Table of contents

Variational and close-coupling methods are compared for the calculation of the 45.0 eV autoionizing resonance of Ne. Analysis of the variational result suggests the importance of the omitted 1s²2s²2p⁴(³P)3p(²D degrees ) d-wave channel for the close-coupling calculation. An argument is presented to justify the importance of this channel relative to the 1s²2s²2p⁴(³P)3p(²P degrees ) d-wave channel arising from the same configuration.

Beam-foil quantum-beat techniques have been used to measure the ³D₁-³D₂ intervals of 1snd (n=3-8) in ⁴He. I. A two-spectrometer technique was used as to obtain simultaneously a time calibration from the 1s3p(³P₁-³P₂) fine-structure separation. The following ³D₁-³D₂ separations (in MHz) were found: 1323.62.3 (n=3), 553.0+or-0.7 (n=4), 284.1+or-0.6 (n=5), 165.3+or-1.0 (n=6), 101.6+or-1.1 (n=7) and 69+or-3 (n=8). These results are compared with previous measurements and theoretical values.

First results on the scalar polarizability alpha ₀ of a number of very highly excited p states of Cs (40<or=n<or=60), produced by one-step photoexcitation with frequency-doubled dye-laser radiation, were obtained by means of electrostatic beam deflection in a two-wire type electric field. Enormous polarizabilities of alpha ₀/4 pi >10⁹ AA³ were measured; the dependence of the polarizability as a function of the principal quantum number n was found to be in agreement with the expected n⁷ behaviour.

The R-matrix theory of photoionization described by Burke and Taylor (1975) is used to obtain cross sections for the photoionization of ground-state carbon and oxygen atoms. The calculations are the first to couple systematically all channels corresponding to the 2s²2p¹, 2s2p^q+1 and 2p¹⁺² configurations of the residual ion. The results are in good agreement with experiment where comparison is possible.

The R-matrix theory of photoionization described by Burke and Taylor (1976) is used to obtain cross sections for the photoionization of ground-state nitrogen atoms. The calculations couple all channels corresponding to the 2s²2p² and 2s2p³ configurations of the residual ion. The results obtained are in excellent agreement with experiment.

The pronounced structures in energy spectra of autoionization electrons arising from near threshold electron impact on rare gases are shown to be due to interferences between contributions from different intermediate autoionizing states, and from the direct ionization process, to the observed electron intensity. By semiclassical calculations based on the 'post collision model' the authors reproduce existing electron spectra for He.

An apparatus has been constructed to observe various interactions between ions. In the present experiment, beams of H⁺ and H^- ions intersected at 8¹/₂ degrees and cross sections for mutual neutralization were measured for barycentric energies between 20 and 3000 eV. The earlier results of Gaily and Harrison (1970) and Moseley et al. (1970) were closely confirmed, but the greater accuracy of the new measurements made it possible to resolve maxima in the cross section curve around 32 and 54 eV. Two further maxima at 120 and 600 eV had previously been observed by Gaily and Harrison.

Beams of ions intersecting at an angle of 8¹/₂ degrees have been used in experiments to measure cross sections for the mutual neutralization of He⁺ and H^- ions for barycentric energies between 35 and 4550 eV. The results agree quite closely with those of a similar experiment by Gaily and Harrison (1970) which explored the energy range 200 to 8000 eV but the improved accuracy and lower energies attained in the present experiment made it possible to resolve maxima in the cross section curve at 90 and 150 eV, in addition to those found by Gaily and Harrison at 350, 980 and 1500 eV.

Radiation from the Daresbury Laboratory 5 GeV electron synchrotron NINA has been used to examine the vibrational structure of the photoelectron spectra of N₂ from 730-600 AA. Variations in branching ratios are observed due to autoionization processes in which transitions to the x ² Sigma _g⁺ state of the ion differ from straight photoionization over the region covered by the Hopfield bands.

Calculations are described which reproduce the well known Ramsauer-Townsend effect in the scattering of electrons by methane. The theoretical model used for the interaction of an electron with methane includes (i) the methane static potential, (ii) exchange effects which are approximated by orthogonalizing the incident wavefunction to bound wavefunctions of the same symmetry and (iii) a polarization potential which is chosen to give good agreement between theoretical and experimental total cross sections.

