Table of contents

New high-resolution studies of the photoabsorption spectrum of Sr I using synchrotron radiation are reported. The principal series has been extended from n=33 to n=84 and an MQDT analysis is presented. An accurate value of the ionisation potential is given as 45932.10+or-0.03 cm^-1. The present study of the odd parity states compares well with previous studies of the even parity states of Sr I by two-photon laser spectroscopy, which do not extend above n=60, and the authors' value of the ionisation potential agrees with an earlier average value obtained from both even and odd parity states.

Spectra of cadmium and indium in the far-ultraviolet wavelength region have been obtained from laser-produced plasma. The authors report the observation of lines belonging to three consecutive ionisation states, Cu I, Ni I and Co I like, as well as unresolved transition arrays belonging to other degrees if ionisation in the wavelength range between 15 to 80 AA, and their interpretation based on ab initio relativistic computations.

Nitrogen and oxygen K X-ray emission spectra from nitrous oxide (NNO), excited by electron impact, have been studied using the grazing incidence technique. Interpretation of spectra was made with the aid of ab initio calculated one-centre intensities and photoelectron spectra. Core electron binding energies were determined from an energy calibration using XUV emission lines.

The trajectory effect for a C_nr^-n (n=2, 3 and 4) potential is estimated and compared with the results obtained according to the approximate method of Vallee et al. (1976), based on the use of effective rectilinear trajectories.

The emission anisotropy of isolated MO transitions was measured in 40 MeV Ni+Ni collisions by detecting K Mo X-rays, emitted at 90 degrees and 0 degrees with respect to the beam direction, in coincidence with characteristic Ni K X-rays. A tenfold increase in the anisotropy was observed for the selected transitions over that associated with the total spectrum. Recent calculations agree with this large anisotropy.

The authors study the 'elastic' scattering of fast electrons by atoms in the presence of a laser field which, although strong by laboratory standards, is weak compared with the atomic unit of field strength. The 'dressing' of the target due to the laser field is shown to produce important effects on the scattering at small momentum transfers, in particular when photons are exchanged with the field. As an illustration, detailed calculations are performed for 'elastic' electron-atomic-hydrogen scattering accompanied by the transfer of no photons and one photon.

The first results of an eleven-state R-matrix calculation on electron-helium scattering are presented. The resonance structure near the n=3 states of helium is examined and resonance parameters calculated. There is good agreement with recent experimental observations of the 2³S and 2¹S metastable-state excitation cross sections. Earlier five-state calculations are shown to have significantly overestimated these cross sections above 22 eV.

A field-theoretic optical potential approach is applied to the elastic scattering process in the e⁺-H(1s) system. The s-wave phaseshift is evaluated below the threshold for Ps formation ( epsilon _i<0.5 Ryd) by using an approximation, Sigma ^SPR, to the optical potential, containing information on the static, polarisation and rearrangement contributions to the full dynamics. The computations identify the virtual Ps formation channel as the leading contribution to the dynamics in the elastic scattering process.

For pt.II see ibid., vol.16, no.16, p.2871 (1983). The circular orbit method (COM) which has been proposed in parts I and II is extended to coupled oscillator systems. The authors also develop a new method (they call it the phase correlation method (PCM)) which enables one to calculate a particular state manifold. They apply the COM and PCM to calculations of state energy in the Henon-Heiles and Barbanis potential systems and examine the efficiency of them.

Structural changes in molecules are studied from the viewpoint of Landau's theory of phase transitions (1969) and of the universality argument about critical exponents. Free energies are constructed for molecules utilising the concepts of order parameter and external variable, and a macroscopic description follows by analogy with usual thermodynamics. These methods are applied among other cases to the study of the chemical potential for the electrons in systems like H₃/H₃⁺ and NH₃/NH₃⁺. Examination of the conformational changes along the isoelectronic family SiH₃/PH₃⁺/SH₃²⁺ reveals that structural transitions in molecules seem to corroborate the hypothesis of a universal critical exponent.

The transitions 4d-4f and 4d-5p of Y V are analysed to determine all the 30 levels of 4p⁴ 4f, the 21 levels of 5p and the 7 levels of 4d configurations along with three new 5d levels.

The L_beta X-ray emission spectra of elements Nb (Z=41) to Sb (Z=51) are determined by a high-resolution double-crystal spectrometer. The L₁ linewidths of the elements Zr to Sb obtained show a drop at Sn due to closing of L₁-L₃M₄₅ Coster-Kronig (CK) processes. A large discrepancy is found between the theory and present experimental data for the L₁ linewidths of the elements Z<or=49 (In). This is interpreted as due to a poor description of L₁-L₃M₄₅ CK processes by theory.

