Table of contents

The 5d to 5, epsilon f 'giant resonance' in the absorption spectrum of ThF₄ vapour has been observed and is compared with previous observations of the corresponding feature in the spectrum of the metal. It is concluded that thorium provides the simplest example yet known of a 5, epsilon f resonantly localised state, the corresponding feature in uranium being complicated by additional structure. Other members of the actinide sequence are not readily accessible experimentally. As a check on their method, the authors have also observed the 4d-4, epsilon f spectrum of LaF₃ vapour and they compare it with those of La I and La metal.

Noble-gas-induced relaxation of photon echo signals is used to determine the broadening rates of the Na D lines. Since echo signals are 'Doppler free', precision measurements are possible at low perturber-gas pressures (<or=1 Torr), and are sensitive to the previously inaccessible central lineshape. Interestingly, the authors results disagree with other recently published broadening rates. The equivalence of linewidth measurements and echo measurements on transitions between states of low electronic angular momentum is discussed.

A dense barium vapour is illuminated by an intense (2 kW), 1 mu s long, dye laser pulse. The hook technique using an N₂ laser pumped dye laser as a background light source is demonstrated as a fast tool to measure the temporal change in level populations of neutral and ionised barium atoms, with a time resolution of 10 ns.

Data are presented for the relative photoabsorption cross section of Ba I near the 5p-5d 'giant resonance'. A new interpretation is proposed for the peak at 581 AA in the Ba²⁺/Ba⁺ ratio, which resolves a number of difficulties. Coupling between two-step autoionisation and many-body processes leads to an asymmetric Fano profile of a novel kind in the double ionisation spectrum.

A procedure based on the rotated-coordinate method (RCM) is proposed to obtain widths for the decay of resonance into individual channels and it is demonstrated for a simple model potential.

Laser-induced collisional ionisation of caesium is observed in a strontium-caesium vapour mixture. In their initial state both atomic species are in the ground level. The ionisation of caesium is achieved when the system is illuminated by an intense laser light, the frequency of which is tuned across the resonance line of strontium. The process involves photoabsorption during the collisional interaction. It is monitored by the observation of optical fluorescence after Cs⁺, e^- recombination. The laser-induced collisional ionisation profile, i.e. the variation of the ionisation yield against the laser wavelength is recorded under various experimental conditions.

A formula for the relativistic correction to the interaction energy between the nuclei of a diatomic molecule in the Born-Oppenheimer approximation, in which the nuclei are considered as classical particles, is found. Application to the Au₂ molecule shows that the correction is small even for highly charged atoms, and can be neglected in most calculations.

Cross sections for Xe²⁺ production from the combined processes of charge transfer and ionisation have been determined for Xe⁺-Xe⁺ collisions, at centre-of-mass energies between 38 and 303 keV by means of an intersecting-beam technique previously developed in the authors' laboratory.

The spin polarisation of electrons scattered elastically by mercury atoms has been measured for collision energies between 8 and 18 eV. The results are discussed in comparison with ab initio calculations by Walker (1971) which show only fair agreement. The scattering at 14 eV incident energy and angles between 90 degrees and 130 degrees is suitable for a low-energy electron spin detector.

The effect of magnetic fields up to 100 T on the dielectronic recombinations of helium ions has been calculated. Below 5 T the rate is unaltered because of a cancellation of effects. At higher fields the rate does fall off slowly. Preliminary calculations including the Lorentz electric field appropriate to 3.4 MeV ions reveal similar behaviour. Contrary to simple intuition, the rate in an electric field is not greatly reduced by Stark ionisation.

A variational method for calculating the correlation structure of atoms is proposed. The full wavefunction is written in the form Psi = Omega Phi where Phi is a sum of determinantal functions. Omega is expanded in terms of one-, two-, three-, etc. body cluster operators. The following approximations are made: (i) the cluster expansion is truncated; (ii) the cluster operators are taken to be local or to involve bilinearly spin, orbital angular momentum and linear momentum operators; and (iii) the cluster operators are expanded in terms of separable operators. The resultant expressions involve at most two-dimensional integrals, and the sums are tractable. Applications are discussed.

A discrete basis set consisting of occupied and excited states in conjunction with many-body perturbation theory has been used to calculate the electric dipole, quadrupole, octupole and hexadecapole polarisabilities in Pr³⁺. Comparison with existing results is made. Electron-electron correlation contributions to the quadrupole polarisability are also calculated and found to be important (about 17% of the lowest order contribution, but of opposite sign). This arises from the use of the V^N Hartree-Fock potential in generating the excited states. Correlation contributions to the nuclear antishielding factor gamma _infinity in various atoms and ions have previously been found of similar magnitude but of the same sign as the lowest contributions to the parameter.

The related problems of optical and radiative collision are examined theoretically. The actual R^-n potentials found in nature are simulated by powers of the hyperbolic secant, which enables one to obtain closed-form solutions for the transition probabilities in the weak-field regime.

An experiment is proposed in which observation of the fluorescence from an atomic beam coherently excited by a pulsed laser and subsequently acted upon by an oscillating field allows one to distinguish atoms which have undergone one transition to a second state and back again to the original state from atoms which have undergone no transition at all. This experiment, for which one might have expected no distinction to exist, is interpreted in terms of the 0 and 2 pi rotations of a spinorial quantity.

