Table of contents

The absorption spectrum of gaseous barium was obtained between 120 AA and 140 AA, the region of the N_IV,V thresholds. The similarity to lanthanum of some of the features observed in barium suggests that the 4f orbital contracts and overlaps the 4d orbit. This contraction produces two terms, ³P and ³D, of the 4d⁹4f configuration below the 4d ionization limits, while the large electrostatic exchange interaction drives the ¹P term of this configuration some 10 eV above the limit. Furthermore, extensive mixing of the 4d⁹(6s²)6p configuration with 4d⁹(5d²)6p and 4d⁹(5d6s)6p produces many weak resonances in addition to the resonances associated with the 4d⁹4f configuration. A suggested classification of these features is given with the aid of known features of the La I spectrum. Finally, from the known X-ray splitting of the N_IV,V threshold and the energy interval between the 6s²6p ²P and 6s²¹S levels in La I and La II, the ionization thresholds of the 4d electron were determined to be 814800 (1000) cm^-1 and 792500 (1000) cm^-1.

The excitation of the 4³S, 5³S, 6³S and 4³D states of helium by electron impact has been studied in the energy region around 60 eV. New experimental evidence has been obtained for the existence of an indirect excitation mechanism resulting from a post-collision interaction (PCI) between the scattered electron and the electron ejected from an intermediate autoionizing state. It is found, and made plausible, that the PCI mechanism via the excitation of autoionizing states primarily leads to the excitation of S-states.

From the calculations of electron scattering by the helium atom, the authors present a rule showing how to choose an arbitrary basis set of Slater-type orbitals for the scattering electron such that the R-matrix becomes convergent.

The authors develop a theoretical analysis which describes coherent nonlinear resonant effects where two optical pulses of different frequencies undergo two-photon self-induced transparency (two-photon SIT) in an inhomogeneously broadened three-level system. The field (Maxwell) and atomic (Schrodinger; density matrix representation) equations are coupled in a self-consistent manner by virtue of the resultant macroscopic polarizations at the two frequencies. This treatment is accomplished under circumstances where atomic coherence plays a predominant role. The resulting equations yield a new two-photon area theorem which gives first the intensity dependent, two-photon absorption law analogous to Beer's law for the one-photon transition. Secondly, a 2 pi Lorentzian pulse solution is derived which propagates without loss at the modified pulse velocity.

For pt.I see ibid., vol.8, no.3, 339 (1975). The transient behaviour of two short optical pulses in two-photon coherent propagation is obtained by calculating numerically the coherent propagation equations which couple the two electromagnetic pulses and the atomic system in self-consistent fashion. The coherent interaction of the two different frequency pulses gives rise to lossless propagation in the absorptive medium. This description is characterized by a two-photon area theorem. Novel coherent propagation effects of peak amplification and multiple pulse breakup are found. These effects are ascribed to two-photon self-induced transparency analogous to that of a one-photon transition in a two-level system. A difference of input intensity between the two different frequency pulses yields curious modulation effects involving sharp peak amplification and marked pulse breakup for the low-intensity input. In contrast, the high-intensity input pulse displays a smooth variation with distance and time. Low-loss pulse transmission above threshold, and pulse advance and delay are found for the present case in contrast to that for one-photon self-induced transparency.

The free-free absorption coefficient of the negative hydrogen ion is calculated in both the length and velocity formulations using the method of polarized orbitals to describe the free electron. The bound-free cross section of Bell and Kingston (1967) is converted to an absorption coefficient and combined with the present free-free results. Comparison with other calculations and the use of certain oscillator strength sum rules suggests that the total H^- absorption coefficient values of the present paper are accurate to better than 10%.

The electron-photon delayed coincidence technique has been used to measure the lifetimes of five of the excited states in mercury, and the results obtained are presented. In all but one case this study constitutes the first determination of these lifetimes. The five levels, together with their measured lifetime, are as follows: 7¹S (31.0+or-0.5)ns, 6³D (10.8+or-0.2)ns, 8¹S (84.0+or-2.0)ns, 7¹D (38.5+or-0.5)ns, 10¹P (51.0+or-4.0)ns.

A modulated beam technique has been used to determine cross sections for one-electron loss sigma _1*2 and total cross sections sigma _D for collisional destruction of 18-50 keV ³He⁺ (2S) metastable ions in H₂, He, Ar, N₂ and O₂. Cross sections sigma _1*2 are shown to be roughly 100 times larger than the corresponding one-electron loss cross sections for ground state He⁺ ions. Total collisional destruction cross sections sigma _D, which are very large ( approximately 10^-15 cm²) and not greatly energy dependent, indicate that electron loss is not the dominant destruction mechanism in the present energy range. The results are shown to be consistent with the requirements of a recently proposed source of polarized ³He ions. Data relevant to the optimization of such a source are presented.

Helium negative ions were produced in collisions between ground state helium atoms and vapours of magnesium and lead in the energy range 20-40 keV. The results suggest the existence of a doublet state of He^-.

