Table of contents

Transition probabilities for the intercombination transitions 1s²2s2p³⁵S₂⁰ to 1s²2s²2p²³P_1,2^e in the O III, Ne V, Si IX, Ca XV and FE XXI ions of the C sequence have been calculated using configuration interaction wavefunctions. The relativistic effects are included in the Breit-Pauli approximation. The transition probabilities considered here are essentially proportional to the square of the mixing coefficient which mixes the ⁵S₂⁰ state with the 1s²2s2p³³P₂⁰ (or ³D₂⁰) state. The authors have empirically adjusted this mixing coefficient by using the experimental energy differences of the ⁵S₂⁰, ³P₂⁰ and ³D₂⁰ states. With this adjustment, good agreement is obtained with a previous calculation which employed more extensive configuration interaction wavefunctions. Results are compared with other theoretical work. For O III, the authors compare the radiative lifetime of the ⁵S₂⁰ metastable state with the experimental value. The agreement is better than 20%.

Two novel experiments producing three-dimensional histograms of time-of-flight correlations have recently been performed at the Daresbury Synchrotron Radiation Source. In the first, the two photoions from the double ionisation of SF₆ were detected; in the second, the photoelectron and the O⁺ ion from the predissociation of O₂⁺. This technique allows a visual insight into the details of dissociative photoionisation which has not been available previously.

An investigation was performed of the decomposition of MnCO₃ by both CW CO₂ laser heating and controlled thermal heating in a derivatograph. The process was shown to have different kinetic evolutions in the two cases even though the final product was the same.

A beam-gas technique is used to measure absolute nd relative cross sections, Q, for the ion pair production reaction Li+Cs to Li^-+Cs⁺-3.27 eV, where the atoms and ions are in the ground state. The measurements are made with Li as a projectile over the laboratory energy range from 1000 to 5500 eV. The data show that Q is a monotonically increasing function of the relative velocity with values ranging from 0.79 to 9.2 AA². The Q curve can be qualitatively explained by the poor coupling in at least the first step of a ladder-climbing process between the incoming covalent and outgoing ionic potentials.

In an electron-polarised-photon coincidence experiment the authors have measured the linear and circular polarisation of the 3¹D to 2¹P decay radiation (667.8 nm) emitted upon electron impact excitation of the 3¹D state of helium. Data are presented for an incident electron energy E=45 eV and electron scattering angles theta _e=31.2 and 40 degrees . The circular polarisation P₃ was found to be positive at both scattering angles.

Using a 40 cm curved crystal spectrograph of the transmission type, the L emission spectra of the light rare-earth elements have been investigated. The high-frequency satellite L gamma '_2,3 has been reported for the first time in the spectra of ⁵⁷La, ⁵⁹Pr, ⁶⁰Nd, ⁶²Sm and ⁶³Eu at wavelengths of 2026.3, 1860.5, 1785.1, 1645.3, and 1579.2 mAA, respectively. The transition energies have been calculated for the satellite using the semi-empirical Auger electron energies in the doubly ionised atoms and they have been found to be in good agreement with the measured values in all the elements in which the satellite has been reported so far.

The high-frequency Stark effect on neutral lithium was investigated by measurements of line profiles in a microwave field. In the experiment, satellite transitions on the allowed line (2²P to 4²D) and on two forbidden lines could easily be detected. The results are in agreement with the theory of Hicks et al. (1972) even for the highest-order transitions if saturation effects are taken into account.

