Table of contents

Accurate configuration - interaction wavefunctions are used to calculate oscillator strengths of resonance transitions , and in S I. The interaction between the and states is represented by both correlation and spectroscopic d functions. The excellent agreement between the present calculation and measurements removed long-standing large discrepancies between theory and experiment for the transition.

We present analytical and numerical results for the steady-state resonance fluorescence spectrum produced by a coherently driven two-level atom in the presence of a broadband squeezed vacuum. Here the central frequency of the squeezed vacuum is off-resonant with the driving field frequency, while maintaining resonance between the driving frequency and the atomic resonance. Numerical solutions of the Bloch equations of the system are obtained under the strong driving field approximation which agree well with the analytic results.

Recently we introduced a general method for generating a represention of the solution of the time-dependent Schrödinger equation in terms of an optimized dynamical basis set. In the present contribution we apply this method to calculate excitation and electron loss cross sections in collisions between -particles and hydrogen. We discuss merits of the adaptive basis set in comparison with the commonly used two-centre-atomic-orbital approach.

Radiative decay of resonances is investigated in detail for electron - ion recombination cross sections of He-like ions of carbon and iron. Relativistic close-coupling calculations are carried out using the Breit - Pauli R-matrix method, combined with a perturbative treatment for radiative and autoionization decay, as well as with the relativistic distorted-wave method. Good agreement is found with the synchrotron-storage ring experimental data for C V by Mannervik et al, and the electron-beam ion-trap data for Fe XXV by Beiersdorfer et al. Radiation damping is considerably more significant for Fe XXV than C V; however, contrary to claims by Gorczyca and Badnell and Robicheaux et al, it is unlikely to be of importance in general since the radiative decay rates are orders of magnitude smaller in atomic species other than the He-like and the H-like ions.

New data for single and double ionization of the rare gases Ne, Ar, Kr and Xe, by positron and antiproton impact are presented and compared with existing data for electron and proton impact. It is found that the current understanding of these processes obtained from studies involving low-Z targets only extends to high-Z atoms provided that certain target-dependent effects are taken into consideration. In particular, the static interaction between the projectile and the undistorted target is found to play a crucial role in the case of the lighter projectiles.

A general method is described for finding algebraic expressions for matrix elements of any one- and two-particle operator for an arbitrary number of subshells in an atomic configuration, requiring neither coefficients of fractional parentage nor unit tensors. It is based on the combination of second quantization in the coupled tensorial form, angular momentum theory in three spaces (orbital, spin and quasispin), and a generalized graphical technique. The latter allows us to graphically calculate the irreducible tensorial products of the second-quantization operators and their commutators, and to formulate additional rules for operations with diagrams. The additional rules allow us to graphically find the normal form of the complicated tensorial products of the operators. All matrix elements (diagonal and non-diagonal with respect to configurations) differ only by the values of the projections of the quasispin momenta of separate shells and are expressed in terms of completely reduced matrix elements (in all three spaces) of the second-quantization operators. As a result, it allows us to use standard quantities uniformly for both diagonal and off-diagonal matrix elements.

A new modification of the second-order polarization propagator approximation (SOPPA) is presented. In the new method, called SOPPA(CCSD), all first-order Møller - Plesset doubles and second-order Møller - Plesset singles correlation coefficients are replaced with the corresponding coupled-cluster singles and doubles (CCSD) and amplitudes. This is in contrast to the earlier coupled-cluster polarization propagator approximation (CCSDPPA) in which only one of the doubles correlation coefficients was replaced in the unrelaxed second-order density matrix. The importance of this modification is investigated for the dipole polarizability, dipole oscillator strength sum rules and second hyperpolarizability of , which is known to be a difficult case for perturbative methods. The SOPPA(CCSD) values are compared with previous and new full CI results for the same basis sets.

Threshold photodetachment of negative hydrogen ions stored in a Penning trap has been studied. The electron affinity of hydrogen is determined to in good agreement with previous experiments. The Wigner law has been found to be valid in a region of above the threshold.

We report an experimental analysis of a structure induced by a Nd:YAG laser embedding the sodium excited state in the ionization continuum. The induced structure is probed from the ground state via a one-photon ionization. The experiment is performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analysing the emitted photoelectrons. The selectivity of this technique provides the best measurement of a laser-induced resonance in the photoionization spectrum reported to date. A good agreement is found between the theory and experimental results.

