Table of contents

The results of a new experiment using high-resolution Doppler-free laser spectroscopy are reported. Firstly, the wavenumbers of the D₁ and D₂ lines of Fr atoms have been accurately measured: sigma (D₁)-12237.409 (2) cm^-1 and sigma (D₂)=13923.998 (2) cm^-1. Secondly, the isotope shifts of the D₁ and D₂ lines have been compared for Fr and Cs atoms. The contribution of the p_1/2 electron to the volume shift has been measured relative to that of the s_1/2 electron as 4.75(25)*10^-2 for Fr and 1.4(3)*10^-2 for Cs. They are discussed in terms of semi-empirical calculations as well as relativistic Dirac-Fock ones.

When a reaction channel has an attractive Coulomb force between the separating particles, it is known that the total cross section in any open channel is predicted to be continuous across the threshold. The author points out that there is a second result of threshold theory that can be cast into a form relating cross sections above and below threshold (and so can be checked by experiment).

New high-resolution observations of the Hopfield series of autoionising resonances in N₂ are reported. They are discussed in relation to recent ab initio calculations of autoionisation in N₂ and are shown to shed further light on the variations of the quantum defect parameters as a function of n. The authors are able to analyse two perturbations, one strong (electronic-electronic) and one weak (vibrational-electronic), with the help of simple two-dimensional Lu-Fano plots based on experimental data. The strong perturbation upsets the results of the ab initio theory at high n, but the weak perturbation does not disturb the predictions for lower n, which is consistent with the fact that better agreement between ab initio theory and experiment for line profiles is obtained at low than at high n values.

It has recently been shown that certain two-photon transitions can be indiced in atoms by applying a static magnetic field at the intersection of two crossed laser beams. The mechanism involves quantum mechanical interference between different excitation pathways. In this letter it is shown that a similar effect takes place in any highly symmetric molecule such as methane or sulphur hexafluoride belonging to a cubic symmetry group. However, in this case, the effect occurs through a non-linear magneto-optical interaction which is quite distinct from the interference mechanism.

Ab initio fully relativistic SCF molecular calculations of energy eigenvalues as well as coupling-matrix elements are used to calculate the 1s_sigma excitation differential cross section for Ne-Ne and Ne-O in ion-atom collisions. A relativistic perturbation treatment which allows a direct comparison with analogous non-relativistic calculations is also performed.

Absolute cross sections for collisional depopulation of 2.5-40 keV u^-1 Rydberg hydrogen atoms have been measured in Ar, N₂, CO₂ and SF₆. The data establish unambiguously that both the Rydberg electron and the proton core scatter quasi-freely. This result confirms Matsuzawa's (1980) theoretical description of fast collisions of Rydberg atoms with neutrals. Consequently, under certain conditions the dominant Rydberg atom depopulation mechanism is that of core collisions and the total scattering cross section of a fast Rydberg atom reaches a natural limit, independent of the principal quantum number n, given by the independent-particle model.

A semiclassical model for He is applied to the study of multiply charged ions colliding with He at intermediate energies. Single and double-electron capture and ionisation and capture-ionisation reactions are studied for incident-ion charge states q=+1 to +50 at 1 MeV amu^-1. For charge states q>or approximately=+10, single electron capture is found to be dominated by a two-electron transition in which one electron is captured and the other is ionised.

Calculations for electron capture to the nl=2l (l=0,1) states in bare ion-atom collisions are presented using a recently developed first Born-type approximation. The evaluations of total cross sections for proton impact on hydrogen and helium targets are compared with other theoretical and experimental results.

Exact quantum reaction probabilities for a collinear model of the F+HH, HD, DD and DH reactions on the MV potential energy surface have been computed using hyperspherical coordinates. The results, obtained up to a total energy of 1.8 eV, show three main features: (i) resonances, whose positions and widths are analysed simply in the hyperspherical formalism; (ii) a slowly varying background increasing for FHD, decreasing for FDH, and oscillating for FHH and FDD, whose variations are interpreted by classical dynamics; (iii) partial reaction probabilities revealing decreasing vibrational adiabaticity in the order FHH-FDD-FHD-FDH.

A method providing ab initio, self-consistent parameters alpha for the X alpha theory has been reviewed. Shake-up energies on the Ne 1s and Ar 2s and 2p levels have been determined by the Z alpha method with theoretical parameters alpha . A few RER (radiative electron rearrangement) transition energies for the elements N, Ne, K, Fe, Si, Ca and Ti are also presented.

