Table of contents

The first study of the quantum dynamics for a highly excited ('Rydberg') H₂ molecule in a magnetic field is presented and compared with the well studied atomic case. Wavefunctions from an accurate quantal calculation have been used to obtain the corresponding Wigner functions by means of a transformation to a basis where the latter are analytical. Unlike the atom, the dynamics of the excited electron in the molecular case experiences two, interfering, energy regimes. This study consider the case near electronic scaled energies -0.5 and -2.3, which would correspond to predominantly regular motion in the atomic case. In the molecular case there is evidence for chaotic behaviour in this energy range. Quantum states for the molecule may divide into those where the underlying dynamics is a perturbed version of the hydrogen atom case, and others where different configuration-space and phase-space structures are apparent.

Many-body calculations of the 4s photoionization cross section of free Mn⁺ in both ground (Mn⁺(3d⁵4s¹⁷S)) and excited (Mn⁺*(3d⁵4s¹⁵S)) states are performed for the first time. Striking differences between these cross sections, caused by a non-trivial manifestation of multielectron correlation, are discovered.

Ionization of atomic hydrogen in collisions with H⁺ and He²⁺ ions is studied by a large-scale coupled-channel calculation. It is demonstrated that high-lying continuum states of the projectile, which have been disregarded in previous calculations, contribute significantly to the ionization of the target atom. The portion of the ionization into these projectile continuum states amounts to 13% of the total ionization cross section in the case of H⁺ impact even in the MeV region, and the question is raised as to the normalization procedure of experimental values to the Born cross section there. The partial cross section of the ionization into the target continuum satisfies the Z² scaling rule (where Z is the projectile charge) at high energies while the component into the projectile continuum states has a weaker dependence on the charge.

Results of a calculation for e-Cs scattering, carried out in a fully relativistic (Dirac R-matrix) framework, are used to calculate the polarization of the light emitted by excited caesium atoms in an energy range from threshold to 2.7 eV. They are compared with previous semirelativistic Breit-Pauli calculations and the available experimental data. The results indicate that the Percival-Seaton theory approximating relativistic scattering amplitudes by recoupling amplitudes from a non-relativistic LS calculation is not valid for low-energy e-Cs scattering.

Dissociation of N₂O into metastable O(¹S) fragments following electron impact has been studied in the energy range from threshold to 500 eV. A novel detector, selectively sensitive to O(¹S), has been used together with pulsed excitation and time-of-flight techniques. The data suggest that O(¹S) production is dominated by a single dipole allowed transition in the parent molecule.

Recent two-state and five-state close-coupling calculations for positron-alkali scattering are used to predict the linear polarization of the resonance line (np)²P degrees to (ns)²S emitted by the alkali atoms Li, Na, K and Rb after positron impact excitation. The collision energies considered range from threshold ( approximately=2 eV) to 40 eV. The results are interpreted in terms of elementary collision theory, including depolarization of the line radiation through fine- and hyperfine-structure effects. Given the size of the polarizations predicted and the recent developments in positron physics, an experimental test of the authors' results should be possible. This should provide an independent test of the theoretical model, in addition to the cross sections considered so far.

The ground state binding energy of two interacting charged particles, confirmed in a two-dimensional quantum disc by an infinite square well potential, is calculated within the framework of the variational method as a function of the disc radius. The results are discussed in relation with the two limit cases R to infinity and R to phi .

The K-shell resonance and dielectronic satellite lines of Mg XI through Mg VIII have been observed in absorption in a near-LTE, radiatively-heated, laser-produced plasma. Measurements of the absorption of 1.3-1.7 keV X-rays from a Dy backlighter layer by 1-2, 1-3 and 1-4 transitions have been made with high spectral resolution. Radiative hydrocode simulations and detailed atomic physics calculations describe correctly the experimental results, except for the 1s²-1s2p resonance line of Mg XI which is always observed in emission. Resonance fluorescence resulting from a collisional-radiative cascade from the Mg XI 1s3p level photopumped by the strong 3d-4f transition arrays of Dy could be the source of the observed emission.

K_beta /K_alpha X-ray intensity ratios of Ti, Cr and Ni in Ti_xNi_1-x and Cr_xNi_1-x alloys have been measured for the concentrations x=1.0, 0.74, 0.55, 0.35 and 0.0 and x=1.0, 0.58, 0.20 and 0.0, respectively, using gamma ray fluorescence of 59.54 keV gamma rays from a 200 mCi ²⁴¹Am point source. For the concentration x=0.20 in Cr_xNi_1-x alloy the authors find an increase in the K_beta /K_alpha ratio of Cr by about 18% over its pure metal value. Similarly for the case of Ti_xNi_1-x alloy they find for x=0.35 an increase in the K_beta /K_alpha ratio of Ti over its pure metal value. For the other concentrations of both the alloys the Ti and Cr K_beta /K_alpha ratios are in agreement with the earlier pure metal results as well as with theory. In both alloys the nickel ratio remains almost the same for all alloy compositions. The large increase for the Cr ratio corresponding to x=0.20 and the increase in the Ti ratio for x=0.35 could be explained qualitatively as due to transfer of 3d electrons from Cr to Ni and Ti to Ni, respectively. However, theoretical calculations are needed for a quantitative interpretation of the data.

