Table of contents

This letter offers an explanation for the existence of two different atomic states with the same configuration in lanthanum 6s²4f_5/2 and europium 6s²4f_5/2⁶4f_7/2.

For Ar VIII there is a discrepancy between experimental results and recent calculations with semiempirical pseudopotentials by Preuss et al. (1981). The authors show that the error is most probably on the experimental side.

The authors report the first observation of spectral line perturbations, due to van der Waals coupling between very excited atoms, in a sample where the average interatomic distance is about 15 times the Rydberg atom radius. Broadenings of the order of intervals between adjacent lines reveal Rydberg-Rydberg couplings no longer negligible compared with the atomic binding energy. The authors analyse the excitation mechanism of this 'dense Rydberg gas' by laser light non-resonant for the unperturbed atoms.

Optogalvanic signals arising from a group of transitions in Zr I have been studied under irradiation of a cold cathode glow discharge with polarised laser light. The signals increase with the application of a magnetic field of a few gauss crossed with the symmetry axis of the polarisation, but the change is not related to the direction of current in the discharge. The occurrence of these magnetic resonances depends on at least partial saturation of the optogalvanic signals. The empirical evidence leads to the conclusion that the resonances result from changes of occupation probability in the excited state upon removal of degeneracy by the magnetic field-a non-linear, level-crossing effect.

A method is proposed for calculating the photoionisation cross sections of many-electron atoms near the ionisation threshold of an inner shell. The method takes into account the variation of the field in which a photoelectron moves due to creation of a deep vacancy and its decay. For example the cross section of the 1s² shell photoionisation in Ar is considered. The results appear to be in good agreement with experiment.

The angular distribution of photoelectrons from the 4d shell of free Ag atoms was measured with the aid of synchrotron radiation in the energy range 20 eV<or=h nu <or=45 eV. The asymmetry parameter beta of the 4d multiplet was found to rise monotonically from 0.20 at 20 eV to 0.95 at 45.2 eV. Hartree-Slater predictions reproduce this trend, but underestimate the absolute values of beta by about 0.5 units. The beta values of the ^1,3D term components were also obtained and found to differ by less than 0.15 units from one another, indicating similar anisotropic (exchange) interactions between photoelectrons and ionic core states.

The angular distribution of photoelectrons has been investigated experimentally for 'normal' multiphoton ionisation and for above-threshold ionisation of xenon at 0.53 mu m. The first is characterised by a secondary maximum, and both show an isotropic term.

The effects of strong driving fields on the ratio (R) of the resonant multiphoton ionisation probabilities in the presence of circular and linear polarised radiation are discussed. It is shown that the maximum value of R can be much larger than the maximum calculated on the basis of perturbation theory ((2N-1)!!/N! for an N-photon ionisation). Numerical results are presented to illustrate this effect.

The absorption coefficient of electrons in a short-range potential is considered as a function of the interaction parameter lambda . It is shown that the absorption coefficient is a smooth (analytic) function of lambda , but its free-free component shows a non-analytic behaviour at bound-state bound-state appearance thresholds. Thus perturbation theory methods with respect to lambda are possible for the determination of the absorption coefficient. In particular these methods are possible in the case when a new bound state arises as a result of a perturbation in the potential.

A theory of the resonances in the bremsstrahlung cross section close to the high-frequency limit for any short-range potential is derived by means of the Jost function formalism. It is shown that the resonance in the bremsstrahlung spectrum can be explained as final-state resonance scattering due to quasi stationary (l>0) or virtual (l=0) levels near zero. Resonances corresponding to l>0 show free-bound transition contributions as the interaction strength is continually increased.

The 4 mu m band radiation from high temperature air investigated by Wray and Taylor in 1969 is interpreted as 5g-4f emission of the NO molecule. Position, fine-structure and intensity of the band are in excellent agreement with previous knowledge on the 4f complex of NO and with theoretical 5g energies and 5g-4f oscillator strengths.

