Table of contents

Some doubly excited P-wave resonances in He converging on the N=3, 4, 5 and 6 hydrogenic thresholds are calculated by a method of complex coordinate rotations. Results are compared with recent experiments and theoretical calculations.

Selective field ionisation of helium n¹P Rydberg atoms with n approximately=41 has been investigated. The helium n¹P states are of particular interest because they have negative quantum defects and thus lie above the higher l levels of their respective manifolds. This leads to unusual field ionisation behaviour. Results are presented for He(41¹P) atoms and effects associated with diabatic passage through zero electric field are noted.

The allowed n=2-2 transitions in titanium ions with three- to nine-electron systems are prominent features in the extreme ultraviolet spectrum of the DITE Tokamak operated with titanium gettering. The appearance of the lines in high order and their proximity to other well documented impurity lines which are used as transfer standards of wavelength has allowed precision wavelength measurements of the Delta n=2-2 transitions. Accurate values of the energy intervals in the configurations 1s²2s²2p^k (k=1-5) are of topical interest for the predictions of forbidden (M1) transitions within the ground configurations.

The photoionisation cross section of Ca I has been calculated in the threshold region by many-body perturbation theory. Correlation effects among 3p⁶4s² electrons were found to be very important. Double-electron resonances dominate the cross section and were calculated using the interacting resonance formalism. Comparison is made with several experiments.

Reports absolute cross section measurements of the ion pair production H_A(2s)+H_B(1s) to H_A⁺+H_B^- using a merged beams experiment. Four centre-of-mass energies have been investigated between 85 and 325 eV. The results are found to be in good agreement with recent theoretical predictions. This work also confirms the important distinction made between 'direct' (like the reaction studied) and 'indirect' (i.e. H_A(2s)+H_B(1s) to H_A^-+H_B⁺) charge exchange.

The parameters of the resonance in electron scattering by potassium ground-state atoms are obtained by the use of experimental data on the self-broadening of the Rydberg states. These parameters are close to the results of a variational calculation. The possibility of the extension of the method to other atoms and molecules is discussed.

The authors have calculated adiabatic polarisation potentials for e^--Be and e^--Mg scattering using matrix Hartree-Fock calculations with extended Gaussian basis sets including diffuse functions. They find that the adiabatic polarisation potentials are much more attractive than the potentials that have been used recently to study low-energy shape resonances in these systems.

The L-shell X-ray relative intensities I(L_alpha )/I(L_l), I(L_alpha )/I(L_beta ) and I(L_alpha )/I(L_gamma ) for U, Th, Pb and the ratios I(L_{alpha +l})/I(L_beta ) and I(L_{alpha +l})/I(L_gamma ) for W have been measured. The L-shell electrons are excited by 59.57 keV gamma rays from ²⁴¹Am and the fluorescent L-shell X-ray intensities are measured with a Si(Li) detector. The experimental results are found to agree well with theory.

The authors investigate two-photon ionisation using a generalised rate equation technique for a model in which the atom is assumed to have only two bound levels (the ground state and an excited state) together with a simple continuum of ionised states. Spontaneous emission is allowed from the excited to the ground state, and the laser is assumed to have a finite bandwidth due to phase diffusion. The photoelectron energy distribution is calculated; it is found that if the Rabi frequency exceeds the ionisation rate the energy spectrum is a sensitive function of the linewidth and spontaneous emission rate, whereas if the ionisation rate exceeds the Rabi frequency then the sensitivity of the energy spectrum to these parameters depends upon the initial state of the system. A quite small value of laser linewidth is found to considerably reduce the heights of peaks and increase the widths. Analytic and numerical results are presented.

The authors consider population trapping in a three-level system which is driven by two applied fields of arbitrary intensity, frequency and bandwidth. An irreversible loss of population is allowed to take place from the common level. The conditions are investigated under which population trapping can occur when spontaneous emission and finite radiation bandwidths are allowed. For the lambda configuration with monochromatic lasers the authors find that increasing the spontaneous emission rate can increase the amount of population trapped in the system, whereas spontaneous emission destroys trapping in the ladder configuration. For the case of uncorrelated lasers, finite laser bandwidths destroy trapping for either configuration.

Two-photon excitation spectra of xenon were recorded for the energy region of 67000-69000 cm^-1 and a pressure range of 50-500 Torr, using frequency-doubled light of a tunable pulsed dye laser. Molecular structures related to the 0_g⁺(³P₁), 1_g(³P₁), and 1_g(³P₂) states of the 5p⁵6s configuration of Xe I were observed. The assignment was based on the polarisation dependence of the excitation rate. The fluorescence behaviour of the excited states is discussed.

Excitation cross sections in new channels remain finite above threshold because of the degeneracy of the levels in the two-particle Coulomb problem. Below the thresholds there are series of resonances distributed according to the geometrical progression law. In cases with one complex eigenvalue lambda , the S-matrix elements in the vicinity of the threshold continuously trace circles on a complex plane: one circle below the threshold, the other above. The properties of the circle are investigated. The logarithmic angular dependence is obtained for the elastic scattering amplitude on excited states at any threshold and for any mass.

