SF₆ and C₇F₁₄ have been measured in a flowing afterglow/Langmuir probe (FALP) apparatus within the range of electron temperature, T_c, from 300 to about 4000 K at a carrier gas temperature, T_g, of 300 K and for the SF₆ reaction additionally at a T_g of 540 K. The above gases were chosen in the light of the results obtained from previous FALP studies under truly thermalized conditions, i.e. for T_c=T_g=T. These previous experiments showed that the beta for the CCl₄ reaction (product Cl^-) and SF₆ reaction (products SF₆^-, direct attachment, and SF₅^-, dissociative attachment, becoming increasingly important with increasing T) were close to the maximum value beta _max, and that the beta for both the CHCl₃ dissociative attachment reaction and the C₇F₁₄ direct attachment reaction increased with increasing T indicating the presence of activation energy barriers in these reactions. Now the present studies indicate that (i) the beta for both CCl₄ and the SF₆ reactions decrease with increasing T_c at a rate predicted by s-wave capture theory, and that the SF₅^- product of the SF₆ reaction becomes increasingly important with increasing T_c, (ii) the beta for both the CHCl₃ and the C₇F₁₄ reactions increase with increasing T_c, but a much greater increase of T_c than T is required to produce a given increase in beta . To explain these experimental observations, a simple model of these attachment reactions has been constructed which is consistent with the experimental data, based on the notion that the energy of the incoming electron is distributed amongst the accessible vibrational states of the molecules, and that this vibrational excitation promotes these particular attachment reactions. A careful analysis of our data indicates that the endothermicity of the dissociative attachment reaction of SF₆ producing SF₅^- is close to 0.12 eV which is consistent with the available thermochemical data.

We have determined experimentally the absolute cross sections for i-electron capture (to projectile) and j-electron removal (from target) ( sigma _q,q-i^j), total electron capture ( sigma _q= Sigma _i sigma _q,q-i), j-electron removal ( sigma _q^j= Sigma _i sigma _q,q-i^j) and i-electron capture ( sigma _q,q-i= Sigma _j sigma _q,q-i^j) for the processes: I^q+(q=10 or 15)+B to I^(q-i)++B^j++(j-i)e (B identical to Ne, Ar, Kr and Xe) at collision energy of 1.5q keV. Charge-state distributions of the scattered ions were measured in coincidence with the recoil ions. The absolute electron-capture cross sections were measured by the initial growth-rate method. The experimental results for sigma _q and sigma _q^j were compared with the predictions of the extended classical over-barrier model (ECBM). The branching ratios of the multiply-excited ions produced by multi-electron transfer have also been determined.

The reduced IERM approach is applied to electron scattering from hydrogen atoms below the n=2 and n=3 thresholds to obtain accurate widths and positions for the resonant states of H^- in this energy region, together with integrated cross sections for n=1 to n=2 transitions. Comparison with standard R-matrix results, where only bound states have been included in the close coupling expansion, indicates the importance of including the continuum channels even at these low energies. The results are in good agreement with coupled pseudostate and recent J-matrix calculations.

Polarized electrons were scattered from unpolarized argon, krypton and xenon atoms with scattering angles ranging from 30 degrees to 125 degrees . For xenon elastic left-right asymmetries were measured at energies ranging from 1.5 to 10 eV. Inelastic scattering asymmetries were observed for argon, krypton and xenon at energies between 10 and 40 eV. For inelastic scattering from argon and krypton it could be shown that exchange, in conjunction with the spin-orbit coupling within the target atom, is the dominant mechanism for scattering asymmetries. For the heavy xenon, on the other hand, this simple mechanism is not sufficient to describe the observed spin effects. A comparison with theoretical data shows good agreement for elastic scattering from xenon, whereas, in the case of inelastic scattering, the theoretical predictions are rather unsatisfactory.

In two recent letters results for threshold ionization of hydrogen by electron impact (Rost, J.M. 1994 Phys. Rev. Lett. 72 1998) and positron impact (Rost, J.M. and Heller, E.J. 1994 Phys. Rev. A 49 R4289) were communicated. The results were obtained by calculating the S-matrix semiclassically in Feynman's path integral formalism. This paper gives a more complete account of the theoretical method. Moreover, it is shown how the ionization cross section of atomic targets other than hydrogen can be expressed with the hydrogen cross section through a scaling relation. This demonstrates the universality of the threshold behaviour.

On the basis of average experimental data we demonstrate scaling laws of electron-impact multiple ionization cross sections and propose expressions for the cross sections for arbitrary atoms and ions.

We present a novel method for obtaining Floquet or time-dependent wavefunctions for atoms in laser fields. This method has the potential to be more stable and/or faster than previous methods. The main idea is to include the effects of the laser field in a gauge that smoothly changes character as a function of distance. This transformation allows us to formally account for the analytic behaviour of the wavefunction as r to infinity and as r to 0. We demonstrate the feasibility of this mixed gauge approach through Floquet calculations on a one-dimensional model problem and three-dimensional calculations for the hydrogen atom. Time-dependent calculations for a one-dimensional model problem are performed with the mixed gauge. This approach may also be useful for exploring analytic and perturbative properties of the wavefunction in the high-field limit.

Results of a previous relativistic distorted-wave calculation for electron-impact excitation of Cs are used to test the validity of the non-relativistic LS approximation in the intermediate energy range of 20-100 eV. We have considered the (6p) ²P_1/2, (6p) ²P_3/2, (5d) ²D_3/2, (5d) ²D_5/2, (7s) ²S_1/2, (7p) ²P_1/2 and (7p) ²P_3/2 excited states. In general, electron excitation of caesium is found to exhibit large relativistic effects.