The 2288 AA cadmium absorption resonance line broadened by pressure effects of cadmium or krypton is analysed in the 2200-3000 AA spectral range. The temperature dependence of the profile is studied and interpreted in the framework of the quasi-static theory. Interatomic potentials are deduced. For the Cd-Cd molecule, the ground-state X ¹0_g⁺ potential curve is determined in its attractive part for R=2.6-6.5 AA; the well depth is 410+or-30 cm^-1 at R_e=3.3+or-0.1 AA. The ¹1_u and ¹0_u⁺ excited-state potentials are respectively determined from 4.6 to 6.5 AA and from 2.6 to 4.6 AA. The blue satellite at 2214 AA is connected to a maximum of the ¹1_u excited-state potential of 1300+or-50 cm^-1 at R_M=4.25+or-0.05 AA; a decrease of the transition moment ¹1_u from X ¹0_g⁺ at R_M is found. For the Cd-Kr molecule, the X ¹0⁺ ground state and the ¹1 and ¹0⁺ excited-state potentials are determined for internuclear separations between 2.9 and 4.65 AA; the well depth of the ground state is found to be 130+or-10 cm^-1 at R_e=3.75+or-0.10 AA. Furthermore, the transition moments are found to be independent of R in this R range.

The 5p²³P autoionising resonances of neutral cadmium were investigated using laser excitation from the 5s5p ³P_J levels populated in a discharge. The profiles were recorded using optogalvanic detection. The narrow autoionisation widths of the 5p²³P₀ and ³P₁ levels were measured for the first time by analysing the absorption profiles for the 5s5p/sup /3P_J'-5p²³P_J transitions. The measured autoionisation widths for the ³P₀ and ³P₁ levels are 0.35 and 1.90 cm^-1 respectively. A theoretical explanation for the different behaviours experimentally observed for the 5p²³P_J levels with respect to the autoionisation process is presented. The non-vanishing width for the 5p²³P₁ level is ascribed to 'purely relativistic effects' which appear as a first-order effect in a relativistic treatment, or as a crossed second-order effect involving spin-orbit and electrostatic interactions in a non-relativistic formalism. Purely relativistic effects associated with breakdown of the Russell-Saunders coupling scheme induce strong cancellation effects which decrease the autoionisation width for the 5p²³P₀ level. Similar phenomena do not appear for the 5p²³P₂ level which possesses a broad width.

Near-threshold ionisation of an atom is investigated using a non-perturbative realistic model. The influence of the Rydberg states on the dynamics of the ground-state population is discussed. In particular, details of its non-exponential decay and of the population trapping as well as the shift of the threshold are calculated.

An investigation is reported of the autoionising decay of the n=18-22 members of the nu =3 Rydberg series, converging to the X¹ Sigma ⁺( nu ²+3) state of NO⁺. Photoelectron spectra for two-colour multiphoton ionisation processes via the A² Sigma ⁺( nu '=3) intermediate state are recorded. Photoelectron analysis of the decaying 22p( nu =3) Rydberg state leads to branching ratios of 1.00:0.06:0.25 for autoionisation into the nu ⁺=2, 1, 0 channels respectively. These branching ratios deviate from the Delta nu =-1 propensity rule for pure vibrational autoionisation. The effect of electronic coupling with a dissociative (valence) continuum is discussed. Experimental branching ratios are compared with results of two-step multichannel quantum-defect calculations by Guisti-Suzor and Jungen (1984) in which vibrational autoionisation and (pre)-dissociation are treated simultaneously.

Well depths of 0.27 meV at R=14 au and 0.19 meV at R=15 au are calculated for He(2³S)Ne(2p⁶¹S₀) and He(2¹S)Ne(2p⁶¹S₀) respectively. These results were obtained by solving a one-electron non-local Hamiltonian model. The active electron interactions with the neon atom and the helium ion core are represented by non-local pseudo- and model potentials, respectively.

Total cross sections for resonant electron capture by protons from metastable H(2s) targets have been computed in a six-state molecular close-coupling formalism. Transitions between degenerate sublevels of the L shell of the target occurring at large internuclear distances, before the exchange reaction, have been taken into account in the impact parameter approximation, expanding the system wavefunction on a basis set of atomic functions. Cross sections are presented for impact velocities between 0.05 and 0.3 au. The results are compared with theoretical calculations for capture from H(2s) by Li³⁺, c⁶⁺ and N⁷⁺.