A non-variational Galerkin method, completely based on B-spline methods is introduced and applied to calculations of resonance positions and widths in H^- and He. The results agree very well with other theories and experiments. Photodetachment in H^- and photoionization of He are also considered.

The satellite structure due to the configuration interaction (CI) and shakeup process from 6s to 7s has been studied in the 4d, 5s and 5p photoelectron spectra of Cs and Ba. Spectra have been compared with each other and with the results of multiconfiguration Dirac-Fock calculations. Theoretically determined total intensities of CI satellites, shakeup probabilities and energies have been found to be in good general agreement with experiment.

Measurements of Faraday rotation spectra for the elements lithium and sodium are reported. The observed patterns are all obtained and analysed under full Paschen-Back coupling conditions. These are believed to be the first accurate measurements for these elements extending to high principal quantum numbers. The course of f-values in the principal series is compared with available calculations, which take account of core polarization effects by the quantum defect orbital method. The authors' measurements reveal that the influence of the magnetic field on the f-values of the high series members grows as B²n⁴. When this is corrected for, the data extrapolate to absorption cross sections in the base of the continuum which agree well with earlier measurements. The authors also apply this correction to earlier data for potassium (Nawaz et al., 1992).

The threshold electron spectra of Ar₂ and Kr₂ were measured using synchrotron radiation. The vibrational structures of the ground states of Ar₂⁺ and Kr₂⁺ ions were clearly observed. The present results for the Ar₂⁺ ionic state confirmed previous ones. For the Kr₂⁺ ionic state, the vibrational term values are determined to be (103695+or-8)+(188+or-1)( nu +1/2)-(0.9+or-0.03)( nu +1/2)² (cm^-1) from the zero vibrational state of Kr₂. The TOF photoelectron spectra of Ar₂ and Kr₂ were measured at several wavelengths of autoionizing states of Ar₂ and Kr₂, using pulsed photons from a synchrotron. In the photoelectron spectra, several vibrational levels were clearly recognized to support the authors' vibrational analysis of the threshold electron spectra.

The ² Sigma ⁺ Rydberg states of ArH are predissociated while the ² Pi states decay radiatively. Theoretical values for the radiative lifetimes of several Rydberg states of ArH and of the width of the A² Sigma ⁺ state of ArH and ArD have been obtained. The results are generally in good agreement with experimental data where available, except for the width of the nu =0 level of ArH where the theoretical value differs from the experimental one by about an order of magnitude.

Experimental results on light-induced drift of (ro)vibrationally excited CH₃F immersed in the noble buffer gas Kr or in the polar buffer gas CH₃Cl are presented. For pure vibrational excitation, the relative change in collision rate is found to be essentially velocity independent. For rovibrational excitation, this quantity can have a significant velocity dependence, as can be concluded from the detuning behaviour of light-induced drift for two transitions of CH₃F immersed in Kr. In combination with earlier observations of anomalous light-induced drift in C₂H₄, these results demonstrate that a sizable velocity dependence of the change in collision rate caused by rovibrational excitation is a general feature for molecular systems. Additionally, while the transport collision rate generally increases with vibrational quantum number, the data suggest that it decreases with increasing rotational quantum number. Finally, the data for CH₃F in CH₃Cl indicate that rotational-state-changing collisions are accompanied by a significant velocity change.

A new potential energy surface (PES) for the first excited singlet state of ClNO was calculated with the coupled electron pair approximation (CEPA) method. This PES could be used without any empirical corrections in subsequent three-dimensional dynamical calculations with a L² amplitude density method of time-independent scattering theory. Absolute photodissociation rates for producing the NO fragment, in particular rotational-vibrational states were calculated for the lowest seven initial vibrational states, of ClNO. A shift of the D and C band peaks to shorter wavelengths with increasing temperature is predicted.

The authors have studied resonance collisions amongst atoms dressed by a strong laser field. The equations of motion have been solved numerically to yield collisional transition rates for various values of the Rabi frequency (V) due to the laser and its detuning ( Delta ) from the atomic transition frequency ( omega ₀). The atoms are assumed to follow classical rectilinear paths and only undergo binary collisions. The resonance interaction is taken to be the first-order dipole-dipole interaction acting within the ground and resonance levels of a pair of atoms. A qualitative explanation is offered for the results obtained by splitting the Hamiltonian into parts that represent the collision-free evolution of the system at the Rabi frequency, the 'normal' resonance interaction, and the 'dressed interaction' responsible for opening new scattering channels. The latter interaction depends upon the relative strengths of the resonance interaction (R), the generalized Rabi frequency ( Omega ) and the ratio V/ Delta .