A diagonalisation procedure of the Foldy-Wouthuysen type is considered for the single-electron Dirac Hamiltonian as a basis for many-body applications. A modified procedure is suggested. In the diagonalisation procedure the Dirac equation is completely decoupled into two equations of the Pauli type. The expressions for positive- and negative-energy projection operators in the transformed basis are then trivial and, by performing the inverse transformation, projection operators for Dirac functions are obtained. Applying a similar transformation to the two-electron Dirac-Coulomb Hamiltonian leads to a diagonalised single-electron part and a non-diagonal two-electron interaction. In principle, the effect of the exchange of virtual, transverse photons (Breit interactions) can also be included in the electron-electron interaction and transformed in a similar way. It is indicated how a diagonalisation procedure of this kind can be used as a basis for relativistic many-body calculations in the coupled cluster formulation in analogy with the corresponding non-relativistic procedure.

Extending the concept of the time-dependent physical spectrum introduced by Eberly and Wodkiewicz (1977) the authors present a formula for the n-fold time-resolved correlation spectrum depending on n time and n frequency arguments. The corresponding measuring device consists of n coupled photodetectors with frequency resolving equipment in front of each. As an application the authors consider the resonance fluorescence radiation of a two-level atom irradiated monochromatically. Under certain conditions the authors derive a simple expression for the n-fold detection probability of photons at time t_i (i=1, . . ., n) with frequencies belonging to the central peak or to one of the sidebands of the fluorescence triplet.

The photoionisation spectrum of atomic sulphur prepared by the reaction of H atoms with H₂S, is presented from its ionisation threshold to 925 AA. Autoionisation peaks assigned to quasidiscrete levels . . . (²D degrees )nd³S degrees , ³P degrees are sharp, whereas those designated . . . (²D degrees )nd ³D degrees are broad. This latter feature parallels the discontinuity observed between first-row and heavier elements in the noble gases and halogens. There is evidence for severe perturbation in the . . , (²D degrees )nd ³P degrees series, which could conceivably be attributed to a proximate 3s3p⁵³P degrees state.

The photoionisation spectrum of atomic selenium, prepared by the reaction of H atoms with H₂Se, is presented from its ionisation threshold to 600 AA. Autoionisation peaks assigned to quasidiscrete levels . . .(²D degrees )nd ³p degrees are sharp, those assigned to . . .(²D degrees )ns ³D degrees and . . .(²D degrees )nd ³S degrees are somewhat broader, and others assigned to . . .(³D degrees )nd ³D degrees are very broad. A few members of the series converging to Se⁺(²P degrees ) are also observed. Two prominent peaks are tentatively assigned to 4s4p⁵³P degrees _2,1, and taken to be the first members of the series . . . 4s4p⁴np, the latter appearing as window resonances in this spectrum. The photoionisation spectrum of H₂Se is also presented.

An original method of determining the nuclear Lande factor for atoms with a diamagnetic ground state is illustrated with the two odd-A isotopes ¹³⁵Ba and ¹³⁷Ba. The method, based on the fast atomic beam laser spectroscopy technique, consists of detecting the in-flight Larmor precession of the nuclear magnetic dipolar moment in a static magnetic field, via the fluorescence induced by resonant laser light. When the magnetic-field amplitude is varied the fluorescence signal exhibits a fringed structure from which the g₁ factor can be deduced.

Photoproduction cross sections of molecular oxygen for O₂⁺ and O⁺ ions in the wavelength region from 1030 to 400 AA have been obtained using a quadrupole mass spectrometer and synchrotron radiation as a continuum light source. Photoionisation and dissociative photoionisation of O₂ in the wavelength region from 750 to 400 AA are discussed. In the yield spectra of O₂⁺ and O⁺, vibrational structures of the b ⁴ Sigma _u^- and B ² Sigma _u^- states of O₂⁺ are observed and the features are considered to be due to autoionisation and predissociation. In the Rydberg series converging to the c⁴ Sigma _u^- (O₂⁺) state, characteristic features of the window-type and asymmetric profiles appear in both the spectra of O₂⁺ and O⁺.

The rate for radiative decay from ortho- to para-H₂ is calculated using the quantum electrodynamical method of Feinberg and Sucher (1971). The calculation is greatly simplified by switching to the dipole length form while the fields are still second quantised. The rate, about 10^-12 a^-1, differs only slightly from that calculated by Raich and Good (1964) and is small enough to be neglected even in interstellar space.

