Table of contents

A new Doppler-free optical-optical triple-resonance (DF-OOTR) spectroscopy is reported for the first time. This high resolution technique is simply carried out using one single mode laser like Doppler-free two-photon spectroscopy, and can reach arbitrary levels in the high-lying g-parity states like the optical-optical double-resonance technique in atomic or molecular systems. It is an effective but simple method for the systematic study of atomic or molecular structures and their intra or internal dynamics.

Experimental data from transmission studies of low-energy electron scattering from caesium atoms are compared with recent theoretical data from a Breit-Pauli R-matrix (close-coupling) calculation. There is strong evidence for the observation of the (6p²)³P₂^e resonance below the threshold for the first excited state, (6p)²P₁2/⁰, of the target atom. In a purely nonrelativistic framework, this resonance corresponds to a stable bound state of the negative caesium ion in the ionization continuum. Consequently, its decay is direct evidence for relativistic effects on the autoionization process.

Heisenberg's correspondence principle is applied to the matrix elements of the rotation operator; in this way, an approximation for the reduced rotation matrix elements d^J_M'M( theta ) in terms of Bessel functions is obtained. It is shown that two distinct approximate forms are necessary to give sufficient accuracy over the entire range 0 to pi of the angle theta if the approximation is to be of value. The two forms are most accurate for theta near 0 and pi respectively, deteriorating as 0= pi /2 is approached; however, they retain a surprising degree of accuracy over the full range, particularly when (M'-M) is small and J large, the case for which the exact expression is most complex. Taken in conjunction with the results of the previous papers in this series, the present work allows Bessel function approximations to be obtained for both Clebsch-Gordan and Racah coefficients; indications as to the likely accuracy of such approximations are given.

A classical description of the two-electron atom, analogous to the quantum adiabatic hyperspherical channel approach, is presented. The classical problems, analogous to the quantum eigenvalue problem for the grand angular momentum operator, and the separated dynamical systems defined by each of the other constants of the motion of the non-interacting system are solved by using the Hamilton-Jacobi method. Some matrix elements of the Coulomb interaction terms of the Hamiltonian for doubly excited helium atoms using the Heisenberg correspondence principle are calculated.

Using calculations of collisional excitation rates in He-like argon (Ar XVII) and intensity factors and other data for nearby dielectronic satellites, X-ray line spectra have been synthesized as a function of electron temperature and density. The Ar XVII excitation rates were obtained from interpolations of R-matrix calculations, and are likely to be the best available at present. The resulting spectra are compared with spectra observed from high-temperature plasmas in the Alcator C tokamak. The agreement between observed spectra and those calculated using measured values of temperature and density illustrates the accuracy of the calculations, though there are small discrepancies which are likely to be connected with wavelengths or intensity factors for the Ar XVI satellites. Thus, argon X-ray line spectra can serve as a means of diagnosing low-density, astrophysical (in particular solar-flare) plasmas for which there is no independent means of measuring temperatures.

An improved method for the angular momentum measurement of autoionizing levels of complex atoms based on photoionization polarization spectroscopy is reported. In this method, a variation of the ion yield is measured as a function of the combination of the polarizations of the lasers used to excite the autoionizing level through a ladder of intermediate resonant states. Here, eight combinations of different polarizations in three lasers are used, which include not only circular but also linear polarizations. It has been confirmed that the angular momentum of the autoionizing level can be determined uniquely by relative ion-yield measurements for any combination of angular momenta of the levels of the excitation ladders by choosing appropriate intensities of the lasers.

The electron-emission spectra of atomic Eu and Sm after resonant 4d to 4f excitation by synchrotron radiation have been measured. Energy levels have been calculated using a multiconfiguration Dirac-Fock code. Strong enhancement of the resonant structures has been found at the resonant excitations below the giant resonance. The present vapour-phase results are in good agreement with the corresponding solid-state results, especially for Eu.

Lifetimes and excitation behaviour of a number of core excited quartet levels in neutral Na, produced by beam foil excitation, have been established. The present results have helped to resolve the discrepancy seen in the published lifetime values of the 3s4s ⁴P_5/2 and 3s3d ⁴P_5/2 levels. Further, a new line having a wavelength of 4343 AA displaying similar excitation behaviour to the known transitions between the quartet states has been observed. Its lifetime is measured to be 0.91+or-0.13 ns. It is suggested that this line is most probably due to a transition between the 3s(¹P)3p ^2/2P_3/2 and 3s²(¹S) ²P_3/2 states.

The spin polarization parameters A and xi , the cross section sigma and the angular distribution parameter beta in outer shell photoemission from HBr were measured in the resonance region of electronic autoionization between the HBr⁺ X ² Pi and HBr⁺ A ² Sigma ⁺ thresholds using photoelectron spectroscopy with circularly polarized synchrotron radiation. In this region the photoelectron spectra display pronounced vibrational excitation of the X ² Pi final ionic states. The energy dependence of the dynamical parameters is strongly influenced by electronic autoionization and predissociation of vibrational progressions of Rydberg states. The data for sigma and A are combined to obtain cross sections for the outgoing partial waves. A comparison of the experimental results with ab initio MQDT calculations of Lefebvre-Brion(1989) indicates that predissociation is more important than theoretically predicted.

