Table of contents

Time evolution of the hydrogen-like Rydberg atom placed in homogeneous electric and magnetic fields with an arbitrary time dependence is considered. Within the manifold of the electron states with the same principal quantum number n, the problem is effectively reduced to the solution of two independent two-state problems. In particular, this reduction allows us to use in the analysis exactly solvable two-state models which are well known in the collision theory. As an example, we consider the case of perpendicular space-fixed fields; the magnetic field remains constant whereas the electric field strength varies linearly with time. The effective two-state problem corresponds to the well known Landau - Zener model. The simple formulae are obtained for the final population of Rydberg states. In the case of time-periodic fields the general properties of the quasienergy spectrum are established.

Long-lived, excited neutral particles, arising from the interaction of monochromatic synchrotron radiation with ground state He atoms, have been observed at photon energies close to the (N = 1,2,3 and 4) ionization thresholds. The measurements have been made using an unconventional experimental arrangement in which charged particles, responsible for the dominant signal in conventional photon impact studies, are prevented from reaching the detector. For , it appears that the formation, via photoexcitation, of relatively long-lived, doubly excited Rydberg states is a necessary step in the production of the observed signal. Four processes that might account for the production of the excited neutral particles are considered. The simplest, that atoms in the initial doubly excited Rydberg states are themselves directly observable, is considered unlikely. However, the lifetimes of the photoexcited double Rydberg states may be increased through the effects of electric fields present in the apparatus. Transitions from these double Rydberg states, occurring either as a result of collisions or by fluorescence, could result in metastable, singly excited neutral particles. The presence of signal related to the existence of long-lived doubly excited states could be significant in the interpretation of other photon impact measurements.

The ionization process of an atom in an ultra-intense laser field is considered in a 2D and 3D classical relativistic approach with special attention being paid to the influence of the laser magnetic field and the breakdown of the dipole approximation. We show the possibility of generation of hot quasi-free electrons in the multi-MeV energy regime for feasible pulsed laser fields of intensity and wavelength . The redshift of the electron oscillation frequency with respect to that of the laser field, due to radiation-pressure-induced motion of the electron in the direction of laser pulse propagation, is used to explain previously unexpected energy boosts.

Photodetachment from in a magnetic field has been studied experimentally using light with energies between 14400 and . Presented here are high-resolution data which exhibit sharp magnetic field structure at thresholds and low-resolution data which show monotonically increasing cross sections. The current work is the first in any atomic or molecular system where sufficient energy resolution has been achieved to observe the shape of the cross section in a magnetic field.

The one-dimensional integral representation for the Fourier transform of a two-centre product of B functions (finite linear combinations of Slater orbitals) with real parameters is generalized to include B functions with complex parameters. This one-dimensional integral representation allows for an efficient method of calculating two-centre exchange integrals with plane-wave electronic translational factors (ETF) over Slater orbitals of real/complex exponents. This method is a significant improvement on the previous two-dimensional quadrature method of the integrals. A new basis set of the form is proposed to improve the description of pseudo-continuum states in the close-coupling treatment of ion - atom collisions.

Angular differential measurements from a new electron - ion crossed-beams instrument suggest dynamic effects play only a minor role in forward scattering from almost fully dressed ions. This is contrary to the predictions of a recent many-body theory, and contrary to the case of low-energy backscattering where large dynamic effects have recently been postulated.

The extraction of orbitals and spectroscopic factors from experimental data in electron-momentum spectroscopy by the high-energy (e,2e) reaction requires a sufficiently accurate description of the mechanism. The distorted-wave Born and impulse approximations, using static-exchange distorting potentials, describe the experimental data well. We investigate the need to include complex polarization potentials.

I describe the current theories for cold collisions between laser-cooled and trapped neutral atoms in light fields. The emphasis is on the dynamical nature of the problem, i.e. on the collisional loss of atoms from the trap or collisional heating of the atomic cloud. The recent work on optical shielding of collisions is also reviewed.

