Table of contents

Transition probabilities for magnetic dipole and electric quadrupole transitions within the ground-state configuration 1s²2s²2p² in O III are calculated using J-dependent configuration interaction wavefunctions. These transition probabilities and rate coefficients of Baluja et al. (1981) calculated using the R-matrix method are then used to calculate the intensity ratio (I(¹D₂ to ³P₂)+I(¹D₂ to ³P₁))/I(¹S₀ to ¹D₂) which is used in temperature diagnostics of planetary and gaseous nebulae. The results are compared with other work.

The lifetime of the Xe II 5p⁴6p ⁴P_5/2⁰ level is measured by selective laser excitation of a fast beam of metastable 5p⁴5d ⁴D_7/2 xenon ions and observations of the fluorescent light intensity as a function of flight time. Comparison of the obtained mean lifetime tau =7.53+or-0.15 ns with earlier measurements confirms the results of the photon-photon delayed-coincidence technique.

The first consistent random phase approximation with exchange (RPAE) calculation of Mn photoionisation in the giant resonance region is reported. In order to be able to perform these calculations, the atomic electrons were separated into two groups called 'up' and 'down' depending on their spin projection. A satisfactory agreement with experiment has been achieved and the autoionisation nature of the experimentally observed maximum in the cross section is confirmed.

High resolution (+or-0.003 AA) absorption spectra of CH₃I have been recorded by using synchrotron radiation in the wavelength range from 1800 to 1130 AA. The spectra show high members of the ns, np and nd series converging on the ²E_3/2 and ²E_1/2 levels of CH₃I⁺ as well as the nd series terminating on a nu '=1 vibrational level of ²E_1/2. All the series have been considerably extended up to n values superior to the corresponding series in atomic Xe I and further detail added to previously known structures. A preliminary interpretation is presented and accurate values for the ionisation potentials are derived.

New observations of spectra resulting from transitions between levels of principal quantum numbers 3 and 4 are reported. The spectra were emitted from plasmas produced through focusing the output of a 4 GW neodymium-glass laser on to plane solid targets. Observations are of spectra of ions in the lithium I isoelectronic sequence between Mg X and Ti XX and of P XI and XII. Theoretical calculations made with a Hartree-XR computer program package extended the data to Fe XXIV.

The two-photon ionisation cross section of potassium is calculated from a quasi-degenerate perturbation theory expansion in the atom-radiation field interaction. The authors assume a narrow bandwidth radiation field of moderate photon flux (10²⁵-10²⁹ photons/cm² s). Resonances are treated by first diagonalising the Hamiltonian among the strongly coupled resonant states and then applying perturbation theory to include nonresonant background effects. All bound and continuum atomic states are calculated in the non-relativistic Hartree-Fock approximation and the various summations over intermediate states are handled by the inhomogeneous differential equation technique. Results for potassium show that resonance structures, including fine structure, are well resolved for a photon flux of 10²⁷ photons/cm² or less. As the flux increases to 10²⁹, the resonance enhancement is substantially quenched.

The observation of a transient population inversion in the resonance line of strontium is reported. Basically the process involves a two-photon excitation of the upper state in a three-level system, including the resonant level as the intermediate one. Both transitions involved emit superfluorescent pulses revealing the achievement of population inversions. Characteristics of the process are investigated in time and frequency domains.

A study of the cross section for electron loss from He⁺ and H, which sums over all final states of the target atom (both bound and continuum), is presented. This study is used as a basis for the comparison of several closure approximations which effect an approximate sum over target final states. The qualitative structure of the doubly differential cross section depends strongly upon the specification of average energy of excitation of the target atom. Doubly differential cross sections due to electrons ejected from both the projectile and the target are presented and regions of velocity space where interference between these two processes may occur are identified.

Radiative collision processes between Li⁺ and He have been studied in the projectile energy range from 0.45 to 4.2 keV. The total cross sections for Li(2p²P) and He(2p¹P) excitations show the oscillations in antiphase with each other. A polarisation measurement was done for the 2p to 2s transition of Li; the degree of the polarisation is found to be relatively small in the energy range studied. The partial excitation cross sections for the magnetic sub-levels 2p₀ and 2p_+or-1 of Li(2p²P) are obtained. On the basis of the sudden approximation, the kinematic effect in the polarisation measurement is discussed in detail.

Absolute electron detachment cross sections are reported for collisions of I^- and the rare gases at energies between threshold and 500 eV in the laboratory frame. The detachment threshold energy and forward rate constants at 4000 and 5000K are reported for each system. The cross sections for I^-+Ne are found to be much smaller than those for the other systems studied.

Measurements of the sigma ₂₁ cross section for Ar²⁺ ions in helium and xenon have been made between 60 and 200 keV. The level crossing theory was found to give cross sections which were much smaller than the experimental values in all the rare-gas targets except helium. An improved version of the Rapp-Francis non-crossing theory (1962) gives reasonable predictions for both heavy and light rare-gas targets and this is probably the dominant electron transfer process at these energies.

The parameter mu ^infinity of the screened Coulomb potential approximating the actual atomic potential was proved to be independent of the incident electron energy at sufficiently high energies. The parameter was recalculated for the Thomas-Fermi atom and also determined for the Thomas-Fermi-Dirac and the Hartree-Fock atoms. Considerable discrepancies between mu _TF^infinity , mu _TFD^infinity and mu _HF^infinity were found for elements with atomic number below 30. The values of mu _HF^infinity are recommended for that range of atomic numbers.