Starting from the multi-level master equation a non-Markoffian rate equation is derived. Explicit expressions for the transition rates between each pair of atomic levels in this equation are derived at it is shown that these may be interpreted as the sum of one-photon, two-photon,..., etc. transition rates connecting the given pair of levels. The non-Markoffian master equation is exact and so can describe long- or short-time behaviour under monochromatic or broad-band excitation. The exact steady-state solutions are obtained by solving a Markoffian rate equation.

Time-dependent spectra are calculated for the optical Autler-Townes effect in a three-level system without Doppler broadening and in the probe field approximation. Analytic expressions are also given for the steady-state spectra taking account of the finite laser linewidths, which are assumed to be due to phase diffusion. Comparison is made with time-dependent spectra in the related phenomenon of resonance fluorescence, and the differences pointed out. The effects of laser linewidths on the amplitude of the spectra are also discussed, and found to be marked. The method used in the calculations is that of generalised multiphoton rate equations.

An induction coupled argon plasma has been used to measure transition probabilities in metal atoms. This work is described and transition probability measurements on several lines of cadmium I are reported. These transitions with their measured transition probabilities are as follows: 5¹D₂ to 5¹P₁, (59+or-8)*10⁶ s^-1; 6¹D₂ to 5¹P₁, (5.5+or-1.2)*10⁶ s^-1; 7¹D to 5¹P₁, (4.7+or-1.5)*10⁶ s^-1; 5³D₁ to 5³P₂, (4.3+or-0.5)*10⁶ s^-1.

For pt.II see ibid., vol.12, no.15, p.2541 (1979). The impact parameter dependence of the Be II 2²S-2²P emission probability and polarisation has been studied for 1.5-15 keV Be⁺-He, Ne, Ar collisions by detection of photons in two directions perpendicular to the incident beam, one orthogonal and to one in the scattering plane, in coincidence with ions scattered through a selected angle theta . The studied range of reduced scattering angles tau =E theta corresponds to impact parameters b in a range of 0.3-1.7 au for helium, 0.5-2.2 au for neon and 1.2-2.3 au for argon. The emission probabilities show that at small impact parameters the excitation takes place at quasi-molecular curve crossings for the Be⁺-Ne, Ar systems. Direct excitation dominates for the Be⁺-He system and for the large impact parameter collisions of the Be⁺-Ne, Ar systems. In the region where the direct excitation mechanism dominates, the coherence parameters lambda (b,E) and chi (b,E) describing the relative size and phase of the quantum-mechanical excitation amplitudes for the M_L=0 and M_L=+or-1 magnetic sublevels have been derived for the Be⁺-He and Be⁺-Ne collisions. The lambda and chi parameters vary little with b or E for He as the target, while strong variations are found for Ne. Loss of coherence due to internal forces and cascade effects are treated in detail.

Excitation probabilities P(b,E) and the coherence parameters, lambda (b,E) and chi (b,E) have been calculated for ²S to ²P excitation of Be⁺(2s) and Mg⁺(3s) ions in collisions with ground-state He and Ne atom targets in the energy range E_CM=0.5-100 keV and the impact parameter range b=0.1-5.0 au. Each ion-atom pair has been treated as a quasi-one-electron three-particle system consisting of the valence electron of the ion, the closed-shell ion core, and the closed-shell rare-gas atom. Close-coupled equations, based upon a rectilinear trajectory, have been solved for the amplitudes associated with a three-state (ns, np₀, np₁) one-centre expansion of the electronic wavefunction. A Hellmann-type pseudopotential has been used for the electron-ion core interaction; for the electron-rare-gas interaction an electrostatic model was chosen. The theory predicts that high energies and/or large impact parameters lambda will tend to zero and chi to tau /2. For finite energies (impact parameters) the function lambda (b) ( lambda (E)) exhibits pronounced oscillations which correlate with rapid changes (phase jump mod Delta chi mod approximately= pi ) of the phase angle chi (b) ( chi (E)).

K X-ray transitions in highly charged neon recoil ions have been observed in collisions of 1.4 MeV/amu Ar¹²⁺, ¹⁴⁺, Ni¹⁶⁺, Kr¹⁸⁺, Xe²⁴⁺, Pb³⁶⁺ and U⁴⁰⁺ with a neon gas target. The spectral lines are attributed to excited states of Ne⁹⁺, Ne⁸⁺ and Ne⁷⁺. Experimental evidence is given for a secondary selective electron capture into outer shells of fully stripped slow target ions.

The authors have studied the so-called 'parity unfavoured' excitation of the (2p²)³P state of helium by electrons. This was done (i) by observation of the inelastically scattered electrons for different scattering angles and (ii) by observation of the radiation at 32 nm emitted by the (2p²)³P state in the decay to the (1s2p)³P singly excited state. The results confirm the theoretical predictions that the differential excitation cross section is zero at 0 degrees and that the cross section for excitation of the M=0 magnetic sublevel is zero at all scattering angles.

For pt.III see ibid., vol.10, no.7, p.1385 (1977). The influence of a dipole surface sub-monolayer on the one-electron resonant processes in slow atom (ion)-surface collisions is investigated. It is found that the electron transition probability is considerably enhanced with respect to the case of a clean surface. However, the probability per collision for the resonant processes may be reduced. The survival probability for the negative ions emerging from a surface, covered by a dipole sub-monolayer, is also investigated.