Table of contents

The polarisation of spectrally resolved Na D lines excited by white, linearly polarised light in a homogeneous magnetic field up to 1000 G was measured by means of the crossed-beam technique. The axis of observation, the sodium beam and the direction of the magnetic field were mutually at right angles. The incoming light beam was directed along the magnetic field and linearly polarised either parallel to the axis of observation or perpendicular to it. A Lyot filter of high transmission and a sensitive photon-counting technique made it possible to detect the small intensities of the resolved Na D doublet excited by the light of a xenon arc lamp. The measured polarisations are found to be in excellent agreement with those calculated by means of Breit's theory and the ambiguities of earlier investigations are completely removed. The importance of our results for electron impact excitation in magnetic fields parallel to the beam is discussed. It is shown that the polarisation of the electron impact excited Na D lines is no longer perturbed by hyperfine structure if a magnetic field of 500 G parallel to the beam is applied.

The authors report the direct observation of an intense ion yield when Na vapour in the wide range of densities 10¹²-10¹⁵ cm^-3 is irradiated by a laser tuned to the 3S_1/2-3P_3/2 resonance and with intensities in the range 3*10³-4*10⁵ W cm^-2. Evidence of superelastic e-Na(3p) collisions is shown above 10¹³ cm^-3. A critical discussion of possible electron seeding and heating mechanisms and a simple theoretical model for the observed ion yields are given.

The author compares two models for the ionisation of dense metal vapours by resonant laser pumping in the light of recent experimental and theoretical results (Salter, 1970). The evidence supports the model based on electron heating by superelastic collisions.

Recent claims (Salter, ibid., vol.12, p.L763) have been made that the trapping of resonance radiation explains both the efficient ionisation of dense metal vapours irradiated by intense resonant laser radiation and previous experiments concerning anomalous behaviour of resonance fluorescence. The authors examine these claims and show that they are incorrectly developed and that basic physical arguments show that resonance trapping cannot explain the phenomena mentioned above.

Including electron self-interaction in the Hamiltonian of the X_alpha method gives the Hamiltonian of the Xi _alpha method, which gives one-electron energies epsilon _{Xi alpha} for the even-Z atoms, Ne to Ar, in good agreement with epsilon _HF. The total energies E in the X_alpha , Xi _alpha and HF schemes are compared.

It is shown that use of the plane-wave approximation to describe (non-relativistic) fast particles does not always lead to the correct transition amplitudes, even in the high-energy limit. As an illustration of the possible difficulties, the photoionisation process is analysed in detail. It is found that use of the plane-wave approximation can lead to high-energy transition amplitudes which are incorrect by a factor of two or more, depending on the atomic potential and also on which form of the electromagnetic interaction is used.

The specific effects resulting from the non-monochromaticity of the external field for resonant ionisation of atoms under multifrequency laser radiation are discussed in two limiting cases: wide and narrow laser light spectra. The fluctuations of the radiation intensity and the final spectrum width are taken into account.

Non-resonant k-photon ionisation cross sections may be calculated by kth order perturbative treatment. This method is generally considered to be good when the detuning from the closest resonance is much larger than the expected shift and width of this resonance. The aim of this paper is to discuss the validity of this criterion for several realistic cases. By diagonalising an effective Hamiltonian built on a few quasi-resonant states, we have calculated multiphoton ionisation cross sections for ground state caesium by linearly polarised light in frequency ranges containing the tuning range of the ruby laser, the Nd glass laser and its first harmonic. The study is carried out up to an intensity of 10¹¹ W cm^-2.

A rotational analysis of the A¹ Pi to X¹ Sigma ⁺ system of ⁷⁴Ge³²S molecule is performed by a direct fit of 645 lines involving six vibronic levels (2-1, 2-0, 3-0, 4-0, 2-10, 3-11 bands). Constants of the A¹ Pi state are determined (in cm^-1): T_e=32888.88(9), Y₁₀( approximately omega _e)=373.87(4), Y₀₁( approximately B_e)=0.16093(2), -10⁴Y₁₁( approximately 10⁴ alpha _e)=9.84(6), -Y₂₀( approximately x_e omega _e)=1.314(6), -10⁷Y₀₂( approximately 10⁷D_e)=1.30(6). New vibrational constants of the X¹ Sigma ⁺ state are also recommended: Y₁₀( approximately omega _e)=574.19(2), -Y₂₀( approximately x_e omega _e)=1.631(3), Y₃₀( approximately y_e omega _e)=-1.6(2)*10^-3.

For pt.II see ibid., vol.10, no.4, p.695 (1977). Differential cross sections have been measured for He(2¹S)+Na at five different kinetic energies between 52 and 207 meV. For the lowest kinetic energy a backward glory is observed. An optical potential is obtained from a simultaneous fit to the data at all energies. A modified Morse potential with a well depth of epsilon =300 meV and a minimum distance of r_m=6.3 au gives the best fit. The well depth is in good agreement with data from Penning electron spectroscopy, but the total ionisation cross section is larger than that determined earlier. The metal beam is generated in a recirculating supersonic oven.

By considering how the infinite-order sudden approximation (IOSA) treats scattering by a fixed rotor the authors investigate the centrifugal sudden element of the method. They show that for the two widely used values of the characteristic orbital angular momentum the IOSA corresponds to fixing the relative orientation of the atom and the molecule at its value before or after the collision. Except for backward scattering, neither orientation is representative of the whole collision and there is no dynamical reason for preferring one over the other. From examination of first-order approximations an alternative orientation is suggested.

