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We study the existence of stable bound states of triatomic molecules containing two helium atoms and one alkali atom X (X = Li, Na) and examine the dependence of their binding energies on the mass of the constituent atoms. The three-body systems are solved within the adiabatic approximation in hyperspherical coordinates using the He - He and He - X pair van der Waals interactions that are available in the literature. The binding energies of these systems are shown to be of less than 1 K for some isotope combinations. For the lighter isotopes it is shown that no bound states exist. We found no evidence of the existence of halo states or Efimov states for these systems.

We demonstrate that the technique of measuring the angular correlation between the photoejected electron and polarized fluorescent photon can be used to determine all the photoionization parameters for the relativistic case. Applying this technique to an autoionization resonance of atomic Sr, values were obtained for the ratios of the three dipole amplitudes and the two phase differences.

We have undertaken a detailed study of the various Rydberg resonance series found in the symmetry for electron collisions with molecular oxygen ions. A five-state valence CI approximation is used in our expansion of the target molecular states within the ab initio close-coupling R-matrix approach to calculate the relevant eigenphase sum and associated cross sections. Rydberg resonances series are detected, identified and analysed. An analysis of our results indicates the presence of and interloper resonances in the scattering symmetry which have been assigned to the appropriate molecular oxygen cation threshold. The presence of these interlopers is seen to cause a perturbation on the relevant spectral series disrupting the regular Rydberg series pattern. The results for several low-lying members of each Rydberg series are analysed in detail and compared with the available theoretical and experimental work.

We have carried out a generalization of the continuum-distorted-wave (CDW) model for single ionization of atoms by ion impact where the interaction of the active electron with the target is represented by a model potential. We apply this model to the ionization of He by proton and highly charged bare-ion impact. Doubly differential cross sections are calculated and compared with the available experimental data at high impact energy. Very good agreement is obtained for emission in the forward direction.

Doubly excited or Feshbach-type resonances in helium-like systems have been calculated using the complex-rotation method. Using Hylleraas-type wavefunctions of up to 1632 terms, we calculate resonance parameters (both resonance positions and widths) below the N = 3 and 4 thresholds of the hydrogenic systems with nuclear charge from Z = 2 to Z = 10. Comparisons are made with the other calculations for in the literature.

Oscillator strengths and lifetime values for Yb II levels have been calculated within the framework of a pseudo-relativistic Hartree-Fock (HFR) approximation combined with a least-squares fitting of the calculated eigenvalues to the observed energy levels. A considerable amount of configuration interaction together with core-polarization effects have been included in the calculations. The quality of the HFR results has been assessed through a comparison with data obtained using the completely independent relativistic quantum defect orbital (RQDO) method. The theoretical lifetimes reported in the present paper are now in good agreement with most of the accurate experimental values available. The origin of the persisting discrepancies is suggested.

We use the complex rotation method to calculate the complex energy of the resonance of a two-electron atom with nuclear charge , being the threshold value (the minimum nuclear charge Z for which remains a bound state). The calculation is performed to verify numerically the dispersion relation proposed by us for the ground-state energy of a two-electron atom considered as a function of Z.

We have measured the radiative lifetimes for the level of and the level of . These measurements were performed by monitoring the temporal behaviour of their associated radiative decays during magnetic trapping mode in an electron beam ion trap (EBIT). Our lifetime results, 8.7(5) ms for and 5.7(5) ms for , are compared with theory.

We consider a two-electron Rydberg atom, whose core is driven by two laser fields in a lambda configuration. The two laser pulses are near resonant to the corresponding transitions but are slightly and equally detuned from the upper level. In the case of sufficiently large detuning, spontaneous emission is negligible in this context. We show that slowly decaying but nondispersing radial Rydberg wavepackets, involving arbitrarily large principal quantum numbers, can be prepared in this system without being affected by spontaneous emission. Ca has the properties required for this effect and is employed as a realistic model in this paper.

Following our recent mass spectrometric study of wavelength-dependent photoionization of laser-excited, polarized atoms near threshold (1997 30 609) we report electron angular distributions for photoionization of polarized atoms at four energies above the threshold. The measurements allow us to both check the atomic alignment produced in the optical pumping process and test in detail results of theoretical many-electron calculations for the relevant reduced dipole matrix elements and phase shifts. In contrast to the situation for (1983 16 2945), photoionization of cannot be simply described by two reduced matrix elements and and a single phase difference . For the term dependence of the Hartree-Fock wavefunctions and correlation effects lead to substantial differences between the reduced matrix elements for the five d-wave channels and between the five existing phase differences. The comparison of the experimental angular distributions with those calculated theoretically shows that good agreement is obtained when the theoretical values of the reduced matrix elements for the d-wave channels are increased (relative to that for the s-wave channel) by a common, energy-dependent factor; this finding is in accord with conclusions from the polarization-dependent ion production data. The correction factors range from 1.4 to 1.1 -846 meV) for calculations which include long-range core polarization effects, while they amount to values between 2.0 and 1.5 when core polarization effects are neglected.