There are a variety of mechanisms leading to double multiphoton ionization. We show that two of them, the shake-off mechanism and the time-dependent Hartree - Fock process are, in fact, identical. In the limit in which electron - electron correlations are negligible they are the only surviving mechanism. For a helium target in a low-frequency limit, they are the dominant double-ionization process but in this domain the process is much too small to be observable.

A new linear equations method for calculating the R-matrix, which arises in the R-matrix-Floquet theory of multiphoton processes, is introduced. This method replaces the diagonalization of the Floquet Hamiltonian matrix by the solution of a set of linear simultaneous equations which are solved, in the present work, by the conjugate gradient method. This approach uses considerably less computer memory and can be readily ported onto parallel computers. It will thus enable much larger problems of current interest to be treated. This new method is tested by applying it to three-photon ionization of helium at frequencies where double resonances with a bound state and autoionizing states are important. Finally, an alternative linear equations method, which avoids the explicit calculation of the R-matrix by incorporating the boundary conditions directly, is described in an appendix.

Angular distributions have been measured of the products of dissociative ionization (DI) of the bent triatomic molecules , and induced by intense ( W ), linearly polarized electric fields generated using 35 ps long laser pulses at a wavelength of 532 nm. Both the and fragments produced upon DI of possess isotropic angular distributions. and fragments from DI of are formed with very marked anisotropies, whereas the fragment resulting from DI of exhibits a weak anisotropy. Attempts are made to rationalize these angular distributions in terms of anisotropy of the induced dipole moments of the ground-state precursors and the resulting interaction potentials, in each case. Good accord is reached between the measured data and the results of an approximate, but ab initio, theoretical treatment which indicates that the relative magnitudes of the components of the induced dipole moment, which lie parallel and perpendicular to the molecular symmetry axis, appear to influence the angular distributions.

In continuation of recently published total capture cross sections for electron transfer from Na(3s) and Na(3p) initial states, we report detailed partial cross sections from coupled-channel calculations using a one-electron model for the collision system. Results are presented for Na(3p) excitation and for capture into various subshells of He where some experimental results are available. We shall focus, in particular, on state-to-state integral cross sections for electron transfer from aligned or oriented Na(3p) states into aligned and oriented states where experimental results may become available. We predict significant alignment and orientation anisotropies in the energy range in singlet as well as triplet channels. We show that previous deviations between experiment and theory in estimating the alignment anisotropy parameter for total capture may be related to the omission of the states in the capture channels.

We describe an implementation of the vibro-rotational sudden approximation eikonal (SEIKON) method to treat collisions between ions (atoms) and diatomic molecules at impact energies such that a semiclassical eikonal approximation can be applied to treat the relative motion of the colliding species, and a close-coupling molecular expansion is able to accurately represent electronic transitions. Our method, which does not make use of Franck - Condon-type approximations, allows us to evaluate vibrationally resolved charge-transfer and excitation cross sections, as well as total cross sections for transitions to the vibrational continuum. The implementation is illustrated with calculations of cross sections for single-charge transfer, vibrational excitation, and transfer dissociation in , and DT collisions. The validity of the Franck - Condon approximation for these reactions is studied.

A large-scale configuration interaction (CI) calculation has been performed for the sulphur atom to obtain excitation energies and oscillator strengths of some electric-dipole transitions. In the calculation, relativistic effects were included through the Breit - Pauli approximation. A fine-tuning technique was used for the adjustment of the diagonal matrix elements to achieve accurate transition energies and coefficients of the CI wavefunctions. The calculated excitation energies are mostly within 1% of the measurements. The agreement between the length and velocity forms of the oscillator strengths is also very good.

Collision-induced dissociation (CID) in atom - diatom non-reactive systems is studied quantum mechanically by treating the rotational and vibrational degrees of freedom in a sudden approximation. The collision problem to be solved is reduced to a one-dimensional Schrödinger equation with respect to the intermolecular radial distance. Therefore, only the scattering phase shift is an important quantity, and a physical insight into the collision dynamics can be obtained by observing the dependence of the scattering phase shift on the molecular orientation and on the internuclear distance of the molecule. The sudden approximation is applied to collisions. The present result indicates the applicability of the sudden approximation to CID in general cases of heavier diatomic or polyatomic molecules.