For pt.I see ibid., vol.17, no.9, p.1735 (1984). The L_x (x= beta _2,15, beta ₆, gamma ₁, gamma _2,3, gamma ₅, eta and l) X-ray emission widths of the elements Nb (Z=41) to Sb (Z=51) have been measured accurately by using a high resolution double-crystal spectrometer. The lifetime of M₁, N₁, N₂₃ and N₄₅ holes of the elements Nb to Sb are determined. They are in good agreement with the XPS linewidths.

By combining results of hook and emission measurements, closed loops for six transitions of Cu I in the visible and ultraviolet regions could be obtained. A numerical procedure suggested by Cardon et al. (1979) was applied to give improved relative gf values for these transitions. Nine further gf values could be linked to these without making any assumptions on the state of the plasma and yielding a set of 15 relative gf values altogether. Hook measurements were performed using a wall-stabilised arc, intensity measurements by means of a hollow cathode discharge.

The isotope shifts of the levels 5p²¹D₂, ¹S₀ and 5sns ¹S₀(n=7, 8) of strontium have been measured between the isotopes 84, 86, 87 and 88 in two-photon transitions from the ground state using two lasers with different wavelengths. These levels occur below the beginning of the Rydberg series, and are known to exhibit very strong configuration mixing. Mass and field isotope shifts are calculated ab initio by means of the MCHF method for these levels and 5s4d ¹D₂, 5s6s ¹S₀. The qualitative agreement between theory and experiment confirms the necessity of taking into account the mixing (up to 50%) with both the 5p² and 4d² perturbers in the wavefunction expansions.

Energies and widths of autoionising states in the helium isoelectronic sequence (Z=5-10) converging to the n=3 and n=4 hydrogenic thresholds are computed. States of symmetry ¹S^e, ¹P^o, ¹D^e are considered. Using the Feshbach formalism the eigenvalues of QHQ are calculated by the truncated diagonalisation method of Altick and Moore (1965) or Lipsky et al. (1977) and the continuum functions by the static exchange approximation. The author computes both total widths and partial widths associated with the various open channels.

The photoabsorption spectrum of neon in the range 500-700 AA has been investigated at high resolution using synchrotron radiation and a 3 m normal incidence vacuum spectrograph equipped with holographic gratings. All of the five Rydberg series 2p⁵(²P₃2/)ns(3/2)₁⁰, nd(1/2)₁⁰ and 2p⁵(¹P₁2/)ns(1/2)₁⁰, nd(3/2)₁⁰ have been extended to high principal quantum numbers yielding accurate values for the ionisation potentials (²P₃2/=173 929.6+or-0.2 cm^-1 and ²P₁2/=174 709.8+or-0.2 cm^-1). In contrast with the corresponding features of heavier rare gases, the ns-nd interchannel interactions and autoionisation broadening are shown to be weak. The behaviour is discussed with reference to multichannel quantum defect and Hartree-Fock theory, and is shown to result from centrifugal barrier effects in the nd channels.

Experimental oscillator strengths are used to obtain empirical configuration parameters in Zn 3d⁹4s²mp (m=4, 5, 6), Cd I 4d⁹5s²5p and Hg I 5d⁹6s²6p, by a method described in an earlier paper (Martin, 1984). All the known levels of Zn I 3d⁹4s²4p and Hg I 5d⁹6s²6p may be satisfactorily described in the single-configuration approximation. The deduced empirical configuration parameters of Cd I 4d⁹5s²5p lead to the reassignment of the experimental ejected-electron spectrum of Pejcev et al. (1972).

The relative distribution of the ions between the 2s and 2p states in the photoionisation of helium near the n=2 threshold is examined. The influence of short-range correlations and the long-range Coulomb potential and the dipole coupling between the 2s and 2p states is separated by applying multichannel quantum defect theory. It is argued that a measured value of Gamma =1.48*10^-4 eV and q=3.55 for the 3- resonance would be consistent with sigma (2p)/ sigma (2s) approximately 3.5 at the threshold.

The ultraviolet bands of diatomic mercury iodide extending from 3095.0 AA to 2110.0 AA have been investigated under moderately high resolution and dispersion. All the previously known systems, namely C-X, D-X and one component of the E-X, could be recorded with good intensity. The vibrational assignments in the C-X system could be confirmed using mercury isotope shifts. A large number of new bands involving higher vibrational quanta have been observed in the reinvestigated systems. The new measurements coupled with computer analysis of this have enabled the authors to determine precise vibrational constants for X, C, D, E₁ and E₂ states of this molecule. A new vibrational analysis is proposed for the E-X system and the nature of the E state is shown to be ² Pi . Approximate rotational constants have also been determined for the C state.