Consideration of the effect of target polarization is extended to inelastic collisions. The accuracy of the usual methods of obtaining polarization potentials by the implicit use of perturbation theory is found to be poor, as tested on hydrogen by the more satisfactory, if tedious, method of diagonalization of the energy matrix.

The angular distribution of electrons scattered elastically from helium atoms has been measured in the energy range from 2 to 19 eV. The experimental data have been fitted by the partial wave formula to determine the scattering phase shifts. From these, absolute values of the differential and the total cross sections were computed. The accuracy of the measured shape of the angular distributions is better than 5%, of the absolute calibration between 20% at low and 5% at high energies. The discrepancy between earlier experiments and theoretical predictions is removed in favour of theory.

Ionization of atomic hydrogen by electron impact is treated by the classical finite characteristic collision time approximation of Bates and Snyder (1973). The cross sections obtained by the displacement version are fairly accurate.

Measurements of the angular differential cross section for the elastic scattering of electrons in neon have been made for incident energies of 100 to 625 eV over an angular range 10 to 150 degrees. The observed relative differential cross sections for energies of 100 to 500 eV have been converted to an absolute scale by measuring differential scattering at each energy in 5% and 10% mixtures of neon in helium and using available cross section data for helium. Results are compared with other measurements and with recent calculations. Agreement with absolute measurements for small angles by Bromberg (1974) is excellent.

Excitation of the lowest triplet state (2³S) near threshold arises from interactions of short range, because of the electron-exchange needed to bring about the change of target spin. The short-range interaction is described by the simplest possible R-matrix which reproduces the ²S resonance observed about 0.5 eV below threshold (i.e. a simple pole term without a background). Only s-waves are considered. The observed singularity ('cusp') in the elastic differential cross section, and the observed nearly vertical onset of the inelastic cross section at threshold are accounted for if one includes the effect of polarization of the atom on a slow outgoing electron which has been scattered inelastically.

Electron loss from hydrogen atoms passing through atomic hydrogen is calculated in the Born approximation. Contribution of all open channels is taken into account by means of the closure approximation and corrections are introduced for the final states 1s, 2s and 2p of the target atom. It is shown that the procedure called 'corrected closure approximation' converges rapidly. Attention is drawn to the possibility of obtaining a close upper bound to the Born cross section. Excellent agreement with experimental data is obtained for impact energies beyond 50 keV, that is in the region where the Born approximation is expected to be successful.

A time-of-flight spectrometer is described and is shown to have sufficiently good energy resolution to allow a detailed investigation of energy losses for scattered neutral particles. This method has been applied to the study of the exchange channels to complement the previous ion studies. The He⁺-He results are discussed in the light of the quasi-molecular description of the collision mechanisms.

A theoretical investigation has been conducted to the exothermic transfer ionization reaction, He⁺+Mg to He+Mg²⁺+e^-. The mechanisms assumed is autoionization of the diatomic ion HeMg⁺. To compute the potential energy curves and autoionization width descriptive of this process, Hartree-Fock single-configuration electronic wave-functions have been constructed for the initial discrete state, He⁺-Mg, and for the final continuum state, He-Mg⁺²-e^-. As the internuclear separation varies from 3 to 5 bohr, the lifetime for autoionization grows from 8.75*10^-14 to 3.2*10^-13 s. The cross section for transfer ionization has been computed according to Miller's classical trajectory theory of Penning ionization. This cross section reaches a maximum value of 12.9 a₀² for a collision energy equal to 1 hartree. Because of the inadequacy of our approximate potential energy function for the initial state, the theoretical cross section the authors have computed here is believed to be unreliable for energies less than this value.

The authors report calculations of 'quasi-diabatic' potential energy curves and wave-functions which extend the earlier results of Gilbert and Wahl (1971) to both smaller internuclear separations and to several excited states. Calculations at small internuclear distance, 0<or=R<or=1 bohr, involving one and three-centre LCAO-SCF techniques provide an ab initio correlation diagram of the type previously estimated by Lichten (1967). Elastic differential cross sections at low and moderate energies are obtained from the computed potentials and effects of spin-orbit coupling and dynamical non-adiabatic interactions are discussed. In addition new differential measurements on the 4s and 4p excitation cross sections of Ar are presented and interpreted in terms of 'quasi-diabatic' crossings of potential energy curves and 'diabatic MO' crossings.

The vibrational excitation probabilities of H₂ in collinear collisions with He are calculated quantum mechanically, using the Fredholm integral method. Important differences are found when the accurate H₂ molecular potential of Kolos and Wolniewicz (1968) is used instead of the less accurate Morse potential used extensively in previous work. The approximation of truncating the infinite set of coupled equations describing the collision problems is tested, and an alternative closure approximation is suggested to provide an indication of the effect of truncation on the results.

The results of an experimental investigation of the electron density decay in a caesium afterglow (0.06<or=p_Cs<or=0.4 Torr, 10⁹<or=n_e<or=10¹¹ cm^-3) are presented and compared with previous experimental results. The authors show that the large recombination coefficients which are measured (up to several 10^-7 cm³ s^-1) cannot be interpreted as due to dissociative recombination of diatomic caesium ions but involve heavier ions such as Cs₃⁺.