Table of contents

We report on fully quantum calculations of the momentum distribution of atoms cooled in the sigma ⁺- sigma ^- configuration. These distributions have sharp quantum features at integer multiples of the photon momentum h(cross)k, similar to those which have been observed experimentally using linearly polarized light by Doery et al (1994). They explained their observation in terms of the band structure of the atoms in the light field which is a different point of view to that which we present here.

Inner shell states of CO have been studied using an electron impact technique at low values of incident energy. The decay of the inner-shell excited triplet state, (1s sigma _c,)^-1(2p pi )^1.3 Pi , has been observed for the first time with vibrational resolution, via its decay to ionic states of the molecule. The vibrational profiles are observed to broaden and shift to higher energies close to threshold. This behaviour is explained as a molecular PCI effect.

Methodologies for systematically including configuration states in calculations for atomic transitions in many-electron systems such as nitrogen are explored. Instead of generating configuration states by replacements from a reference configuration, promotions from a reference complex are considered and the notion of layers is introduced. Our methodology provides for an effective strategy for handling systems with many electrons in a partially filled shell. Theoretical transition data is reported for transitions among a number of low-lying quartets in nitrogen. Comparisons with known experimental data and other theoretical calculations are included.

The ground-state energies of helium-like uranium, helium-like holmium and neutral helium have been calculated within the multiconfiguration Dirac-Fock model. The convergence problem of the MCDF method for two-electron heavy ions is studied. The results are compared with values obtained by others using projection operators for the negative energy continuum. A small but significant discrepancy in calculated energies is found.

Singlet-triplet anticrossings of 1s3d Stark sublevels of He I were investigated by inducing resonance transitions from the 1s2p ³P level to the 1s3d Stark states at electric fields 80 kV cm^-1 <F_z<100 kV cm^-1. The measurements were performed using a beam of metastable helium atoms perpendicular to a 588 nm laser beam from a tunable dye laser. From the energy separations of closest approach of four anticrossings we deduced the radial integrals h_off^3p=(3660+or-200) MHz and h_off^3d(652.6+or-1.0) MHz of the singlet-triplet off-diagonal matrix elements of the spin-orbit coupling for the 1s3p and 1s3d configurations. They agree with recently calculated values.

By selecting B-spline basis functions systematically in variational calculations, we obtain the accurate wavefunctions and energies for n^1,3 P and n^1,3D states of He for n<or=9. In comparing with our previous results e.g. 2-5^1,3P, it is indistinguishable between our present and previous results. The present results were obtained with smaller number of basis functions. We can therefore use the accurate wavefunctions to calculate oscillator strengths for the transitions mS-nP, mP-nD, m, n<or=9, in He. The results of f values are accurate to within 0.01% at least. The error estimate is based on the uncertainty of the difference of the corresponding energy levels as in our previous work. Our results agree with the previous highly accurate calculated results of Kono and Hattori (1986) impressively, which are likely the most accurate theoretical results. We also give hybrid f values of transitions between m¹P and m¹D (or between 1¹S and m¹P) for m<or=9. Our results also agree with some recent accurate experimental results as good as those of Kono and Hattori.

A recently proposed algorithm for the determination of the continuum wavefunction employing a B-spline basis set expansion and a least-squares approach is extended to a full multichannel formulation employing a general close coupling expansion of the continuum wavefunction. Photoionization of the He atom up to the n=4 threshold and H^- up to the n=3 threshold are described with a pure close coupling wavefunction (16 and 9 channels respectively), and an accurate full CI expansion for the ground state. Excellent agreement with previous accurate calculations is obtained for total and partial cross sections, resonance parameters and angular distributions. For He a nine-channel wavefunction allows photoionization to be described up to 2 keV. Modification of the boundary conditions to describe long Rydberg series is successfully tested. The proposed algorithm appears to be a general and flexible approach for the description of multichannel continuum wavefunctions by basis set expansion.

We have observed the autoionizing Rydberg series converging to the F⁺ 2p⁴(¹D₂) limit by way of photoelectron spectrometry with synchrotron radiation using a monochromator bandpass of 0.012 nm. The energy data are in good agreement with earlier results. The relative intensity of the 2p4(¹S)3s(²S_1/2) feature arising from the 2P_3/2 ground state was found to be small, and previous discrepancies concerning this resonance could not be resolved.

Using resonant two-photon excitation of a metastable Ne(2p⁵3s 3P_0.2) beam by two single-mode CW dye lasers, we have measured reduced widths Gamma _r=n*³ Gamma _n for the four Ne(²p⁵(²P₁2/) nd' (K)_s) autoionizing resonance series ((K)_s=(3/2)₃, (3/2)₂, (5/2)₂, and (5/2)₃). The resulting values are Gamma _r=167(7), 350(7), 87(2), and 73(3) cm^-1, respectively, in good agreement with predictions by a semiempirical multichannel quantum-defect analysis involving complex quantum defects.