For electron capture in asymmetric ion-atom collisions the projectile-target charge ratio is a natural expansion parameter. Recently Macek and Taulbjerg (1981) have claimed that the anomalies of the Coulomb T matrix introduce corrections to the impulse approximation which do not depend on this ratio. The authors show that this is due to the approximations they introduced, and that more accurate estimates lead to a dependence of their correction on the charge asymmetry. This correction may nevertheless be important even for rather asymmetric systems.

Cadmium vapour enclosed in a cell was excited continuously by a beam of slow electrons. The Hanle effect was investigated for visible 5-10 ¹D₂ to 5 ¹P₁ and UV 5-10 ³D₃ to 5 ³P₂ transitions. Lifetimes of the D levels were measured from Hanle curves which were free from the cascade effect.

Resonant excitation of Ps to the higher P states is found to influence strongly the probability of gamma emission by annihilation. A considerable population can be, at the same time, transferred to the upper states. Numerical results for the excitation of the 2 ¹P and 2 ³P states and the annihilation of the 1 ¹S and 1 ³S states are presented for a number of field strengths of interest.

Dynamic dipole polarisabilities for neon and argon have been calculated using the R-matrix approach presented and discussed in a previous paper. The dynamic dipole polarisabilities are calculated for the ground states of the atoms for photon frequencies ranging from zero up to at least the first pole. A comparison is made between the results of calculations which omit and include the R-matrix surface contributions and values derived for the oscillator strengths for transitions from the ground state of each atom are presented.

Relative oscillator strengths for 37 lines of neutral calcium have been measured to an accuracy of +or-5% using the absorption method. The oscillator strengths are placed on an absolute scale by reference to recent lifetime measurements. All lines have lower energy levels in the range 2.5 to 3.0 eV above the ground level and the set of lines includes most of those important for the determination of the calcium abundance in late-type stars. In addition the same equipment has been used to study collisional broadening by helium of 17 lines chosen from among those with newly measured oscillator strengths. These measurements provide interesting insight into the dependence of the collisional damping parameter on total angular momentum within a spectral multiplet and on the configuration of the upper level.

Semi-analytical methods are presented for computing quantum form factors in the general non-hydrogenic case. For transitions between hydrogenic states, semiclassical and quantum form factors are compared in a large domain of momentum transfer. The free electron model is applied to the l-changing Rydberg-atom plus rare-gas collisions between nearly degenerate states on the one hand, and to inelastic n-changing Rydberg-atom plus rare-gas collisions on the other hand. It is shown that the free electron model can be justified in the first case for high values of n(>or=15) and that it breaks down in the second case when large momentum transfers are involved.

Reports measurements of the cross sections for collisional quenching of highly excited rubidium nS (32<or=n<or=45) states by helium. A time-resolved selective field ionisation technique is used for this study. The experimental results are compared with the values calculated using an approximate scaling formula.

The semiclassical approximation is extended to the case of ionisation of atoms by relativistic charged particles. The theory is formulated both for a relativistic and a non-relativistic electron description. For total cross sections the theory is shown to be equivalent to the relativistic plane-wave Born approximation. Numerical results for the impact parameter distribution of the ionisation probability for the K shell are given.

The average X-ray production cross sections and energies for Ar projectile K_alpha , K_beta and K REC (radiative electron capture) rays were measured for C target foils of effective thicknesses 8-450 mu g cm^-2. A model is constructed which describes these yields, the several parameters employed being fitted to the data by a chi-square fit program. The parameters involved represent the main Ar K vacancy production (a single cross section parameter, sigma _v'), the Auger and X-ray decay modes for projectiles within and after the foil, and the in-foil radiative and non-radiative electron capture (REC and NREC, respectively) decay modes. Variations in these parameters with projectile energy are estimated, and in-foil collisionally induced configuration alteration effects are considered. From the fit parameters it is possible to estimate several projectile characteristics, e.g. Ar K vacancy and K_alpha and K_beta X-ray production cross section, in-foil and post-foil average fluorescence yields and the NREC cross section. From the energy measurements the average Ar projectile configuration of an ion with a K vacancy in the thin target foil limit is estimated.

