Table of contents

Variational cellular method (VCM) calculations for F₂ have been carried out at several internuclear distances. The ground and excited state potential curves are presented. The overall agreement between the VCM results and ab initio calculations is fairly good.

The authors draw attention to the fact that the work presented by Aydinol (1981) was the result of a collaboration between the authors and Aydinol, and that the paper was submitted for publication without the authors knowledge. The same work has since been presented, with Aydinol as a co-author, in Hippler et al. (1981). The data published by Hippler et al. (1981) differ from those given by Aydinol (1981) in that they include an important correction (of 5 to 20%) to the absolute subshell cross sections. This correction is due to the change in the detector efficiency in the energy range 4.1 to 5.4 keV.

A new type of global operator to be used in evaluating matrix elements of two-particle delta functions is introduced. It is based, like the Trivedi (1980) one-particle operator, on the Poisson equation and is easier to apply than the method of Hiller et al. (1978). After a test in the helium isoelectronic sequence, the new method is applied successfully to the interesting problem of hyperfine structure in muonic helium.

An algorithm for calculating atomic properties, based on the topological theory of molecular structure, is described and applied. For any given molecular system atoms are rigorously defined in terms of the topological properties of the system's charge distribution rho (x) in three-dimensional space. The essential feature of this distribution is that its only local maxima, the attractors of the associated gradient vector field grad rho (x), occur at the positions of the nuclei. The region of space occupied by an atom, the atomic basin, is the space traversed by all gradient paths of grad rho (x) which terminate at its nucleus. This property is used to construct a coordinate transformation which maps each atomic basin onto three-dimensional space. The Jacobian J, of this transformation depends solely on the second derivatives of rho (x). The determinant of J is obtained by solving a first-order differential equation governed by grad² rho (x). Any atomic property may then be calculated by an integration of an associated single-particle density over the basin of the atom. Numerical integrations of some atomic properties of diatomic and polyatomic molecules are reported.

Presents a solution of the Thomas-Fermi-Dirac-Weizsacker equation with three-centre boundary conditions for the molecules H₂O and SO₂. The solution is compared with previous results using the simple Thomas-Fermi model and with LCAO/MO calculations.

The ground-state energy of a hydrogen atom in magnetic fields up to 10¹⁰ G has been calculated accurately by a combination of Rayleigh Schrodinger perturbation theory and a simple variational procedure. The wavefunctions are expressed conveniently in spherical polar coordinates and contain only two variational parameters. A similar calculation for the low-lying excited states is successful only up to much lower fields (about 10⁸ G), but can be improved in a straightforward fashion by including higher order perturbation corrections.

Measurements of the even-even isotope shifts in seven Sm spectral lines made using tunable dye laser heterodyne spectroscopy are presented. Anomalies in the isotope shifts in the level at 19 192 cm^-1 in the Sm I spectrum reported previously have been confirmed to within experimental uncertainty by data from the line at 564.75 nm which couples to that level. Results from the line at 565.28 nm show equal and opposite deviations, from straight lines fitted through King plots, to the anomalies produced by the data from the 19 192 cm^-1 level. This has been interpreted as demonstrating that the upper level of the 565.28 nm transition, which is at 19 174 cm^-1, is mixed with that at 19 192 cm^-1. An explanation of the mixing in terms of second-order perturbations due to the isotope shift operators is presented. Other closely spaced pairs of levels do not show anomalies to within present experimental uncertainty.

In a recent study of the ejected-electron spectrum of zinc vapour following excitation by electron impact (see Back et al. (1981)), a number of lines were reported which could not be attributed to the autoionisation of known Zn I levels, viz. levels which had been excited in photoabsorption experiments. Most of the new lines are identified by comparison with ab initio Zn I calculations by the author. In the course of the analysis some anomalies encountered in previous comparisons between theory and photoabsorption measurements are explained. In particular the energy adopted for the Zn I 4p5s ¹P₁ level is changed significantly. Four Zn I configurations, 3d⁹4s²4p, 3d¹⁰4p5s, 3d¹⁰4p5p and 3d¹⁰4p4d are analysed in detail.

Dense barium vapour (>10²⁰ m^-3) produced in a heat pipe is irradiated by a laser light pulse (10⁷ W m^-2, 1.5 mu s) tuned to the resonant transition at lambda =553.5 nm. The transient population densities of various Ba I and Ba II states are measured, time resolved (<10 ns) by means of the hook method. As one result the strong ionisation, as described by other authors, is observed in its temporal development. For a quantitative interpretation a rate equation model is developed including a simplified treatment of radiative transfer. Most of the observed phenomena are described sufficiently by the authors calculations. The ionisation, especially, is simulated satisfactorily, showing that two-photon ionisation is the dominant process.