Li₂ molecules in a vapour cell were excited by dye laser irradiation into the J'=11, 13, 15, 17, 19, 21, 23, 27, 29 rotational levels of the A¹ Sigma _u⁺, nu '=15 vibronic state. Collision-induced rotational transitions out of these levels occurring in the presence of a foreign gas (He through Xe) were monitored by means of the fluorescence spectrum. Effective cross sections for transitions up to Delta J=20 were measured. They exhibit a strong + Delta J/- Delta J asymmetry, the cross sections for upward and downward Delta J transitions from the same level differing by up to a factor of 2.5. For Delta J=+or-2 this effect can be treated by time-dependent perturbation theory. Cross sections were calculated by adapting formulas given by Verter and Rabitz (1974) to the present case and using realistic potential parameters. It was found that the relative magnitudes of the measured cross sections, including the asymmetry, are well reproduced by the calculation. Application of the surprisal theory to the measured Delta J dependence of cross sections shows that the 'exponential energy gap law' is not an adequate description.

The spectrum of K alpha , K alpha ^h and K beta X-rays from H-, He-, Li- and Be-like sulphur, excited during the passage of 65 MeV sulphur ions through thin carbon foils, has been measured. The observed structure agrees quite well with the structure predicted by theoretical transition energy and intensity calculations. A low-energy component of the He-like K alpha line has been ascribed to the presence of a 3s spectator electron. The present results are compared with those obtained in laser-plasma experiments and those from previous beam-foil measurements.

The charge exchange reaction O+H⁺ to O⁺+H is reinvestigated after a detailed examination of the interatomic potential curves and the couplings responsible for the process. The long-range behaviour and the problem of the connection between the long-range and the short-range potentials are considered. The charge exchange collision is then treated in a close-coupling formalism and we take account of the fine-structure excitation process which can occur simultaneously. The results are in accordance with the experimental measurements of Fehsenfeld and Ferguson (1972) and are much smaller than the previous theoretical estimates.

For pt.I see ibid., vol.12, p.2553, 1979. The N⁺-Ne collision is experimentally studied in the 500-2000 eV energy range. Identification of the various direct and exchange processes is achieved using energy loss techniques. At low energy, the collision is dominated by charge exchange into the (²D) and (²P) metastable states. A simple model is derived which accounts for the relative population of these two states. Contrary to C⁺-Ne collisions, large excitation of Ne and charge exchange into Rydberg states of N are found. Qualitative interpretation of the present findings is carried out in the quasimolecular framework.

The single-electron excitation and ionisation processes in collisions of atoms with multiply charged ions are considered using the dipole approximation for the atom-ion interaction and a close-coupling method based on atomic states. In the case of ionisation the application of the close-coupling method is simplified by the introduction of an effective oscillator strength for the coupling of the discrete initial state with the continuum.

A two-state approximation based on atomic wavefunctions is used to calculate cross sections for electron capture by He²⁺, Li³⁺, Be⁴⁺ and B⁵⁺ from atomic hydrogen in the ground state. The velocity range covered is from v=0.44 to v=2.8 au which corresponds to a laboratory energy range of from 5 to 200 keV amu^-1. Reasonable agreement is obtained with the experimental data for He²⁺, Li³⁺ and B⁵⁺.

Charge transfer between H₂⁺ ions in specific vibrational states and argon atoms has been investigated. H₂⁺ ions were produced by photoionisation and state analysis was achieved by the registration of charge transfer events in coincidence with energy-analysed photoelectrons. Results at 20 eV are interpreted with the aid of a simple theory which incorporates the effects of changes in different vibrational levels and the ²P_3/2,1/2 states of Ar⁺. The implications of these results for the variation of vibrational distribution in molecular beam experiments are discussed.

The differential cross sections for the excitation of the O(³P-³S) transition for incident electron angles of 100 eV to 500 eV at scattering angles of theta =2.5 degrees to 20 degrees are presented. The cross sections are made absolute by normalising the generalised oscillator strength to the known optical oscillator strength by extrapolating the generalised oscillator strength curve to the zero value of the momentum transfer.

The authors present collision strengths for the 3s²3p²P^o to 3s3P²⁴P^e transition in S IV calculated using the R-matrix method. At low energies the cross section is dominated by resonances which are considered in detail for two partial waves. Collision rates for a Maxwellian distribution are larger than previous values and in better agreement with those implied by observations of S IV emission lines in the solar spectrum.

A partial-wave analysis for orientation parameters in atomic collision processes has been formulated by using a coordinate system with its z axis perpendicular to the collision plane. The result is interpreted in terms of the intuitive model of grazing collisions. Details are illustrated for the electron impact induced Na(3²S to 3²P) transition.

The R-matrix method describing the scattering of low-energy electrons by complex atoms and ions is extended to include terms of the Breit-Pauli Hamiltonian. An application is made to the astrophysically important 1s²2s²¹S₀^e-1s²2s2p ³P₁^o transition in Fe XXIII, where in the most accurate calculations carried out all terms of the 1s²2s², 1s²2s2p and 1s²2p² configurations are included in the expansion describing the collision. This gives up to 28 coupled channels for each total angular momentum and parity which are solved on a CRAY-1. The collision strengths are increased by more than a factor of two from their non-relativistic values at all energies considered.

The formation of negative-ion resonances associated with inner-shell excited states of the molecules, N₂, NO, N₂O, CO and CO₂ has been investigated. The resonances have been observed in the yield of positive ions resulting from excitation of the target molecules by electron impact. In the case of N₂ structure has also been observed in the detection channel corresponding to low-energy scattered and ejected electrons. The positions of these resonances have been found by comparison with known features in the metastable excitation spectra of N₂ and Ar. These positions are compared with the excitation energies of the excited states with which these resonances are associated.