A systematic study of the doubly differential cross sections for the electron emission occurring in fast strongly asymmetric collisions is presented. Protons and atomic hydrogen (0.5 MeV) were collided with the heavy targets krypton and xenon under single collision conditions. The ejected electrons were observed at laboratory angles in the range 0 degrees <or= theta _f<or=180 degrees and with energies E_f from 25 to 1400 eV. In addition to the familiar strong ejection in the forward direction ( theta _f approximately=0 degrees ) arising from capture and loss to continuum states there is pronounced emission in the backward hemisphere for the case of atomic hydrogen projectiles. This is accompanied by large variations in the angular dependent singly differential cross section and in the energy and width of the electron loss peak as measured at particular angles. These fluctuations in the ability of a heavy target to ionize the structured projectile are a manifestation of a Ramsauer-Townsend effect and can be understood within the framework of quasi-free electron scattering by the target potential. We give a relativistic quantum mechanical model based on the electron impact approximation which shows good agreement with experiment for these strongly asymmetric collision systems.

Relations are derived for the photon-scattered particle angular correlation in the case of collisional p-state excitation from an initial p-state. The treatment is aimed at optimization and analysis of the charge exchange experiment H⁺+Na(3p_0.+or-1) to H(2p_0.+or-1)+Na⁺. The spin polarization of the active electron in case of optical pumping with circularly polarized laser light is taken into account.

Electron-redistribution processes in collisions of He²⁺ ions on H₂ are studied for energies from 1 to 25 keV amu^-1. One-electron capture and target excitation cross sections are determined by photon-emission spectroscopy. At energies exceeding approximately 5 keV amu^-1 capture into excited states is the dominant charge-transfer process while at lower energies one-electron capture into the ground state dominates. The latter process is found to be accompanied by dissociation of the target and excitation of the resulting hydrogen atom. This can be explained qualitatively with the classical overbarrier model under the assumption that during the collision binding energy is exchanged between the electrons, which is a kind of auto-excitation process.

We have extended our previous work on electron-capture total cross sections for the Li³⁺+Li(2s) system by considering a larger molecular basis and taking into account the electron-translation effects. In the present paper we also include differential cross section results, computed in the energy region of the total cross section maximum. The modifications considered do not introduce major changes in the general trend of the total cross section, but its magnitude is enhanced by about 10%. These results are in general good agreement with those obtained by Kumar et al. (1991) except for the oscillatory structure taking place in the lower part of the energy range considered here. The origin of these oscillations and their effects on the differential cross section are discussed.

The uniform semiclassical approximation of Crothers (1986) is used to evaluate a final-state wavefunction for the electron-impact threshold ionization of helium. And using an explicitly correlated initial bound-state wavefunction for helium, absolute singlet triple-differential cross sections are calculated. These results are compared favourably with the original results of Crothers, and with experiment (with Selles et al., 1987) for excess-of-threshold energy E=1 eV with Rosel et al for E=2 eV.

The electron impact ionization of CO₂ molecules has been studied in order to determine the dissociative paths leading to charged fragments. The electron impact energy (EIE) ranged in the interval 40-130 eV. Time-of-flight spectroscopy was used to obtain the kinetic-energy distribution (KED) extending down to thermal kinetic-energy fragments. We have evaluated the contribution of indirect processes leading to charged fragments through CO₂ autoionization. The threshold for C²⁺ production from CO₂ has been measured and a possible production channel is analysed.

Total electron impact ionization cross sections for CH₄, C₂H₄, C₂H₆, C₃H₆ (cyclo-propane and propene) and C₃H₈ were determined in the energy range from threshold to 3 keV. Results are compared with the available data. The measured cross sections are found to correlate strongly to the total electron number and the molecular dipole polarizability. In connection with the total number of molecular electrons and the molecular dipole polarizability, two scaling methods for the total ionization cross sections are presented. The isomer effect is also discussed.

The complete valence-shell electron separation-energy spectra and momentum distributions are measured for 1,2-propadiene by high momentum-resolution electron-momentum spectroscopy at a total energy of 1500 eV. Many-body calculations of the separation energies and spectroscopic factors using Green function methods are carried out and, for the binding energies, compared with the experimental data. The measured momentum distributions are compared with those calculated in the plane-wave impulse approximation (PWIA) formalism using two respective SCF orbital wavefunctions, the second of which includes d-functions in the basis set. The agreement between the measured momentum distributions and the PWIA-SCF orbital-momentum distributions is, in general, fair, although for the outermost orbitals the SCF wavefunctions underestimate the density at low momentum. The inner valence 1b₂ and 1a₁ orbitals are found to be severely split by final-state correlation effects.

A numerical investigation of electron potential scattering in the presence of a high-frequency, high-intensity electromagnetic field has been carried out using classical mechanics. A Yukawa potential has been considered to simulate the electron-atom interaction. The high-frequency approximation, in which the electron moves in the so-called dressed potential has been employed. Both scattering angles and energy transfer from the field have been calculated. Strong fields lead to marked reductions in the scattering angle. Except at high frequencies large energy transfers are obtained, in the approximation that the energy transfer can be evaluated using trajectories in the dressed potential. Preliminary calculations on the validity of the high-frequency approximation show that it yields, within about 30%, the maximum energy transferred as the field phase is altered, but is poor for the detailed variation with phase.