The spectroscopy of chlorine dioxide, OClO, has been investigated using the techniques of dissociative electron attachment (DEA), near-threshold electron energy loss (EEL, 0-10 eV) and optical absorption (2.5-10.8 eV). Mass and energy analyses of the ions formed in the DEA processes show that at energies above about 3 eV, the product ions are predominantly near-thermal , suggesting that the accompanying oxygen atoms are excited electronically. All features observed in the EEL experiments are in regions of optical absorption, indicating that the optically dark doublet and quartet states have energies which overlap optically allowed transitions. The optical spectrum itself is in good qualitative agreement with previous measurements, but there are differences in relative absorption cross sections. Alternative interpretations of some spectral features are proposed.

Threshold electron and total ion yield spectra are presented for the near C and O 1s edge photoexcitation of n- and iso-propanol. To our knowledge these are the first such data for iso-propanol, and represent the first direct measurement of the core ionization energies of these molecules by threshold photoelectron spectroscopy. In particular, the C 1s energies for n-propanol are found to be lower (by about 0.5 eV) than determined indirectly in a previous study. Application of multiple coincidence techniques has enabled determination of the branching ratios of the dissociative multiple ionization channels uniquely associated with each core edge in both molecules. These show evidence for a greater degree of energy localization in n-propanol than in iso-propanol, and for the existence of long-lived excited states, allowing intramolecular energy transfer to occur.

The final state integral alignment for the electron transfer process is investigated in the keV energy range. Experimentally, polarization analysed photons from the Li I - transition are detected in coincidence with scattered Li neutrals in the 1-25 keV impact energy range. Over most of this range, we find good agreement with 26- and 35-state atomic orbital (AO) basis coupled-channel calculations, as well as with 28-state molecular orbital (MO) basis predictions. Limits of validity of the two theoretical approaches are also discussed in terms of the predicted state-to-state transfer cross sections.

The effects of excitation and orbital alignment on the electron transfer process in - collisions are investigated experimentally and theoretically over a wide range of impact energies. Experimentally, the integral alignment parameter and the ratios between the integral transfer cross sections for targets to that for a target have been determined using an optically prepared target. The results are in good agreement with earlier data at lower energies of Campbell et al. Theoretically, the integral cross section results of our 26-state two-centre atomic orbital close-coupling calculations are in good agreement with all experimental data, including the absolute electron transfer cross section measurements of Daley and Perel. These processes are now well understood over more than three orders of magnitude for the impact energy range.

The R-matrix method on a Lagrange mesh is a very simple approximation of the R-matrix method with a basis. By analysing an exactly solvable example, we observe that the mesh approximation does not reduce the accuracy of the R-matrix bound-state energies and phase shifts. This property is obtained with two different meshes, the shifted Legendre and shifted Jacobi meshes, which correspond to equivalent polynomial bases. Their comparison shows that the orthogonality of the Lagrange basis functions is not as crucial as was previously assumed: the Legendre mesh, which corresponds to a nonorthogonal Lagrange basis, is at least as accurate as the Jacobi mesh based on an orthogonal basis. We also emphasize the surprising origin of a known property of the R-matrix method: the results are much more accurate with basis functions without uniform boundary conditions because the quality of the matching is realized by a few highly excited eigenfunctions, with weak physical content, of the sum of the Hamiltonian and Bloch operators.

We show from simple conductance considerations in the intermediate flow regime, i.e. in the pressure regime adjoining molecular flow, that a nonlinear behaviour of the flow rate as a function of backing pressure behind a nozzle is to be expected. For Knudsen numbers a second-order polynomial fits the experimental observations well. The results have been applied to all available data in the literature.

A metastable state of helium with an attached positron is predicted to exist with a total energy of . The state is stable against dissociation into with a binding energy of . This state is formed by the attachment of a positron to the metastable He 1s2s state. The He ( state has a decay rate of . The state can only decay by a process with an approximate rate of . The structure of these states are best characterized as a positronium atom orbiting a 1s core at large distances.

Vibrationally elastic cross sections for electron scattering from difluoromethane are calculated at the collision energies 3-30 eV. No elaborate study for the system has been published to date. Present calculation is based on the fixed-nuclei approximation. As for an interaction potential, effects of an ab initio electrostatic, electron-exchange and target polarization are taken into account. The latter two effects are considered by means of a model potential approximately. The closure formula is employed to obtain a converged cross section. The agreement between the present differential cross section and the experimental one is generally good. The integral and momentum transfer cross sections are estimated theoretically for the first time.

Survival of the excited triplet states and the spin-forbidden transitions including the magnetic dipole (M1) and magnetic quadrupole (M2) decays in the two-electron ion embedded in a variety of Debye plasmas are investigated theoretically. The dependence of the transition energy on the plasmas screening strength exhibits remarkably opposite patterns for the principal quantum number conserving and the non-conserving transitions. The magnetic multipole decay rates of the lower-lying states are seen to remain less affected at lower values of the screening magnitude.