The analysis of the first observed singlet transition ( Sigma - Sigma ) of Si₂ is reported. The lower state of this transition is identified with the lowest ¹ Sigma _g⁺ state derived from the ( pi _u3p)⁴ configuration. New bands are also reported for the two H ³ Sigma _u^--X ³ Sigma _g^- and the K ³ Sigma _u^--X ³ Sigma _g^- transitions.

Compares the experimental values for shifts of some rotational transitions of OCS and CH₃CCH (propyne) measured by Luijendijk (1977) with the calculated values obtained from the Anderson-Tsao-Curnutti-Frost theory and the modified Murphy-Boggs theory as formulated by Cattani. The authors review the broadening and shifting of the J=0 to 1 transitions of the methylhalogenides CH₃Cl, CH₃Br and CH₃I. In particular, they conclude that the theories mentioned predict the right temperature dependence of the shift parameters for these transitions in the temperature region investigated (150-350K). Finally, they give a list of all shift measurements on rotation and inversion lines as far as they are known to us.

Proposes a single translation factor multiplying a truncated expansion on molecular orbitals to describe the collision of a slow charge with a one-electron atom in the impact parameter approximation. Proposing a quadratic polynomial dependence on electron coordinates, the translation factor reproduces the asymptotic flux of the electron density and it also generates the translation velocity of the electron associated with the state of motions of the inertial observer; transition probabilities are therefore Galilean invariant. The coefficients of the polynomial are determined by a variational procedure, which determines the field of velocities produced by the phase factor adjusted to describe both the density flux inside each state of the expansion due to its time dependence, and the flux associated with transitions between states.

A crossed-beam technique in which the products from ionising collisions are identified by time-of-flight spectroscopy and counted using a coincidence technique has been used to investigate the ionisation process X^q++H(1s) to X^q++H⁺+e. Cross sections have been determined for C^q+ with 2<or=q<or=4, N^q+ and O^q+ with 2<or=q<or=5 at energies within the range 12-195 keV amu^-1. Cross sections for different ions with the same q are found to agree closely both in magnitude and energy dependence. At high velocities, where cross sections have decreased below their peak values, cross sections scale according to q^1.46 and are in reasonable accord with theoretical values based on the classical-trajectory Monte-Carlo (CTMC) method. The results are also considered in relation to corresponding cross sections for electron capture and total cross sections for electron removal from H predicted by the CTMC method.

L-X-ray cross sections have been measured for several targets (Ag, Cd, Sn, Er, Yb, Hf, Au, Pb, Th and U) in the energy range 250<or=E_p<or=400 keV using a low-energy proton accelerator. By measuring the Rutherford backscattering spectra simultaneously with the X-ray spectra, the actual target thickness averaged over the beam spot area is obtained with high accuracy yielding overall errors in the cross section measurements as low as 10%. The results are compared with calculations in the plane-wave Born and the perturbed stationary state approximations with Coulomb deflection, binding and relativistic corrections included.

Monte-Carlo calculations have been carried out to determine how the termolecular ionic recombination coefficient is influenced by inelasticity in the ion-neutral collisions, specifically by the interchange of translational and rotational energy. The effect was found to be slight, indeed negligible for most purposes.

Examines the role of electronic relaxation in the description of K-K charge transfer for ion-atom collisions with bare or one-electron projectiles in the intermediate velocity region. Total differential K-K transfer cross sections for the system F⁹⁺Si are calculated within the coupled-state impact parameter method, starting from a relaxed molecular Hamiltonian and from a frozen two-centre Hamiltonian. By comparing the results of the two molecular expansion calculations with each other and with those of the atomic orbital expansion method. The authors test the various assumptions about relaxation of passive as well as active electron orbitals in a single-electron molecular orbital picture. A comparison with experimental data favours the description with relaxation in neither active nor passive orbitals for collision energies above 400 keV amu^-1.

The polarisation potential for electron-atom scattering is investigated analytically in momentum space. Contributions from discrete and continuum excitations are treated in detail. The equivalent local polarisation potential V (P) is derived, and calculated for the case of hydrogen at several energies. Contributions to the polarisability, defined as -8 pi times the derivative of V(P) at P=0, and terms of range as long as r^-4 are zero unless the corresponding potential contains a singular integral, which is investigated in detail. A contrast is drawn with the adiabatic potential, which is local but different from the equivalent potential.

A two-centre, fixed nuclei, close-coupling approximation, including exchange and polarisation effects, is employed to calculate cross sections for the sum of pure elastic and rotational excitation processes in the scattering of electrons by molecular hydrogen. Calculations are performed for impact energies less than 60 eV and for this entire energy region good agreement is obtained with cross sections derived from total cross section measurements.

