Table of contents

Within the first-order perturbation theory the influence of the atomic core on the Stark structure of alkali-metal atom Rydberg states is investigated. A simple formula is derived which connects the energy separations at the level quasi-crossings with the parabolic quantum numbers and the quantum defects of the unperturbed atom.

Some properties of symmetrical collisions observed in the decay of the helium autoionising states by electron emission are reported. They have been seen in the He⁺ on He and the He collisional systems within the 7 to 140 keV energy range. Comparison of the excitation processes for the fast and slow atoms is made through the measurement of ejected-electron line shape and angular distributions for various autoionising states. At low energy the symmetry of the two systems is well verified; discrepancies from the symmetry requirements are observed in the angular distributions for the neutral-neutral system at high collision energy.

In the first Born approximation the ionisation probabilities of the ns states (n=1 and 2) of hydrogen-like ions by nuclei corresponding to the transitions with the different changes in the orbital angular momentum projection of the relative motion h(cross) mu have been calculated at a fixed impact parameter rho . It has been shown within the 5% accuracy that the total ionisation probability and cross section are determined by the transitions with mod mu mod <or=2 n.

The continuum distorted-wave approximation (CDW) is used to calculate total cross sections sigma _i^CDW for ionisation in fast H⁺-He⁺ and He²⁺-H collisions. The former results are added to those for electron capture from He⁺ by fast protons and the total cross section sigma ^CDW(He²⁺) for alpha -particle production is compared with the corresponding experimental data of Angel et al. (1978). Surprisingly, a lack of agreement is noticed at high energies where the method is the most adequate. Similar discrepancies between theory and experiment have previously been found in the first Born, binary encounter and classical trajectory Monte Carlo approximations. To see how sigma _i^CDW varies with the projectile charge Z, the ratio of cross sections for ionisation in He²⁺-H and H⁺-H collision is calculated. At incident energies below about 100 keV amu^-1, a significant departure is observed from the Z² dependence of the cross section predicted by the simple first Born approximation.

It is pointed out that all higher order odd Born amplitudes calculated in the closure approximation vanish in the forward direction for elastic scattering of structureless charged particles by hydrogen atoms. The consequences of this result on elastic scattering of charged particles by complex atoms and on other approximate theories are also discussed.

Elastic e^+or--atom scattering phaseshifts eta can be analysed within the context of the modified effective range theory (MERT). A scheme for the analysis of the data is developed which should be good for low energies, perhaps up to the first inelastic threshold, even if the phaseshift varies through a range of a few pi , so that tan eta or cot eta has a number of poles; the previous formulation of the MERT is not then applicable. Having extracted the effects of the long-range potential, one uses the fact that cot eta _c ( eta _c is the phaseshift associated with a truncated potential) is a meromorphic function of k², where k is the wavenumber. The method is applied to the e⁺-He s-wave phaseshift.

The electron impact excitation of the 1s to 2s transition in He⁺ is studied in the energy range 40-102 eV using the algebraic variational method with a small pseudostate basis. Results are compared with experiment and with other calculations.

The variational method which previously has been presented for single-channel scattering is discussed for the case of multi-channel scattering. The technique is illustrated by a calculation of scattering parameters for a two-channel square well problem.

For pt.II see ibid., vol.10, no.6, p.1015 (1977). Mean static dipole polarisabilities are calculated for the ground states of neutral atoms from Al to Cl and for singly ionised Si⁺ to Ar⁺. The polarisability is expressed in terms of the first-order correction in the wavefunction (due to the static electric dipole perturbation) which is determined variationally as a configuration interaction expansion. Near-degeneracy effects are included. The results are comparable with those obtained from the coupled Hartree-Fock scheme. The first-order wavefunctions may be used as target pseudostates in electron-atom scattering.

Spectroscopic intensity measurements were carried out using an atmospheric pressure wall-stabilised arc operated in a mixture of argon and sulphur hexafluoride. The temperature (14500-17000K) was derived from Ar I and Ar II line intensity measurements in conjunction with the electron density (0.7-1.1*10¹⁷ cm^-3) obtained from the half widths of Stark broadened lines. Experimental F II/F I and S II/S I line transition probability ratios were determined. A new absolute scale for F I line transition probabilities based on published F II lifetime measurements is presented.

Alignment of the L₃ shell in argon has been measured for electron and proton impact in the region of electron threshold and compared with previous experimental data and theoretical calculations. The measured alignment at electron threshold is negative in the proton impact case. For electron impact, an increasingly positive alignment is found as the velocity is decreased. However, for electron impact velocities less than 20% above threshold, no alignment analysis was possible because of observed Auger line broadenings. By studying the broadening as a function of velocity above threshold and comparing electron- and proton-induced spectra for equal velocities, the authors have described the broadening as arising from a post-collision interaction between the ionised 2p and emitted Auger electrons.