A new method of calculating elastic and inelastic cross sections for electron-atom and electron-ion scattering at intermediate energies is described. In this method the total wavefunction is expanded as a sum over channel functions plus a sum over L² integrable functions. Cross sections between channels included in the first expansion are obtained by averaging the T matrix over pseudo-resonances arising from the L² integrable functions included in the second expansion. It is shown that an appropriate averaging procedure allows for loss of flux into open channels which are omitted from the first expansion. The method is discussed in detail for a two-channel model and is shown to give good results for elastic scattering in this case when only a few L² integrable functions are included. The method is then applied to the astrophysically important transition 2s²¹S^e-2s2p ³P^o in C III where the cross section at intermediate energies is found to be substantially below the previous best estimates.

The R-matrix calculations of O'Malley et al. (1979) are simplified and extended to the intermediate impact energy range in which the He ground-state wavefunction is coupled with a ¹P pseudo-state to include the full static dipole polarisability. Phaseshifts, differential, integrated and momentum transfer cross sections for electrons elastically scattered from helium are reported for the impact energy range of 5 to 200 eV. At low energies the present calculation essentially reproduces the results of O'Malley et al. (1979) for 5<or=E<or=16.5 eV. At intermediate energies good agreement between the present calculations and the most recent measurements is observed.

For pt.I see ibid., vol.14, p.307 (1981). The R-matrix calculations of Fon et al (1981) on electron-helium elastic scattering are extended to consider the elastic scattering of electrons from neon in which the Ne ground-state wavefunction is coupled with a ¹P pseudo-state to include the full state dipole polarisability. Phaseshifts, differential, integral and momentum transfer cross sections for electrons elastically scattered from neon are reported for the impact energy range of 4 to 200 eV. A search has also been carried out for the 'critical energy' and 'critical angle' where maximum spin polarisation of the incident electrons may be expected after scattering from neon in the elastic scattering channel. Satisfactory agreement between the present calculation and experimental measurements is achieved.

The effective-range theory (ERT) for slow electron scattering by quadrupole molecules is developed. A low-energy expansion for the K matrix in the adiabatic approximation is obtained with an accuracy to the order k³ ln k. Modified formulae taking into account the polarisation potential are also derived. Closed expressions for eigenphase sums in the limit of small energies are presented. The threshold behaviour of the excitation cross sections is investigated. The ERT results for scattering from N₂ and CO₂ and H₂ molecules are compared with the data obtained by close-coupling calculations.

Electron scattering by aluminium atoms below the threshold of the 4²S state excitation is considered. The K matrix and cross sections are calculated by the close-coupling approximation including the 3²P and 4²S states. The values obtained are compared with the results of the effective range theory (ERT) for scattering by quadrupole atoms. The range of applicability of ERT and the possibility of its use for very small energies are discussed.

The total and differential cross sections for the excitation of the 3p^-14s¹ P state in argon and also the correlation parameters lambda and chi have been calculated using the distorted-wave Born approximation. Different variants of the distorting potential are used. It is shown that the inclusion of the imaginary part of the optical potential in a semiphenomenological form allows one to improve the agreement between the calculated total and differential cross sections and experiment.

Dissociative attachment from metastable oxygen (a¹ Delta _g) has been studied with an electron impact spectrometer and a microwave discharge. Three processes are observed. The first dissociates to the first limit, has a cross section maximum of 3.8*10^-18 cm² at 6.5 eV with respect to the ground state and is a result of formation of the well known ² Pi _u resonance. The second dissociates at the second limit and is the consequence of a ² Sigma _g⁺ resonance. The cross section is estimated to be 1.8*10^-18 cm² at its maximum which occurs at 8.5 eV above the ground state. The third is very weak (cross section maximum less than 10^-20 cm² at 9.5 eV above ground), dissociates to the third limit and ² Sigma _g⁺ symmetry is proposed for the resonance.

Cross sections for absolute total ionisation between 10 and 100 eV, dissociative attachment between 0 and 10 eV, and negative ion-pair formation between 10 and 40 eV of the bromine molecule have been measured by electron collisions using a parallel-plate type interaction chamber. The maximum of the ionisation curve was found to be at 83 eV with a peak value of 10.25*10^-20 m². Six dissociative attachment processes peaking at 0.00, 0.50, 1.45, 3.75, 5.30 and 8.50 eV with cross sections of 17.7, 1.50, 2.80, 4.94, 0.42 and 0.325 units of 10^-22 m², respectively, were detected. Negative ion formation cross sections with a maximum at 18.0 eV and a value of 5.26*10^-22 m² were measured. The relative error, except for the first attachment maximum close to the zero electron energy, is +or-0.05.

The model dependence of the calculated field ionisation rate of hydrogen is presented. The importance of correctly treating both the Coulomb field of the proton and the three dimensionality of the problem is demonstrated. A new, accurate, analytic method for field ionisation rate calculation is derived and justified. This method may be developed for systems more complicated than the hydrogen atom in a uniform field such as field ionisation of gas atoms near a metal surface or post-ionisation. Previous uniform field ionisation rate calculations for hydrogen are discussed and an explanation of errors in some of these is given.