The use of distributed Gaussian basis sets in reducing the total basis set truncation error in matrix Hartree - Fock and second-order many-body perturbation theory calculations is investigated for the ground state of the water molecule at its equilibrium geometry. A distributed basis set of even-tempered Gaussian functions centred not only on the atomic nuclei but also on the O - H bond centres and at the midpoint of the line H - H is shown to give a matrix Hartree - Fock energy of . For diatomic molecules, distributed basis sets of this type have been shown to yield matrix Hartree - Fock energies which approach an accuracy of . The present distributed basis set, which includes functions of s, p, d and f symmetry, is employed in a second-order many-body perturbation theory study of correlation effects recovering 97.6% of an estimate of the exact second-order correlation energy given by Klopper. The effects of higher harmonics in the basis set are investigated and a basis set, which includes functions of s, p, d, f and g symmetry, is shown to be capable of recovering 98.6% of the exact second-order energy. The reliability of simple extrapolation techniques to estimate the effects of basis functions of h symmetry and higher is investigated and shown to support 99.8% of the estimate of the exact second-order correlation energy component.

An analytical polynomial formulation of the exponential of fully symmetric matrices is proposed. This polynomial formula is derived using the Cayley - Hamilton theorem and the polynomial Euclidean division. The application field of these developments is time-dependent wavepacket calculations involving multiple potential energy surface crossing. The use of this polynomial formulation in time-dependent calculations is directly connected to the choice of the split operator, proposed by Feit and Fleck (1982 J. Comput. Phys. 47 412) as a numerical propagation scheme. We show in this paper that this analytical polynomial formula is numerically more efficient than the scheme proposed by Almeida and Metiu (1988 Chem. Phys. Lett. 146 47) where the exponential of the kinetic operator is evaluated in the momentum and diabatic representation and the exponential of the potential operator is evaluated in the coordinate and adiabatic representation. The use of an optical potential ( negative imaginary potential) to absorb the wavepacket near the edges of the grid point is quite simple to implement as it appears as a multiplicative factor in front of the polynomial expression. Our scheme is exemplified on the photodissociation of a diatomic molecule involving three potential energy curves.

The atomic emission spectrum of four-times ionized argon, Ar V, has been observed in the vacuum ultraviolet (VUV) region. The - transition array has been studied for this ion. This is the first study of the - transition array which occurs in the Si I isoelectronic sequence. Seventeen new lines that belong to this transition array were classified. All five levels of the configuration were determined. The configurations are interpreted by fitting the theoretical energy expressions to the observed energy levels using a least-square-fitting technique. The identifications are supported by Hartree - Fock relativistic calculations.

Binding energies of helium in a homogeneous magnetic field of strength up to 10 au for S and P symmetries are presented. The calculation is based on a block-tridiagonal representation of the field-free atomic Hamiltonian, whose bandwidth is independent of L. The method is generally applicable for two-electron systems in strong cylindrically symmetric fields.

Recent high-resolution measurements of the - bands in PN have provided additional information about the perturbations of the low-lying vibrational levels resulting from interactions with nearby valence states. The primary interaction arises from the excited state. Multireference configuration-interaction calculations were performed on the and states of PN in order to gain some insight into the probable accuracy of theoretically predicted spectroscopic constants for the state, including , and .

Following Martin and Salin we discuss how to demonstrate the presence of electron - electron dynamical correlation during the collision of a fast ion with an atom. Specifically in this paper we calculate proton - helium and antiproton - helium single- and double-ionization reference cross sections when , the electron - electron interaction, is set to zero during the collision but retained in the initial and final states. We present and critique three ways of obtaining these reference cross sections. Correlation measured in this way seems to be more important for proton projectiles than antiproton projectiles; it is especially needed at small impact parameters and for projectile energies below 350 keV.

The field - field correlation function and the conditional covariance for the ionic microfield in a plasma are calculated within a virial cluster expansion of the two-time generating function. Results for the Debye potential are given. The short-time, high-field behaviour is discussed within a regularization procedure.

The evaluated conditional covariance provides direct access to the probability density of a field strength jump which appears in the well known model microfield method. The resulting stochastic process would describe the time evolution of the ionic microfield on the basis of the real motion of the plasma particles instead of assuming Markovian behaviour.

Seven triply excited states of lithium-like beryllium and carbon are calculated with the multichannel saddle-point and saddle-point complex-rotation methods. Relativistic effects are included using first-order perturbation theory. The widths are studied with one open channel at a time as well as fully coupled open channels. The predicted Auger branching ratio is compared with the observed spectra. The radiative transition rates are also calculated. These results are compared with the theoretical data in the literature.