For pt.I see ibid., vol.25, no.17, p.3641-54 (1992). The transitions between the Zeeman magnetic sublevels of atomic hydrogen which are induced by ion collisions when a uniform magnetic field is applied are theoretically studied. This work takes explicit account of the orientation dependence of the collision process in anisotropic space. The process plays an important role as a disalignment mechanism of atomic hydrogen in plasmas accompanied by an external magnetic field. In solving the problem, a peculiar symmetry property of atomic hydrogen is used.

The characteristic X-ray spectra of cobalt (Z=27) have been obtained as a function of energy in a range from 0.7 to 2.0 MeV amu^-1 for incident protons, deuterons and ³He ions. K X-ray production cross sections sigma _x were measured by a comparison with the simultaneously measured Rutherford scattering cross section. Uncertainties inherent in this method are minimized to approximately 8%. For protons, the measurement was extended at energies up to 3.0 MeV. The results are compared with the predictions of the perturbed-stationary-state with energy-loss, Coulomb deflection, and relativistic corrections (ECPSSR) ionization theory and first Born approximation. The values obtained for the ratios K beta /K alpha and sigma _x are tabulated. Comparisons of the authors' data with those from other measurements in earlier works are discussed.

Measurements by Mezyk et al. (1989,1991) have shown that the rate coefficient for Xe₂⁺+Cl^-+Xe to XeCl*+2Xe is enhanced relative to that given by the universal curve that describes termolecular recombination of structureless atomic ions. This is attributed to Cl^- exciting internal modes of Xe₂⁺ during passage through the perihelion part of the orbit. The electrostatic tidal effect is simulated by the Monte Carlo technique, excellent agreement with the data of Mezyk et al. being obtained. It is reasoned that the tidal action cannot enhance the rate of termolecular recombination of small valence-bound ions like O₂⁺ and O₂^- appreciably. However its influence is pervasive because complex ions are commonly present when the ambient gas density is enough for termolecular recombination to be of interest: for example, O₄⁺ is the main positive ion if the ambient gas is pure oxygen. Enhanced mutual neutralization may obscure the experimental evidence.

Using the single-particle amplitudes from a 20-level coupled-channel calculation with ab initio relativistic self consistent LCAO-MO Dirac-Fock-Slater energy eigenvalues and matrix elements the authors calculate within the frame of the inclusive probability formalism impact-parameter-dependent K-hole transfer probabilities. As an example the authors show results for the heavy asymmetric collision system S¹⁵⁺ on Ar for impact energies from 4.7 to 16 MeV. The inclusion probability formalism which reinstates the many-particle aspect of the collision system permits a qualitative and quantitative agreement with the experiment which is not achieved by the single-particle picture.

The influence of an intense laser field on the rate coefficients of various elementary electron-ion processes is studied. Prospects for applications, such as the creation of population inversions in the X-ray region of the spectrum, of these laser-assisted effects are discussed.

The authors present collision strengths and electron impact rate coefficients for some transitions in N III calculated using the R-matrix method. Collision strengths are presented in the LS coupling scheme. Some of the collision strength spectra show extensive autoionization features across thresholds. Maxwellian averaged collision rates were calculated at a range of temperatures for each prominent transition in the N III spectra. These are calculated to be used for planetary nebulae and solar/late-type stellar studies in related work.

Absolute emission cross sections, for the VUV spectral features at 130.4, 102.7, 98.9 and 87.8 nm, have been measured for electron impact on atomic oxygen in the energy range from threshold to 100 eV. An RF discharge source containing NO was used to prepare a high density atomic oxygen beam. Significant differences were found with earlier measurements on these transitions. The data are compared with available theoretical results and with level excitation cross sections obtained using inelastic electron scattering techniques.

Direct and indirect dissociative recombination are compared. If the attractive potential energy curve of the ground state of the ion is not crossed by a suitable repulsive potential energy curve of the molecule formed by neutralization the indirect process can be much faster than the direct process. However, in the case of, for example, HCO⁺ its rate coefficient alpha (I) is several orders of magnitude less than the measured value even though the radiationless transition involved is of the extremely rapid single-electron type. A new process, multi-step indirect dissociative recombination, that entails the system passing successively through several intermediate states in Delta nu =1 step is introduced. Examination of the formula for its rate coefficient alpha (I*) obtained from high-order perturbation theory shows that the final pre-dissociation step is rate-limiting. Extensive computations are done on the overlap integrals between the bound vibrational wavefunctions of the ion and the wavefunction for the separating molecular fragments.

In this paper a complete scheme for calculating ionization cross sections for molecular targets is given, and for the first time the cross sections for the e⁺+H₂ to e⁺+e^-+H₂⁺ processes are calculated in the energy region 30-200 eV using the CPT model. The results show some conformity with recent experimental data.