Quantum-electrodynamic perturbation theory is used to investigate the relativistic energy of a many-electron atom. The corresponding formulae are reduced to a quite convenient form in which only radial integrals and phase multipliers depend on 1s²2sp² and 1s²2p³. They are in fairly good agreement with experimental data.

By studying the response of a two-level system to a narrow-band light excitation completely modulated in amplitude, a new manifestation of the dynamical Stark effect is predicted. It can be observed not only through the modulation rate or the phaseshift of the scattered light but also through the mean total intensity. Thus its experimental observation should be particularly easy.

The model of Armstrong and Feneuille (1975) for phaseshift measurements of atomic lifetimes using monochromatic excitation is solved for the case where the laser frequency is not necessarily on resonance with the atomic transition. It is found that even quite small detunings (of the order of those which may be difficult to avoid in practice) produce significant changes in the phaseshifts and strength of the modulated component.

The strong field-induced quantum beats observed in beam-foil studies of Ly- alpha radiation are obtained in a conventional metastable-hydrogen quenching experiment. The phase relation between the Stark shifted 2s_1/2-2p_1/2 Lamb-shift oscillations and the much more rapid 2s₁-2p_3/2 fine-structure oscillations depends on the detailed way in which the quenching field is switched on. Apart from a phaseshift, the results agree with a non-perturbative theoretical calculation which assumes that the field is applied suddenly. Various frequency components of the time-dependent radiation intensity are identified with specific hyperfine transitions or groups of transitions. No adjustable parameters are used for the initial state amplitudes.

Describes an experiment which allows the study to be made of the relaxation of metastable atoms with non-thermal equilibrium velocities. This relaxation is mainly due to elastic collisions. The velocity perturbation is created by a short and intense layer pulse. The evolution of the perturbation is studied by absorption of delayed probe pulses provided by an optical delay line. The 5¹D₂ to 6¹P₁ transition of barium I is studied at 1.5 mu m. Pressure conditions are such that elastic collisions are mainly due to atoms of a carrier gas mixed with the barium vapour. Results obtained with krypton and xenon are presented. The specific time constant for the velocity relaxation is described in the frame of the strong collision model.

Measurements on the angular distributions of the 4d, 5s and 5p photoelectrons from xenon have been made using synchrotron radiation from a 5 GeV electron synchrotron. The importance of including intershell correlation effects is demonstrated by the results on the 5p electrons. The data indicate not only the limitations of the single-particle theories, but also, in the case of the 5p electrons beyond the 4d ionization threshold, the inadequacies of the present RPAE theory.

Differential cross sections for excitation of the n=2 states of atomic hydrogen by electron impact are calculated in a hybrid pseudostate close-coupling distorted-wave model at energies from 13.6 to 54.4 eV. The total (2s+2p) differential cross sections are in good agreement with absolute measurements. Elastic differential cross sections are given at 54.4 eV. Sets of parameters which give a complete description of the radiative decay of the excited 2p state and of the angular distribution of the scattered electron are presented for four energies with a view to simulating coincidence experiments.

The optical potential method of Feshback (1962) is applied to describe the effects of post-collision interaction in electron impact excitation of helium near the thresholds of autoionizing states. The resulting formalism is a generalization of the standard theory of resonances. The shake-down model of King et al. (1975) is obtained from an exact expression after several drastic but well defined approximations.

The doubly differential cross section for electron loss from the projectile in fast atomic collisions is obtained from a transformation to the laboratory frame of the corresponding Born cross section for ionization of electrons in the projectile frame. The ejected-electron velocity distribution is always centred close to the projectile speed but the character of the ejected electrons is different in forward and backward directions. In forward directions a cusp-shaped velocity distribution arises. In backward directions the description of the distribution as due to elastic scattering of the ejected electrons from the target nucleus is justified.

Charge-exchange processes in the H⁺Xe system have been investigated in the ranges 0.2< theta <3 deg., 60<or=E<or=600 eV. Time-of-flight analysis of the scattered neutrals, allows the identification of the lowest electron-transfer channels. Charge exchange into higher excited states is observed. The relative differential cross sections exhibit an oscillatory behaviour as a function of the product E theta . The interpretation is carried out in the quasi-molecular framework. Theoretical investigation of the H(1s)+Xe⁺(²P₃2,1/2/) channels involves the three lowest mod Omega =0⁺) states of the (XeH)⁺ molecule. Differential cross sections are calculated using the Landau-Stueckelberg approximation and the solutions of three quantal close-coupled equations. The structures in the differential cross sections are attributed to inelastic rainbow effects and to Stueckelberg-type interferences.