We have developed an alternative R-matrix with pseudostates (RMPS) method for incorporating a two-electron continuum description into the wavefunction of electron - ion collision calculations. This method is similar in spirit to various recent treatments of the pseudostate expansion, most notably the R-matrix approaches of Meyer and Greene, and Bartschat et al. Our approach differs in that we: (1) utilize a direct sum of several bases: the physical target orbitals, additional MCHF pseudo-orbitals for the He ground state, R-matrix continuum orbitals, and, on occasion, Laguerre orbitals, and (2) rely on a different approach for the creation of an orthonormal basis. Since we use the Belfast codes that are based on a Wigner - Eisenbud R-matrix treatment, we also need to introduce a modified Buttle correction. The method is first tested on the s-wave (Temkin - Poet) model for electron-impact excitation and ionization of . Then it is applied to the calculation of cross sections and angular distributions for the photoionization - excitation process (n = 2,3). We find that these results are greatly improved by the inclusion of the two-electron continuum description, which is more important for the ground state, but plays a role in the final photoionized states as well. A highly correlated multiconfiguration Hartree - Fock expansion is used to represent the ground state, and from the configuration interaction coefficients, asymptotic ratios are determined and compared with other theoretical results. Results are also compared with recent high-energy measurements for the n = 2,3 cross section ratios and angular distribution parameters. This pseudostate expansion allows us to predict photo - double-ionization cross sections, . Of particular significance, we find good agreement between our length and velocity gauge results, indicating that our ground-state correlation is sufficiently converged for the present system.

Stimulated radiative recombination has been demonstrated experimentally in and collisions using a merged electron - ion beams apparatus with field ionization detection of the excited neutral products. Enhancement of the recombination over spontaneous recombination to form the n=11, 12 and 13 levels of atomic hydrogen and the n = 11 and 12 levels of atomic helium by factors of between 1000 and 3000 have been found using a laser power of 8 W. Evidence for the resolution of fine-structure levels has been seen for the case of helium.

Measurements of the single and the ratio of double to single ionization cross sections for positron impact on neon, krypton and xenon are presented from near threshold to 1000 eV. By multiplying the values of each target by the appropriate , new data for the corresponding double ionization cross sections have also been obtained. The ratio between positron and electron maxima is generally seen to be lower for higher target atomic number, as expected from the static interaction between the projectile and undistorted target. Comparison with available data suggests that Ps formation accompanied by the production of singly or doubly charged ions is a significant channel in positron - atom scattering.

Experimental data for single, double and triple ionization of Ne, Ar, Kr and Xe by 30 - 1000 keV antiproton impact are presented and compared with existing proton data. It is found that the current phenomenological understanding of ionization developed from studies of light targets generally apply to heavy atomic targets as well. Inner-shell ionization followed by Auger decay is found to be an important channel for multiple ionization of the heaviest targets, and the projectile charge dependence of their multiple ionization cross sections must be understood as stemming from the underlying inner-shell ionization cross sections.

Soft x-ray spectra (10 - 70 Å) from carbon and aluminium plasmas produced by high-intensity laser pulses have been studied as a function of laser pulse duration (130 fs - 1.3 ps) and intensity ( - ) for two different laser wavelengths (248 and 800 nm). The x-ray energy emitted from the plasma into specific lines and into the continuum has been measured by means of an absolutely calibrated x-ray spectrograph and thus allows the water window x-ray yield from plasmas produced by long and short wavelength laser radiation to be compared directly. Comparison was made between the dependence of the x-ray energy and width of the x-ray lines on the laser pulse duration, wavelength and intensity. The experimental results have been related to hydrocode simulations.

Basis-set all-electron Dirac - Fock calculations for the ground state of the molecule are presented. Four-component molecular spinors are taken to be linear combinations of kinetically balanced spherical Gaussian spinors centred on the nuclei. The nuclei are modelled as spherical Gaussian distributions of positive charge. An uncontracted basis set with 28s, 28p, 12d and 8f basis spinors on the Tl centre, and 22s and 16p basis spinors on the F centre, yields an equilibrium bond length of ; the dissociation energy is found to be ; experimental values are, respectively, and .

Hinds and Sandars have shown that the electronic contributions to the hyperfine interaction due to a nonzero proton electric-dipole moment (EDM) can be characterized by two matrix elements and , respectively, arising from nuclear structure and extent, and magnetic interactions. The best estimates of these quantities from the models of the present paper are and .