Charge transfer and excitation cross sections are calculated for Na⁺+Li to or from Na+Li⁺ collisions. Large bases consisting of travelling atomic orbitals on both centres are employed for a range of projectile velocities 0.1<or= nu <or=1.4 (au). For low velocities ( nu <0.25) good agreement is found with the recent molecular-orbital calculations of Allan and Hanssen (1985). For velocities lying beyond 0.25 the authors predict lower cross sections, partly because a common translational factor was used in the molecular-orbital calculations. The authors also find good qualitative agreement with the experimental data of Daley and Perel (1969), which shows the oscillatory structure to be present in the velocity dependence of the charge transfer.

The authors report on the observation of a laser-induced charge exchange process in a Sr⁺-Ba mixture. Two maxima occur in the cross section profile when the barium atoms are ionised to the final 5p ²P_1/2 level. The photon energy for each maximum differs from the energy defect of the reaction partners at infinite separation. In the framework of the quasimolecular model, a complete analysis of the profile is derived. The first peak corresponds to the expected assisted signal while the second one is interpreted as a satellite line.

Unexpected structures have been observed in the Sn K and Ag K X-ray yields in 1.4 MeV N^-1 U+Sn and Pb+Ag collisions at impact parameters below 200 fm. Related gas target measurements suggests that these effects are mainly due to electron capture following strong ejection of outer-shell electrons in such close collisions. At larger impact parameters, the K-vacancy production is established to be dependent on target density and thickness.

Presents the results for measurements of the absolute total cross sections for electron detachment sigma _e(E) and charge transfer sigma _i(E) in low-energy collisions of Cs^- with He, Ne, Ar, Kr, Xe, D₂, N₂, O₂, CO, CO₂, SO₂, N₂O, CH₄ and SF₆. In the case of the rare gases, the apparent energetic threshold for detachment is rather high (near E=50 eV); a similar observation has also been made for the Na^- and K^- projectiles. As before, a curve-crossing model well describes the observed sigma _e(E). Structure in sigma _e(E) for the CO₂ target has been attributed to charge transfer to a metastable state of CO₂^-(²A₁). Similarly, in the case of N₂O, both sigma _e(E) and sigma _i(E) exhibit behaviour which suggests that charge transfer to a temporary negative-ion state of that molecule is involved in the collisional dynamics. Charge transfer is also observed to be an important exit channel for other molecules; for example, the value of sigma _i(E=30 eV) is about 500 a₀² for the SF₆ target.

Differential cross sections for electron capture into the 2S state of hydrogen are presented for the reactions H⁺+H₂ and H⁺+D₂. The results are for laboratory collision energies between 3.3 and 24 keV and scattering angles between 30 and 90'. The measurements expand on the results previously presented.

Angular dependences of the elastic scattering and the vibrational excitation of N₂ have been measured for impact energies between 0.1 and 1.5 eV in the angular range from 15 to 135 degrees . Absolute differential and integrated cross sections are presented. The elastic scattering can be described reasonably well by the modified effective range theory for 0.1 eV. The integral vibrational excitation cross sections are in good agreement with swarm results.

Eigenphase sums and resonance positions and widths are calculated for low-energy elastic impact of electrons on NO molecules using the R-matrix method with numerically defined continuum orbitals. Calculations are performed using both the simple close-coupling and the close-coupling plus short-range polarisation models using up to six electronic states or pseudostates of the NO target. The open-shell, ² Pi , target is found to show increased sensitivity to polarisation effects which are better represented by coupled-states calculations than by a one-state model with an optical potential. Comparison is made with the available experimental data on e-NO scattering.

Total cross sections for 0.7-400 eV positrons and 1.400 eV electrons colliding with CH₄, C₂H₄ and C₂H₆ molecules have been measured using a retarding potential-TOF (RP-TOF) method. The cross sections were obtained by a normalisation method rather than absolute measurements. The minima of the total cross sections for positron scattering by these hydrocarbons were distinctly shown in the region from 4 to 5 eV. The total cross sections for electron collisions with these hydrocarbons were compared with previous data and with positron collision data.