We present results of perturbative calculations on the two-photon ionization of beryllium above the second ionization threshold: Be⁺(2p). As in earlier work, we use a discretized approach (utilizing B-spline functions) for the representation of the atomic spectrum. Besides two-photon ionization, the case of above threshold ionization, where one photon is already sufficient to ionize Be, is presented.

Photoionization of the CO molecule and inner-valence states of CO⁺ between 22 and 45 eV have been studied by means of photoelectron spectroscopy using both synchrotron radiation and He II radiation. Vibrational structure has been resolved in many bands up to 45 eV. CASSCF (complete active space self-consistent field) and MRCI (multireference configuration interaction) calculations of potential curves in the 22-30 eV range have been performed and these have been used to predict vibrational levels and Franck-Condon factors. In this energy range three valence states, D ²II, 3² Sigma ⁺ and 3²II have been identified, and spectroscopic constants have been determined for the first two of these. Above 30 eV, all valence states have been found to be repulsive. In addition to the broad bands expected for these states, several progressions of narrow lines are observed most probably reflecting transitions to Rydberg states.

Symmetry-adapted perturbation theory is used to calculate the interaction potential between Na⁺ ions and He atoms with high accuracy over a wide range of ion-atom separations. This potential is used to compute the transport coefficients of Na⁺ ions in He gas. A comparison of the calculated results with the most recent transport data validates the claimed accuracy of the potential.

We investigate linear and non-linear absorptive and dispersive properties for a homogeneously broadened molecular two-level system including simplified versions of vibrational structure and intramolecular coupling. The Born-Oppenheimer electronic energy curves for this simplified molecular model consist of two intercrossing harmonic oscillators potentials with different force constants. These electronic states are coupled and inclusion of the intramolecular coupling within a vibronic model renders the corresponding coupled states. We have analysed the influence of the intramolecular coupling parameters on the optical response of the system up to third-order in the external field. The critical quantities for this analysis are the transition and permanent dipole moments (both in magnitude and relative sign) which depend strongly on the intramolecular coupling. Some simple exact results are derived for the important case of exact degeneracy. Finally, the relevance of the optical response of the change in force constants associated with the two uncoupled states is examined.

We report experimental and theoretical results for the basic autoionization processes He*(2³S)+H/D(1²S), leading to Penning ( to He+H⁺/D⁺+e^-( Theta , epsilon )) and associative ionization ( to HeH⁺/HeD⁺( nu ⁺,J⁺)+e^-( Theta , epsilon .)). The experimental electron energy spectra, measured at thermal collision energies (E_rel approximately=50 meV), show an angular-dependent behaviour which is due to the interplay between the orientation of the collision complex at the moment of autoionization-as referred to the relative velocity direction-and a strongly non-isotropic internal angular distribution for electron emission from the excited quasimolecule. The ab initio work on He*(2³S)+H/D includes electron angular momentum dependent complex (resonance-to-continuum) coupling elements, revealing substantial contributions of autoionization into the electronic d wave. Comparison between the angle-differential experimental electron spectra, measured with an energy resolution of 30 meV, and the corresponding theoretical distributions, obtained in the framework of the local complex potential theory, yields good agreement for both isotopes.

We report a theoretical study of excitation, charge transfer and ionization in collisions between helium ions He⁺ and atomic hydrogen in the impact energy range between 2.5 and 250 keV amu^-1. The colliding system was treated in a full two-electron formulation as well as in a pseudo-one-electron approximation to the total Hamiltonian. The wavefunctions were expanded in terms of two-centre atomic orbital bases within the semiclassical impact parameter close-coupling method. The results demonstrate advantages and limitations of either approach and allow a detailed comparison to be made with a wide range of available experimental data.

We present calculation of the electronic energies and dynamical couplings for the quasimolecule CHe²⁺, necessary to evaluate charge transfer cross sections in C²⁺+He collisions. Using the MELD ab initio program, an all-electron configuration interaction method has been employed to calculate the molecular wavefunctions. The program has been modified to implement a numerical differentiation technique to evaluate the radial couplings. We discuss the usefulness of a one-electron picture to explain the mean features of the electronic-energies correlation diagrams. Electron translation factors have been included in the calculation of the (strongly origin-dependent) dynamical couplings.