The observed positions of three resonant states of the two isotopes I⁷⁹Br and I⁸¹Br are used to specify more precisely the vibrational quantum numbers of states involved in predissociation of IBr. The value nu =2 given by Child et al. (1973) is now found to be five.

Two triplet-triplet transitions having a common state are observed in the emission spectrum of the P₂ radical and are photographed under high dispersion. These are the c³ Pi _u-b³ Pi _g visible system and the b³ Pi _g-a³ Sigma _u⁺ IR system. The Franck-Condon factors are used to fix the vibrational numbering: a modification of the previous vibrational numbering of the b³ Pi _g state is carried out. The omega _e and omega _e chi _e constants of the three states are calculated. Rotational analysis of the three sub-bands (4,1) of the b³ Pi _g-a³ Sigma _u⁺ transition is achieved. The rotational and spin-doubling constants of the nu "-1 level are calculated. The nu '=4 level is studied by treating such sub-band separately. The effective Lambda doubling and rotational constants are determined. A more complete treatment using an iterative method is carried out and the fine structure of the b³ Pi _g nu '=4 level is interpreted satisfactorily.

The study of 'anomalous' fluorescence series in the spectra of laser-excited ⁸⁰Se₂ molecules has given a new value of the Omega -doubling constant lambda " in the X state. The deduced value of lambda ' in the B state is further confirmed by the values of the Lande factors in this state. The results are 2 lambda "=+(512.9+or-2) cm^-1 2 lambda '=+(78.4+or-2) cm^-1.

Some twenty series excited in gaseous ⁸⁰Se₂ by lines of the Ar⁺ laser have been photographed at moderate resolution. They arise from transitions B0_u⁺-X0_g⁺ B1_u-X1_g and from cross transitions B1_u-XO_g⁺ and B0_u⁺-X1_g. Measurements in the range 6< nu "<12 and 58<J<202 lead to Delta nu ₁₀(X1_g-0_g⁺)=509.5+2.125₆( nu +¹/₂) cm^-1 and Delta nu ₁₀(B1_u-B0_u⁺) becomes 75.45 cm^-1. Some consequences of the new values are briefly discussed.

An apparatus is described which is suitable for studying dissociative excitation of molecules into metastable fragments and results are presented for electron impact on CO₂. Both Rydberg and non-Rydberg contributions have been observed in the resultant fragments. Among the processes which are probably being observed are those which yield the following fragments, CO(X¹ Sigma _g⁺)+O(HR), CO(a³ Pi )+O(⁵S), CO(a³ Pi )+O(HR) and C(³P)+O(⁵S)+O(HR). The results are compared with those of other workers.

Phaseshifts and total cross sections for the elastic scattering of low-energy (0-13.6 eV) electrons by molecular fluorine are presented. The phaseshifts are obtained by an approximate technique based on the weak asymptotic coupling of orbital angular momenta and are calculated solely from the results of a discrete basis set diagonalization of the molecular Hamiltonian. Correlation and polarization effects are not treated. The elastic cross section is dominated by a Sigma _u⁺ shape resonance at about 2.2 eV in the static-exchange model.

The vibrationally excited populations of ground-state N₂ in a flowing afterglow have been determined from energy-loss spectra in the energy-loss region 7.0-10.5 eV. The data of White et al. (1976) for energy-loss spectra in the energy-loss region 10.5-12.5 eV have been re-analysed and confirm a non-Boltzmann population distribution of the vibrational levels. An electron energy-loss spectrum of the afterglow is also presented for the electron energy-loss range 16.5-18.5 eV.

A spectroscopic determination of the relative populations of the vibrational levels of the nitrogen C³ Pi _u state has been performed using a reflex discharge device. The populations have a nearly Boltzmann distribution. A comparison of the relative populations, calculated by means of Franck-Condon factors, with the experimentally measured ones shows the relative roles of the populating processes considered; electron collisional excitation of the molecule in its electronic ground state is by far the most important process. The population and vibrational temperature variation with discharge current as well as the population variation with pressure are given; except in the vicinity of the extinction induction value, no evidence was found of a dependence of the population or vibrational temperature on the magnetic field.