The impact parameter (b) dependence of the K X-ray production probability (P(b)) in 28 MeV Cu-Fe collisions has been measured as function of the thickness (d) of the target foil. The shape of P(b) is found to be independent of d. This observation suggests that the main 2p sigma excitation mechanisms proceed via couplings to orbitals connected with the projectile L shell.

Charge transfer coupling elements for collisions involving the avoided crossing mechanism have been computed for many years by inserting an asymptotic electronic wavefunction into an expression involving a surface integral. Here a second-order wave-function (the next correction) is derived for that purpose.

Discusses a variational functional for scattering theory which has been recently proposed by Takatsuka and McKoy (1980). It is shown that this functional can provide results with a purely discrete set of functions which are approximately equivalent to those obtained by Lucchese et al. (1980) from the first iteration of the iterative Schwinger method. Applications to the scattering of electrons by systems including CO⁺ and LiH illustrate this relationship and other features of the method.

The threshold behaviour of differential and total electron-atom ionisation cross sections is investigated by means of the limit procedure in the integral expression of the ionisation amplitude. The case of total angular momentum L=O is considered for which the integral expression of the amplitude is derived. The semiclassical final-state wavefunction is investigated by means of numerical integration. The results confirm the non-linear threshold law found by Wannier and uniform distribution of electron energies.

Triple differential cross sections for electron impact ionisation of the 3s shell of argon are examined for incident electron energies between 100 and 400 eV. Theoretical distorted-wave results are presented for different models of the scattering process and comparison is made with existing experimental data. The sensitivity of the theoretical results to atomic wavefunctions, atomic potentials and optical potentials is examined. An attempt to obtain an elementary parametrisation for the cross section is also reported.

A series of different Coulomb distorted-wave (CDW) approximations have been used to evaluate the cross section of Ca⁺ for excitation by electron impact at low energies. The problem has been formulated in the LS coupling scheme, with the target ion being represented by a core plus atomic electron (frozen-core approximation) and exchange between the atomic and the incident electron being neglected. For a given number of states the best of the CDW approximations used gives results which are in very good agreement with the close-coupling calculations, with a significant reduction in the computing time required.

The threshold behaviour of the ionisation of atoms by positron impact has been analysed. The total cross section is shown to rise as the 2.650th power of the excess energy; for ions this exponent is larger. The escaping electron and positron are emitted predominantly into the same direction with a uniform electron-positron energy distribution. These effects emerge explicitly from electron-positron correlations.

The exact steady state solution for the reduced atomic density operator for a model of N identical two-level atoms occupying the same site (the Dicke model) and driven by a CW off-resonant laser field is presented. Exact expressions are given for the atomic observables and atomic correlation functions of arbitrary order. Exact thermodynamic analysis (N to infinity ) shows that the critical phase transition behaviour found in the resonant case is destroyed due to non-resonant effects. For large N and increasing detuning parameter, quantum fluctuations tend to vanish and hence the atomic observables tend to assume their semiclassical values. Also it is found that for a fixed arbitrary detuning parameter and increasing N these fluctuations disappear for the whole range of the driving field strength. The authors explain why it is not possible in the limit N to infinity to recover the correct asymptotic limit for the resonant case from the results of the dispersive case.

For pt.I see ibid., vol.14, no.14, p.2519 (1981). The authors extend their earlier theory for the motion of a trapped ion in a travelling wave of light from the low-intensity in detail. A numerical procedure to give the rate of change of the mechanical energy of the ion due to the light pressure force is presented, and a general analytic expression for the steady-state energy is derived. Exact solutions are compared with existing approximations.

Computer simulated experiments are reported in which the ion loss due to mutual neutralisation and that due to ter-molecular recombination are separated. It is found that the presence of an ambient gas may increase the mutual neutralisation rate coefficient by a factor of as much as 10⁴ and that it greatly reduces the range in which the rate coefficients for different mutual neutralisation processes lie. It is also found that mutual neutralisation may suppress ter-molecular recombination to a marked extent unless there is a low probability of a transition ensuing from a double traversal of the ionic-covalent crossing.