A crossed-beam coincidence technique, previously developed in this laboratory and used in measurements of the ionisation of H by protons and multiply charged ions, has now been applied to studies of non-dissociative and dissociative ionisation of H₂. Cross sections sigma _ni for nondissociative ionisation and cross sections sigma _di for the sum of the dissociative ionisation channels have been separately determined for 38-1500 keV amu^-1 H⁺, 31-550 keV amu^-1 He²⁺, 30-388 keV amu^-1 Li^q+ with 1<or=^q<or=3, 16-193 keV amu^-1 C^q+ with 2<or=q<or=4, 14-166 keV amu^-1, N^q+ with 2<or=q<or=5 and 12-145 keV amu^-1 O^q+ with 2<or=q<or=5. Measured cross sections are dependent on and increase with q. Peak values of sigma _ni and sigma _di shift to higher velocities as q increases. Peak values of sigma _i= sigma _ni+ sigma _di contain contributions from sigma _di ranging from about 10% for H⁺ to about 30% for N⁵⁺ and O⁵⁺ impact. In all cases the contribution of sigma _di to sigma _i becomes dominant at low energies. In a few selected cases a fast-ion coincidence technique is used to show that the main contribution to these large values of sigma _di arises from the transfer ionisation process. The ratios R= sigma _i(H)/ sigma _i(H₂) of the previously measured cross sections in H to the present values in H₂ are obtained. At the high-velocity limits, R is decreasing in all cases and has values ranging from about 0.45 for 1500 keV amu^-1 H⁺ to about 0.61 for 145 keV amu^-1 O⁵⁺.

The authors report the first measurements of the cross sections for collisional angular momentum mixing of very highly excited sodium nd (20<or=n<or=46) states by argon, krypton and xenon using selective field ionisation. The authors' experimental results are in fair agreement with a scaling formula obtained by Hickman that provides a rapid estimate of the l-mixing cross section for rare-gas collisions. The influence of an applied electric field on the l-mixing mechanism has been investigated. Marked changes in both the absolute value of the cross section and the final states distribution are reported. These results can be accounted for by simple energy transfer arguments based on the free interacting electron model.

Highly excited and resonant states are created in a rubidium vapour in order to study the collisional ionisation with the ground state and between these different excited species. A mass analysis allows one to separate the atomic and molecular channels. Penning ionisation with a resonant state is shown to have a great efficiency (rate coefficient greater than 10^-8 cm³ s^-1) for all the highly excited levels under study (6d, 8s, 7d). The associative ionisation rate coefficient between these excited atoms and a ground-state atom lies in the expected range 10^-10 to 5*10^-10 cm³ s^-1 and seems to be independent of l j and values.

Cross section calculations of the quasi-resonant ion-ion charge exchange reaction A^Z++B^(Z+1)+ to A^(Z+1)++B^Z++ Delta epsilon (1<or=Z<or=5, Delta epsilon <<1 au) are performed for some Tokamak plasma impurity ions. It is demonstrated that, in the 10-50 keV energy range, the rate of production of A^(Z+1)+ by the above reaction may become comparable with the electron-A^Z+ ionisation rate when the ratio of the concentrations of the B^(Z+1)+ ions and the plasma electrons is of the order of 10^-1-10^-2.

Cross sections for total one-electron capture and emission of important VUV lines produced by one-electron capture in collisions of Ne⁺ and Ne²⁺ ions with Li atoms have been measured at ion impact energies between 5 and 30 keV, for q=1, and 20 and 60 keV, for q=2. Excitation cross sections were derived and compared with empirically calculated data assuming one-electron transitions in the quasimolecular two-state approximation. In deriving the cross sections both metastable primary ion beam fractions and long-lived secondary ion states were quantitatively dealt with.

Cross sections of the reaction H^-+e to H⁺+3e have been measured for interaction energies between 17.8 and 1000 eV. Comparison with the only other measurement shows present results to be much smaller. The discrepancy ranges from a factor four at low energies to a factor 12 at high energies. The influence of positive ion trapping in the electron beam has been carefully investigated in the course of this study as it could explain the large discrepancy.

The authors have determined the anisotropy parameters for the electron impact excitation of the 2¹P state of helium in the energy range from 26.6 to 40 eV and in the angular range from 30 degrees to 110 degrees . The results of the 29.6 eV measurements are in agreement with the results of McAdams et al. (1980) although the small-angle minimum in the value of lambda as a function of the electron scattering angle seems somewhat more pronounced in the authors' results. The authors have derived an alternative expression for the coincidence rate, based on a different choice of the quantisation axis (perpendicular to the scattering plane instead of in the direction of the incoming electron beam). The new expression displays a more transparent relation between the observed radiation pattern and the anisotropy parameters of the excited atom. In particular it allows a more straightforward determination of the errors in the derived parameters.

Absolute total and differential rotational and rotational-vibrational cross sections have been measured for N₂ (weak quadrupole), CO (weak dipole), and H₂O (strong dipole). In the beam experiments, the range of collision energies of the electrons was chosen between 0.5 and 6 eV, and the range of scattering angles from 15 to 105 degrees was covered. The overall energy inhomogeneity of the electron spectrometer was between 10 and 18 meV FWHM, depending on the experiment. Direct and resonant rotational excitation has been measured. In the case of N₂ the rotational branches with Delta J=0, +or-2, +or-4 have been fitted to energy-loss spectra and for each branch a cross section is given. The authors have proceeded similarly in the case of CO (for dipole excitation Delta J=0, +or-1, for excitation via the ² Pi resonance Delta J=0, +or-1, ..., +or-4) and in the case of H₂O ( Delta J=0, +or-1). For N₂ and CO the results are compared with calculated angular dependences. Finally, the cross sections are listed according to the different types of interaction potentials and interaction mechanisms.

Observations of gain on the hydrogen-like carbon ion Balmer- alpha transition, C VI H_alpha , at 182 AA are examined for consistency and possible sources of error. It is concluded on purely experimental grounds that the only result consistent with the full set of data is that amplification is observed, and that this should be ascribed to gain by stimulated emission. The data are compared with the values obtained by computational modelling and shown to be in satisfactory agreement provided the incomplete burn through is taken into account in accord with the numerical predictions. It is found that the model severely underestimates the gain observed: reasons for this are given.