The kinetic energy spectra of the fragment protons and deuterons produced in the dissociative photoionization of and by 29.5 - 60 eV photons have been measured perpendicular to the E vector of the radiation. It is shown that the superexcited state and the repulsive state play a dominant role in the photoionization process and autoionization state widths have been determined.

Single-centred coupled states calculations of cross sections for excitation and ionization of atomic hydrogen by antiprotons are compared to previous calculations and to experiment for projectile energies in the range 1 - 300 keV. A noticeable disagreement between experiment and theory is found for the antiproton ionization results in the range 30 - 80 keV.

An angular momentum (AM) theory is developed to calculate the relative populations of final rotational states after collision between a diatomic molecule and an atom having a narrow, superthermal velocity distribution as produced by, e.g. photolysis of a precursor species. Probability densities are derived from semiclassical expressions for energy and angular momentum assuming the classically impulsive limit with the repulsive wall modelled by a hard ellipsoid. The treatment given is general and therefore applies to molecules in which the centre-of-mass does not coincide with the centre of the potential coordinates. A transfer function for RT is derived and applied to the H+CO system. Analysis of the data allows the anisotropy to be extracted which is in good agreement with an ab initio potential surface. The method described allows one to rapidly assess the contributions from the elliptical core of the potential and from other features of the potential, and would permit more sophisticated representations of the topology to be incorporated.

By means of electron spectroscopy we have studied the charge equilibration pathways following collisions. Partial electron spectra have been determined in coincidence with the charge state of the resulting Ar ion. The electron spectra obtained in coincidence with and target ions show evidence for target excitation. The electron spectra in coincidence with , , target ions show that in all three cases the energies of autoionization electrons lie in the same range. The starting levels of the decay pathways compare well with the predictions of the overbarrier model. Moreover, it is shown that, regardless of the number of electrons initially captured by the ion, the charge state of the carbon ion is eventually only reduced to four or five.

The energy dependence of the nonradiative electron capture cross section is discussed for relativistic collision energy. A simple analytic expression for the cross section is obtained for inner-shell transitions using second-order perturbation theory. We have confirmed that the leading-order term is found to have the following energy dependence: . This is attributed to a combination of kinematic features of the process and retardation effects. For ultra-relativistic collision energies and small nuclear charges, electron capture without change of spin is the dominant transition.

We present total cross sections in the laboratory energy range for electron capture by protons in collision with . We describe this process theoretically by performing a close-coupling (CC) calculation using the symmetrized variational (SV) continuum distorted-wave (CDW) collision ansatz. We discuss the SVCDW results in conjunction with another CC variational CDW calculation, with CDW perturbation theory, and also with close-coupling calculations which involve a representation of the continuum by a set of pseudo-states which may include pseudo-states of the united atom. Moreover, we compare the SVCDW results with the available experimental data. The SVCDW results are found to be in very good agreement with the experiments of Peart et al, Rinn et al and Watts et al at all energies, and consequently to be at variance with the experimental results of Angel et al. The SVCDW results are also found to be in good accord with a CC calculation involving Sturmian-type pseudo-states. We investigate this agreement further by comparing impact-parameter profiles at 50 and .

Cross sections of electron-impact detachment of negative ions and of mutual neutralization between positive and negative ions are calculated using a model based on the semiclassical theory for ion - atom collisions but modified to include Coulomb trajectory effects. It is shown that the method allows us to obtain accurate results for collisions down to low energies. The Coulomb repulsion between the electron and the negative ion accounts for the rapid decrease of detachment cross sections at low energies, and the Coulomb attraction between positive and negative ions accounts for the rapid increase of neutralization cross sections at low velocities.

Valence and Rydberg bound states of CO are studied ab initio using the UK molecular R-matrix code and electron - scattering calculations. Results are presented for singlet and triplet and states of CO at . Various models for both the target and the full calculation are tested and the stability of the final models is demonstrated. Final calculations employ complete active space (CAS) target states, with and without further excitation, and up to 13 target states in the close-coupling expansion. The best model underestimates the ionization potential of CO by 0.1 eV and finds quantum defects systematically slightly higher than those observed, by up to 0.07. The prospects for further improvement of these calculations are discussed.