The temperature dependence of the absorption profile of Hg-Ar and Hg-Kr mixtures in the range 30-1500 cm^-1 about the centre of the Hg 1850 AA resonance line is studied in the temperature range 393-1123 K. Interatomic potentials for Hg-Ar and Hg-Kr in their ground electronic state ¹0⁺ as well as in the excited states ¹0⁺, and ¹1 originating from Hg(6¹P₁) are deduced from the data for internuclear separations R between 3.3 and 5 AA. Furthermore the transition moments are found to be independent of R in this R range. Some features of the profile such as satellites or undulations are discussed.

The authors have measured the absorption coefficient in the impact region of the first potassium resonance lines broadened by rubidium atoms. For the broadening constants gamma _K*-Rb they have obtained the values (6.2+or-0.5)*10^-9 s^-1 cm³/atom and (7.8+or-0.5)*10^-9 s^-1 cm³/atom for the stronger and weaker components, respectively. The theoretical values calculated from the Lindholm formula are 6.0*10^-9 s^-1 cm³/atom for the stronger and 7.0*10^-9 s^-1 cm³/atom for the weaker component. The corresponding theoretical effective C₆ constants are 21.6*10^-30 s^-1 cm⁶ and 15.3*10^-30 s^-1 cm⁶, respectively.

The authors report measurements of detached electron energy spectra yielded in H^- collisions with He, Ar, H₂, N₂, O₂ and CO₂ in the energy range from 4.5 eV to 4 keV. This work allows in particular the study of the dynamics of charge exchange to shape resonances in H^--N₂, CO₂ and O₂ collisions. No charge exchange is observed for H₂. Further spectroscopic information on O₂^- is obtained.

The total K-shell ionisation cross sections were measured relative to nuclear Coulomb excitation cross sections for ₆₄Gd, ₇₄W and ₉₀Th targets bombarded by ⁴He and ¹⁶O projectiles. In the case of ₇₄W, projectiles of ¹²C and ¹⁹F were also applied. The experimental results are compared with the predictions of the SCA-LAL model, and the overall agreement is very good. The data are presented as reduced cross sections and compared with a 'universal' cross section function.

It is proposed to solve a coupled-channel scattering equation representing the effect of all but a few channels by an optical potential. This optical potential is expanded on an L² basis and it is shown how the resulting pseudoresonances can be eliminated using the 'equivalent weight' hypothesis of Heller (1973). These ideas are applied to a model problem posed by Burke, Berrington and Sukumar (1981) and accurate singularity-free solutions are obtained. The extension of the method to electron-atom scattering is discussed.

The electron impact excitation of the 2¹S state of helium has been studied using the method of Das (1981) in the energy range 50-500 eV. The results for differential and integrated cross sections show good agreement with the experimental measurements over the entire angular scattering region.

The 3s-3p impact excitation of spin-polarised Na atoms by unpolarised electrons has been studied. The fluorescence light emitted in the decay of the excited state exhibits linear polarisation caused by the collisional induced 3p alignment, as well as circular polarisation resulting from the spin orientation of the 3p electron at the time of the decay. Since the exchange interaction causes a spin flip, the degree of circular light polarisation is a measure for the contribution of the exchange channel to the total excitation cross section. At small impact energies where exchange becomes important, the authors found a decrease of the circular polarisation. They analysed the hyperfine coupling between the excited electron and the polarised nucleus, and calculated the light polarisation from theoretical excitation cross sections. Comparison of experiment and theory shows reasonable agreement.

The secular approximation is used to calculate an analytic expression for the intensity correlation function for two atoms undergoing cooperative decay. The analysis is valid for high incident field strengths and arbitrary frequency detuning.

A simple analytic expression for the double optical resonance spectrum obtained when a two-level atom is excited by a noisy pump laser is derived for two different models of laser noise, the phase diffusion model, and a model in which the laser bandwidth arises from a random telegraph signal in the laser phase. The double optical resonance spectrum is identical for both models and therefore double optical resonance experiments cannot distinguish between them. The importance of transients in cases where the laser bandwidth is comparable with or less than the atomic linewidth is stressed. Experiments which can distinguish between the two sources of bandwidth are identified.