The authors have observed the six-photon ionisation of H₂ with four-photon resonances on the E, F ¹ Sigma _g⁺, nu _E=0 levels, using laser intensities in the range of 10¹¹ W cm^-2 and photon energies around 3.07 eV. Contrary to all expectations, there is evidence for significant H⁺ production; both H⁺ and H₂⁺ ion resonance spectra are dominated by the Q lines of the four-photon transitions E, nu _E=0 from X, nu =0. From the electron energy spectra, which show a strong nonFranck-Condon behaviour, it appears that the H⁺ ions are formed by photodissociation of H₂⁺ ( nu ₊=2). At higher intensity (10¹² W cm^-2) the authors demonstrate the absorption of two extra photons by the H₂ molecule into the X ² Sigma _g⁺ ionisation continuum.

Inelastic collision cross sections and low-temperature rate coefficients (15<or=T<or= 300K) are presented for the process of collisional excitation of ortho-NH₃ by para-H₂ in its rovibrational ground state ( nu =j=0). The results have been obtained with the quantum mechanical coupled-channels technique, using an ab initio NH₃-H₂ potential-energy surface. Comparison is made with previous calculations of rate coefficients for the excitation of ortho-NH₃ by He.

Total L-shell ionisation cross sections have been investigated for the collision systems Ge-Ge, Kr-Kr, In-In, Mo-Mo, and Xe-Xe by measuring the X-ray yield in the impact energy range 50 keV<E_lab<3 MeV. Energy steps between 2.5 and 40 keV were selected in order to get detailed information about the ionisation threshold behaviour. The influence of multiple collisions on the ionisation process was studied by comparing the X-ray yield obtained from gaseous and solid targets. The experimental data was compared with theoretical results deduced from coupled-state, as well as simplified two-state, calculations. The authors found that in addition to the well known 4f phi -4f delta -4f pi -4f sigma rotational coupling with the initially unoccupied 4f phi MO, further couplings with orbitals originating from the M shell have to be included in order to explain satisfactorily the observed structure in the threshold cross sections.

A new version of the classical over-barrier model for multiple-electron capture in slow collisions between ions and atoms is described. This version accounts for the different ways in which an exchange of a certain number of electrons can occur. Since no adaptable parameter is involved this model has predictive power. Model predictions of absolute multiple capture cross sections are compared with experiment and are found to be in good agreement. Some dynamic effects are accounted for by introducing a velocity-dependent minimum energy uncertainty. Predicted widths of energy gain spectra and of the spectra of autoionisation electrons are found to be in reasonable agreement with experiment.

It is shown that, unlike the Oppenheimer-Brinkman-Kramers approximation a first-order capture theory satisfying Coulomb boundary conditions (B1B) yields remarkable agreement with experimental data, both for symmetric (p-H) and asymmetric (p-Ar) collisions in the intermediate velocity regime. This result suggests that a correct treatment of first-order capture ought to precede a detailed treatment of higher-order corrections.

The authors evaluate the first-order Born perturbation theory for electron capture from atoms by swift ions, with proper allowance for the asymptotic conditions for this three-body Coulomb problem as derived by Belkic et al. (1979). The authors show that the results differ strongly from those of the conventional Oppenheimer-Brinkman-Kramers approximation. Hence the magnitude of the higher terms in the Born series have to be re-evaluated. Some information on this topic is obtained by a comparison between the authors results and those of higher order approximations.

Cross sections for electron capture by H(2s) atoms in collision with H₂, He, Ne, Ar, Kr, and Xe have been measured in the energy range 1.6-5 keV. The results are compared with those of Dose and Gunz (1972), Roussel et al. (1977) and Hill et al. (1979).

Describes a theoretical study of the polarisation properties of the superradiant light emitted on a j to j' transition between degenerate levels. Two specific orthogonal polarisation vectors are associated with each set of initial conditions and the corresponding field components are shown to evolve independently during the whole linear-emission period. Due to a quenching effect, the polarisation of the emitted light is predicted to be identical to one of the two specific polarisations of the superradiant system if the intensities of the two field components are not equal during this period. If, on the other hand, the two intensities remain equal, a spontaneous symmetry breaking is expected to occur and consequently the polarisation of the superradiant light is expected to vary randomly from pulse to pulse.

The mobility of positive charge carriers in water vapour has been measured using an ion swarm time-of-flight technique. A pressure range from 40 to 290 Pa and a reduced electric field from 10 to 950 Td have been adopted. The calculated zero-field reduced ion mobility is in good agreement with Langevin's polarisation limit theory and equals 0.69 cm² V^-1 s^-1. The quadrupole mass analyser was applied as the ion detector.