In this work the multiply differential cross section for the low-energy electron-impact ionization of atomic hydrogen is considered in the coplanar asymmetric energy-sharing kinematics at an incident energy of 27.2 eV. The emergence of structures in the measured spin-unresolved cross sections is explained in terms of isolated two-body final-state interactions and three-body coupling. The cross section shows two peaks originating from `classical' sequential two-body collisions. The position of these peaks is determined by two-body final-state interactions. In addition, it is demonstrated that the signature of three-body interactions is carried by the magnitude and ratio of these two peaks. The direct and exchange amplitudes as well as the spin asymmetry are also considered.

Using a crossed electron - molecule beam ion source in combination with a quadrupole mass spectrometer we have studied the electron energy dependence of the dissociative attachment process (at electron energies from about 0 - 2 eV and with an energy resolution of about 60 meV) in a target gas range of about 300 - 420 K. Utilizing the improved energy resolution of the present experimental set-up it was possible to determine the accurate shape and magnitude of the cross section function in this low-energy range. This leads to the conclusion that close to 0 eV energy (below about 0.15 eV) the reaction proceeds via s-wave capture, whereas at higher energy (above 1 eV) autodetachment significantly determines the shape of the cross section. Moreover, the present measurements allow us to clarify previously reported differences in the absolute cross section, in the number of peaks and in the energy positions of these peaks. Finally, by analysing the measured strong temperature dependence of the cross section peak at about zero electron energy the activation barrier for this dissociative attachment was determined to be in good agreement with thermochemical data deduced from swarm experiments.

Using a gas-evaporation technique with an induction-heating method, Zn fine particles were prepared. Transmission electron microscopic observations showed that Zn with spherical form are about 80 nm in diameter. X-ray diffraction revealed that the particle is composed of a Zn core and a ZnO coating. Raman scattering from the Zn fine spherical particles coated with ZnO of variable thickness was measured, and a broad peak was observed. The Raman peak shifts to the lower-frequency side with the increase of oxide thickness. To explain the experimental results, the frequencies of the optical-surface phonons of a metallic sphere with a dielectric coating were derived. In particular, the dependence of the frequencies on the thickness of the coating was investigated. From a comparison of experimental results with theoretical analysis, it was concluded that the broad Raman peak comes from the surface-phonon scattering.

An interpretation of the wavepacket collapse (WPC) is given. We find a connection between the WPC and the measurement precision limitation caused by quantum fluctuation. A quantitative relation between the decay of the off-diagonal density matrix elements and the measurement precision limitation is obtained for any measurements.

We investigate the scaling properties of the Ne-like collisional laser through a semi-analytic scaling model. The model is found to reproduce the temporal development of the electron density profile and temperature remarkably well, although some aspects of the driving laser wavelength dependences are not completely satisfactory. The model is useful as a means to calculate approximate plasma conditions when detailed hydrodynamic calculations are inappropriate.

We use the scaling laws to identify the existence of two threshold incident intensity conditions for lasing. The first of these is straightforwardly that the absorbed energy is sufficient to generate the required electron temperature. This constraint reproduces the threshold intensities for the elements which have been modelled in detail. The second constraint is that the density gradients are low enough to allow propagation of the lasing light, leading to a minimum energy required to produce a plasma of adequate scale length. At low Z the second constraint is found to completely dominate the energy requirements (an important consideration as interest is developing in these elements as a means to reduce the required driving power). This is true even for the J = 0 - 1 transition which has the shortest wavelength of the lasing lines in low-Z ions. In calcium (Z = 20), in which the refraction constraint defines the threshold, comparable incident intensities to germanium (Z = 32) are required, in which the temperature constraint is applicable. We conclude that elements in the vicinity of iron (Z = 26) are likely to be the most efficient low-Z schemes.

In this recent article, the caption for figure 5 was incorrectly printed. The correct caption with the figure appears below.

 Graph

Figure 1. The energy dependence of the electron detachment cross section. The full curve represents the completely dynamical calculations. The remaining curves represent conventional adiabatic calculations. The broken curve is based on the rate function pertaining to the hybrid model by Ostrovsky and Taulbjerg (1996) while the chain curve is based on the present dynamical width of the resonance. The data points represent the experimental results by Andersen et al (1995).