The high resolution vapour phase M_4,5N_4,5N_4,5 Auger spectrum of indium has been measured from atomic indium and from its mono- and trichlorides. The atomic spectrum reveals fine structure not present in molecular or solid state spectra and has been analysed with the aid of optically known final state energy levels of the 4d⁸5p configuration. Relative intensities and energies of the lines calculated for closed-shell atoms in the ground state using the non-relativistic mixed coupling theory have been compared with experimental values obtained for molecular spectra. Good agreement has been found between experimental and calculated results. Comparison between the present atomic and earlier solid state Auger spectra of indium shows that the kinetic energies are 13.4+or-0.5 eV higher for the metal due to the extra-atomic relaxation.

Photodissociation cross sections of H₂⁺ by the electronic transition from the bound 1s sigma _g state to the continuum of the weakly bound 2p pi _u state have been calculated over the accessible range of wavelengths lambda lambda 500-1200 AA. The partial cross sections sigma _{nu j} obtained for all the vibrational levels ( nu =0-18) and the rotational level J=1 were summed over nu with an appropriate weighting factor rho _{nu J} to get the total cross section sigma . The cross section sigma for the 1s sigma _g to 2p pi _u transition was found to be much larger than that for the 1s sigma _g to 2p sigma _u transition over the range of wavelengths lambda lambda approximately 600-1000 AA. Special features of the sigma to pi transition are discussed.

The present photoionisation study for the N(2s²2p³²D⁰, ²P⁰) metastable states extends the calculation on the N(2s²2p³⁴S⁰) ground state by the same authors. In both works the initial bound state and the final continua are described by collisional R-matrix expansions over the parent valence states N⁺(2s²2p², 2s2p³), represented in turn by correlated wavefunctions. This model accounts consistently for the initial-state valence correlations and the final-state dynamical coupling between the 2p and 2s ejection channels. Amongst the autoionised Rydberg series which precede the opening of each dissociation channel, the calculation points out strong valence transitions N(2s²2p³²D⁰, ²P⁰) to N*(2s2p⁴²P^e) which modify considerably the oscillator strength density over a wide energy range ( approximately=1.15 to 1.45 Ryd). All relevant results of the calculation-total photoabsorption cross sections, quantitative analysis of the prominent resonances, partial photoejection cross sections and angular distributions-are presented in graphic or tabular form over a wide spectral range (0.9 to 3.0 Ryd, i.e. 1000 to 300 AA).

Isotope shift data of the visible (3500-6200 AA) and the ultraviolet (2100-2600 AA) bands of the molecules ²⁸Si³²S and ²⁸Si³⁴S confirm the vibrational assignments proposed by Thomas and Bredohl et al. (1947) and further show that both the groups of bands belong to the E¹ Sigma ⁺-X¹ Sigma ⁺ system of the SiS molecule. The vibrational assignments of the 2500-3900 AA bands of the D¹ Pi -X¹ Sigma ⁺ system are also confirmed from the present studies of the emission spectra of the isotopic molecules.

A molecular beam of PbS molecules was excited by a CW Ar⁺ laser in single and multimode operation. Fluorescence spectra induced by the 4880 AA line were recorded and four fluorescence series originating from the nu =7, 8 and 9 levels of the A0⁺ state were analysed. Using a pulsed dye laser, lifetimes of the nu =4, 5, 6, 7 and 8 level of the A0⁺ state, the nu =0, 1, 2 and 3 levels of the B1 state and the nu =21 level of the a1 state were measured. The electronic transition moments of the a-X, A-X and B-X transitions were calculated and the lifetimes of the a, A and B states of PbS are compared with those of PbO.

The expressions for the singular parts of the non-adiabatic matrix elements H_ss'(k,R) and Q_ss'(k,R) connecting the states mod ks) and mod ks') of the continuous spectrum of the two-centre problem are obtained. The numerical calculations are performed for the sets of quantum numbers q=0, 1, ..., 7 and m=0 in the interval of values k=0.2(0.1)1.0(0.2)2(1)10 and R=0.1(0.1)20. The asymptotic behaviour of the matrix elements is found as R to 0 and R to infinity .

The probability of forming Ag K vacancies in 60 MeV I collisions with centre-of-mass scattering angles theta _cm between 20 and 144 degrees was measured. The probability varies approximately as sin² theta _cm, as predicted for 2p sigma -2p pi rotational coupling but the magnitude of the probability is much higher than expected.