For pt. I see ibid., vol. 27, p. 5011-26 (1994). Charge transfer cross sections for C²⁺+He collisions are calculated in the range of impact energies 1<or=E<or=40 keV employing a molecular expansion and an impact parameter formalism. The results are compared with available experimental data and the mechanisms of the charge transfer processes are discussed.

Charge-exchange collisions of 5 keV N₂⁺ on Cs are examined using translational spectroscopy to identify the internal energy states of the fast neutralized products. Optical pumping of the reactant, N₂⁺(A from X) or Cs(²P from ²S), is used to change selectively the electronic configurations and the reaction energy defect, while monitoring the resulting changes in the product state distribution. The N₂⁺/Cs charge exchange yields N₂ Rydberg states. A simple model of their formation would involve the transfer of the single outer electron in Cs into a Rydberg orbit built on the N₂⁺ core. However, the present studies identify a surprising and highly efficient reaction mechanism in which the electronic configuration of the N₂⁺ reactant is also changed during the electron transfer. This facile two-electron process, which occurs for both near-resonant and non-resonant reaction channels acid has not been observed in other molecular charge-transfer reactions, is believed to be related to the very small energy separation between the ground and first excited electronic states of N₂⁺.

Total cross sections (TCSs) for 0.7-400 eV positrons and 0.8-400 eV electrons colliding with hydrogen chloride (HCl) have been measured using a linear transmission method. The data were corrected for the effects of forward scattering using the differential cross section (DCS). The magnitude of the correction was very large in the low energy region. The method of the correction is given in detail. The TCS curve of electrons has a small hump near 3 eV. The TCS for positrons is somewhat lower than that for electrons in the low energy region. In the intermediate energy region, the values of the TCSs for positrons and electrons approach each other.

The hyperspherical close-coupling method is applied to calculate both the elastic and positronium formation cross sections for positron collisions with atomic hydrogen at low energies. By treating the hyperradius as a slow variable, the Schrodinger equation in the body frame at each fixed hyperradius is solved using the higher-order finite-element method and the resulting hyperradial equations are solved using the diabatic-by-sector method. The coupled hyperradial equations are integrated to a large hyperradius where the wavefunctions are matched to the known asymptotic solutions to extract the scattering matrix. Both the elastic and the positronium formation cross sections are calculated at energies below the H(n=2) excitation threshold for J=0, 1, 2 and 3. It is shown that the present hyperspherical close coupling method gives results comparable to those obtained by elaborate variational methods.

The total cross sections (TCSs) of positrons scattered from He, Ne and Ar atoms below 100 eV have been satisfactorily analysed from the partial cross sections. For He, the sum of the partial cross sections coincides with the TCS within a few per cent below 45 eV and 5% above this energy, though the sum has around 11% error. However, for Ne, the sum is in good agreement with the TCS above 30 eV. For Ar, the sum coincides well with the res below 30 eV, above this energy the difference of the sum from the res increases with increasing energy. A simple analysis of the elastic cross section curve for He was attempted, employing the data used in the partitioning. The presence of cusp in the Q_el curve is not clear just above the threshold of positronium formation.

The stimulated hyper-Raman scattering (SHRS) emissions associated with the excitation of the 4P_3/2 state in sodium vapour have been observed in both the forward and backward directions as the pumping laser was tuned near the 3S-4D two-photon resonance. The results show that though the forward SHRS process is greatly reduced by a destructive interference between it and the four-wave mixing (FWM) process induced by itself, the forward SHRS emission has a positive gain and can still be observed under a certain condition.

The characteristics and properties of a resonant nearly degenerate four-wave mixing signal are studied in the frequency space defined by the pump- and probe-field frequencies. Simultaneous effects of the intense pump field (perturbative treatment at all orders in the amplitude of the total field), chemical concentration of the solution, inhomogeneous distribution of resonance frequencies, and propagation of the electromagnetic fields are studied. Discussion and interpretation of the results are based on different characteristics of the non-linear signal within the absorptive and saturative regime, and on the behaviour in the frequency space of the component of the propagation-vector mismatch. Results obtained in this model are compared to previous spectral homogeneous-line results and to local cases.

High harmonic generation has been investigated in the atomic gases Ar and Xe as well as in the diatomic molecular gases H₂, D₂, N₂ and O₂ using a linearly polarized picosecond dye laser (594 nm) and a linearly polarized subpicosecond Ti:sapphire/Nd:glass laser (1053 nm). The harmonic conversion efficiencies are compared by choosing different gas pairs as a function of three parameters: (1) field-free ionization potential (2) mass and (3) static average polarizability. It is shown that the ionization potential and mass are not sensitive parameters for harmonic efficiency, but the polarizability is an important factor.