The validity of the unitarised Born (B II) approximation for representing the T-matrix elements for electron-polar-molecular collisions is investigated for a number of polar systems. For intermediate and high values of the angular momentum (usually l>or approximately=5), the B II results are in excellent agreement with those of a more elaborate close-coupling calculation. The method provides an efficient, economical means for calculating the higher angular momenta T-matrix elements that are needed in the completion formulae for polar-molecular cross sections. The methods have an advantage over such comparable schemes as the fixed-point-dipole technique in that contributions from potential moments above the dipole can be simply included.

The 1s to nd excitations in e^--H collisions are studied within the framework of the Glauber theory using a recently proposed method of reduction of the Glauber scattering amplitude. Calculations are performed in the incident electron energy range 15-400 eV for the state n=4, and for the asymptotic limit n to infinity and tabular results of properly scaled differential and integrated Glauber cross sections are presented. The Glauber results are compared with the existing theoretical predictions. A unified graphical representation of the integrated 1s to nd (n>or=4) Glauber cross sections above 50 eV normalised to the scaled asymptotic value at 100 eV presented.

The five-state R-matrix calculations of Berrington et al. (1975) on the excitation of helium by electrons have been revised and extended to higher electron impact energies. Inelastic scattering cross sections are presented for all transitions between the 1¹S, 2³S, 2¹S, 2³P and 2¹P states of helium, and elastic cross sections are presented for the n=2 states.

An optical-potential description is used to study the effects of post-collision interaction (PCI) following the near-threshold electron impact excitation of autoionising states in helium. After some drastic but intuitively plausible approximations to the exact expressions published by Nienhuis and Heideman (1976), a model is obtained which lends itself to practical calculations. Results have been obtained on: (i) angular momentum exchanges during PCI, (ii) angular distributions of ejected electrons and (iii) lineshapes of PCI structures in the excitation of Rydberg states. These results are in accordance with the intuitive picture of the PCI mechanism and are confirmed by the trend in the experimental observations.

Calculates the electron impact differential as well as the integral cross sections for the excitation of the lowest lying autoionising level in caesium (Cs) atomic system within a multi-channel T-matrix equation with an asymptotic Green's function approximation in the low bombarding energy region. The autoionising level has been assumed to arise from the inner-shell excitation of an electron to the valence shell and no account has been taken of the electron-electron couplings in the target atoms. The authors also calculated the alignment parameter ( lambda ) within the first Born and the asymptotic Green's function approximations for the Cs5p⁵6s²²P_3/2 autoionising state at incident energy 30 eV.

Investigates the dynamics of the transient, simulated hyper-Raman effect, subject to a number of simplifying assumptions. The density matrix and probability amplitude formalisms have both been utilised for this purpose. Soluble cases of this three-photon process, involving coherent, ultrashort-pulse propagation in a non-linear medium, have been analysed in detail. In particular the authors find for the system which is far from saturation, and which allows for laser pump depletion, a solution free from ringing. This is in marked contrast to its ordinary Raman counterpart.

The concentration of the nitrogen metastable molecule N₂(A ³ Sigma _u⁺) is determined in a pure nitrogen glow discharge as a function of the pressure (from 0.1 to 2 Torr) and of the discharge current (up to 30 mA cm^-2) using the optical absorption of the first positive system bands. The absorption maximum is of the order of 2% corresponding to a N₂(A ³ Sigma _u⁺, nu =0) concentration of about 1.4*10¹² cm^-3. Measurements of the concentration of nitrogen atoms, the electronic field are also reported. A kinetic model indicates that the N₂(A) molecules are created by electronic excitation from ground-state molecules and lost mainly by diffusion to the walls and collisions with N(⁴S) atoms.

Measurements are reported of population densities in the energy level systems of neutral and singly ionised titanium. The discharge under investigation was a high-current hollow cathode operated in argon or neon as a buffer gas. The measured kinetic and population temperatures of the neutral species differ by more than a factor of two. For the metal ions there is direct evidence of the effects of charge transfer and Penning collisions producing departures from a Boltzmann distribution. For the Ar-Ti hollow-cathode system inversion between the populations of the 5s and 5p configurations has been found.

The process of Auger de-excitation of slow metastable atoms, colliding with solid surfaces is considered within the Penning ionisation mechanism. The corresponding transition probability is obtained in closed analytical form. De-excitation probability calculations for the He⁺(n)+Mo(100) (n=2, 3, 4) and He(2³S)+Mo(100) colliding systems are performed and compared with the previous theoretical results.