The impact broadening and shift of the sodium resonance lines by noble gases have been redetermined with improved instrumentation and data reduction allowing for asymmetries. An increase of width, shift and asymmetry with the heavy noble gases is noted, as is a D2 to D1 width ratio greater than one for helium and neon, but less than one for argon, krypton and xenon. Comparisons are drawn with different published calculations to show that the widths are accounted for by the Pascale-Vandeplanque potential whether with a fully quantal or a semiclassical impact broadening theory. On the other hand, the Nikitin-Roueff potential does not work well in this case. Available calculations of the shift are less satisfactory and no predictions of the observed asymmetries have been made.

Cross sections for electron capture by H⁺ and H⁰ and electron loss by H^- in Cs have been measured in the energy range 0.1-2.0 keV. The near-resonant electron capture cross section sigma ₊₀ is larger than 1.0*10^-14 cm² over the whole energy range investigated. The electron loss cross section sigma _-0 displays a rather weak energy dependence, changing by less than a factor of two in the energy range 0.2-2.0 keV. The capture cross section sigma _0- is the smallest in magnitude of the three cross sections measured and displays the strongest dependence on energy. Good agreement is found between the present results and previous measurements at higher energy. The measurements are compared with recent calculations of these reactions.

By measuring the angular distribution of the L_l/L_gamma intensity ratio of gold following proton (E=0.6-1.5 MeV) and He⁺ (E/M=0.2-1.1 MeV amu^-1) impact ionisation, the A₂ alignment parameter of the L₃ subshell has been determined. In contrast to the monotonic behaviour of A₂ predicted by the PWBA at low projectile velocities, the authors' experimental data for He⁺ impact exhibit a minimum at E/M approximately=0.4 MeV amu^-1 which can be explained by the deflection of the projectile in the Coulomb field of the target nucleus.

A classical model is proposed for the electron capture process in collisions of highly charged, fully stripped ions with atomic hydrogen at low and intermediate impact energies. The basic electron capture mechanism is supposed to consist of classically allowed, over-barrier transitions that occur at large internuclear separations of the colliding partners. The cross section calculations are performed for ionic charges Z=8, 10, 14, 18, 26 and 36 in the impact energy range E=0.5-100 keV amu^-1. Electron transition probabilities versus impact parameter are presented for the collisions of Ar¹⁸⁺ with H(1s). Comparison is made with other available theoretical data.

The relative ionisation cross sections sigma ₃/ sigma ₂ of tungsten ionised by electrons with energies between the L₂ and L₁ threshold values have been measured in present work. The experimental results were compared with the theoretical predictions.

Total cross sections have been measured for electron scattering on Ne, Ar, Kr and Xe in the impact energy range between 25 and 750 eV. The method used has been introduced by Blaauw et al. (1980) for the same measurement with He and N₂ as targets. It is a linearisation of the Ramsauer-type experiment, which enables good angular and energy resolution; the accuracy in the total cross sections obtained is better than 5%. Substantial differences are found with experimental data of other groups, which can often be attributed to shortcomings in their experimental set-up. The best agreement is obtained with the recent data of Kauppila, Stein and co-workers (1980). Above 100 eV our Ne and Ar data can be compared with theoretical calculations, of which the optical model results of Joachain et al. (1979) rapidly converge to the present ones. Good agreement is also found with the semi-empirical total cross sections of de Heer et al. (1979).

The molecular multichannel quantum defect theory, initially developed to describe the escape of an electron from the molecular core region, is extended to include dissociation of the molecule into two atoms. The mixing between the two types of channels results from electronic interaction between a Rydberg series and dissociative valence states of the neutral molecule. This interaction is introduced as an additional phaseshift into the continuum parts of the wavefunctions. The resulting unified treatment of (pre)ionisation and (pre)dissociation is applied to the process of dissociative recombination, following the approach of C.M. Lee (1973). The 'indirect' process proceeding via electron capture in vibrationally excited Rydberg states is taken into account fully by the inclusion of closed electron-ion channels in the calculation. Model calculations are presented and the influence of various physical parameters on the dissociative recombination profile is tested.

Charge exchange with neutral hydrogen is examined as a recombination mechanism for multicharged impurity ions present in high-temperature fusion plasmas. At sufficiently low electron densities, fluxes of atomic hydrogen produced by either the injection of neutral heating beams or the background of 'thermal' neutrals can yield an important or even dominant recombination process for such ions. Equilibrium results are given for selected impurity elements showing the altered ionisation balance and radiative cooling rate produced by the presence of various neutral populations. A notable result is that the stripping of impurities to relatively non-radiative ionisation states with increasing electron temperature can be postponed or entirely prevented by the application of intense neutral beam heating power. A time-dependent calculation modelling the behaviour of iron in recent PLT Tokamak high-power neutral beam heating experiments is also presented.