Table of contents

Absolute two-photon detachment cross sections were determined for irradiation of oxygen, copper, silver and gold negative ions at a wavelength of 1064 nm. A 12 ns Q-switched ND:YAG laser and ion beams of keV energies were employed. Results were derived from measurement of the onset of depletion saturation at higher laser intensity. The respective cross sections in units of 10^-49 cm⁴ s are 0.42 (-0.16, +0.19) (O^-), 1.6 (-0.6, +0.7) (Cu^-), 3.1 (-1.1, +1.3) (Ag^-) and 0.39 (-0.16, +0.19) (Au^-).

The eikonal partial wave theory for the electron impact excitation of positive ions is formulated and then demonstrated in the 1s to 2s, 2p excitations of hydrogen-like ions.

Electron-sodium scattering is calculated at 20 eV by the coupled-channels optical method with eight channels (3, 4, 5s; 3, 4, 5p; 3, 4d) explicitly coupled and the ionization continuum represented by the half-on-shell optical potential. In comparison with experiment differential cross sections for 3s and 3p, 3p angular cross sections data and asymmetry are well described. Some discrepancy remains with the 3s asymmetry. The calculation is compared with less detailed calculations to identify the effects of different sets of channels.

The alignment A₂₀ of Zn⁺ (3d^-12D_3/2) ions formed by electron impact has been measured via the linear polarization of fluorescence radiation gamma (5894 AA) in the energy range of primary electrons from 25 eV down to the threshold at 17.5 eV. From the alignment value obtained at threshold, (A₂₀)_th=-0.32 (3), it is concluded that two-electron partial waves with L=0, 1 and 2 (and possibly with L)2) contribute to the Wannier threshold cross section.

Destruction of Xe₂⁺ molecular ions by electrons in He-Xe plasma is investigated in the electron temperature range T_e=0.03-2 eV. The rate constant for the Xe₂⁺ ions dissociation by electrons is measured for the first time. This process appears to be more effective than the dissociative recombination at electron temperatures T_e)1 eV.

The laboratory frame rotational close coupling approximation has been employed to calculate electronic excitation of H₂ (X ¹ Sigma ⁺_g to B ¹ Sigma ⁺_u) by positron impact. Two electronic states (X ¹ Sigma _g⁺ and B ¹ Sigma _u⁺) and three rotational states in both channels are retained in the calculations. The electronically elastic, excitation and rotational cross sections in the energy range 15 to 75 eV have been reported. The coupling effect reduces the electronically elastic results. Present laboratory frame results are compared with body frame results of Mukherjee et al (1991). Above an incident energy of 25 eV the present electronic excitation results differ significantly from those of Mukherjee et al.

The N=1 and 2 series of the ¹P⁰ helium states have been investigated by partial-wave analysis, and by observing the one-body densities, the radial-radial correlation functions and the shape-densities. The details of electron-electron correlation have been revealed. The features of structure of different subseries have been compared with each other. A broad radial-angular correlation region extending beyond the radial-radial correlation region has been found in the (1, 0)^- and (-1, 0)⁰ states. It is also found that the anisotropism and the extent of distribution of the one-body density are closely related to the classification scheme.

It is commonly believed that an atomic system with a 2s electron gives a lower energy than that of a system with a 2p electron. The author shows that in a doubly-core-excited four-electron system, the energy of the 2s²2p²³P state is substantially higher than that of the 2s2p³⁵S ⁰one. In the case of Be I, it is higher by 0.48 eV. The energies for the 2s2p³⁵S⁰ and 2s²2p²³P states are calculated for Z=3 to 10. Relativistic corrections are included. The reason for this 2s-2p inversion is the exchange interaction, which is particularly important in the formation of a bound state or resonance in a negative ion.

New photoabsorption data of gallium in the 4s-subshell excitation region is presented covering the spectral region between 800-1500 AA. The observations were made in the first order of a 1 m vacuum spectrograph equipped with a 1200 lines/mm grating using synchrotron radiation as the background source of the continuum. Long overlapping Rydberg series have been observed converging to four limits built on the 4s4p parent ion configuration: 4s²4p ²P₁₂ to 4s4p(³P_0.1.2 , ¹P₁)np. A few transitions from the thermally populated 4s²4p ²P_3/2 level have also been detected. It is remarked that the level structure in the photoionization region is dominated by the autoionizing resonances.

Atomic transition rates and f values have been calculated in the triplet and quintet systems and for some intercombination lines of neutral oxygen. Configuration interaction was included in the calculations performed with the CIV3 code of Hibbert (1975) for all the transitions connecting the n=3 and n=4 energy levels. In order to reproduce the observed energy splittings between the energy states, small empirical adjustments were introduced to the diagonal matrix elements. Comparisons with previously published f values and radiative lifetimes are discussed and the subsequent astrophysical applications of the results are briefly mentioned.

The authors present an approach that allows the study of the influence of velocity-changing collisions (VCC) on the spectrum of resonance fluorescence. They give a theoretical framework yielding general, though quite complicated results. Working in the dressed-atom basis and describing the VCC by a strong-collision model, they focus their attention on the particular case of well separated spectral lines, which leads to relatively simple expressions. The results allow the authors to compare the effects of Doppler shifts and VCC on the profiles of the central line and on the spectra sidebands. They predict that VCC cause a narrowing of the spectral lines, which they attribute to Dicke narrowing. This prediction is also confirmed by numerical results. The adopted method of dealing with VCC in terms of collision operators can be applied to a larger class of collision models than just the strong-collision model used here.

The authors propose a method to detect and study the non-linear dynamic Lorentz shift in an atomic vapour by applying a controlled intense fixed frequency optical field detuned from resonance and then measuring the spectral reflectivity of a weak excitation which is scanned through the atomic resonance. Even though the nearby pump is intense the reflectivity response is not power broadened, has a Lorentzian response and exhibits a non-linear shift from which the dynamic Lorentz shift can be extracted.

A study has been made of photodissociation processes in O₂ which produce autoionizing fragments. Tunable VUV radiation was used to obtain photoelectron spectra. These contain structure due to electron emission from states of atomic oxygen decaying to the ground and first excitated states of the ion. The investigation has concentrated on the region of the thresholds for the production of the atomic autoionization, and spectra have also been recorded up to 60 eV photon energy.

A joint experimental and theoretical study of the double-differential cross sections (DDCS) for ejection of electrons from helium by 1.00 and 1.84 MeV amu^-1 p⁺, He²⁺, C⁶⁺ and O⁸⁺ is presented. Electron-emission angles from 20' to 160' are surveyed, and the electron energies range from 4 to 2500 eV. Single-differential and total cross sections, obtained by integrating the DDCS over angle and electron energy, are also presented. Large departures from the q² dependence (where q is the projectile charge) predicted by the first Born approximation are observed in the cross sections. The deviations, which are seen to increase with projectile charge are discussed and compared with calculations, using the first Born approximation and a continuum-distorted-wave eikonal-initial-state model.

Experimental results for the integral cross sections for the emission of Na(3p to 3s) and Na(4p to 3s) light after H+Na collisions at energies from 15 to 1500 eV are reported. The data are compared with model predictions.

Charge transfer into He⁺ (nl) states (n)5) in collisions (2-13 keV amu^-1) of He²⁺ on atomic hydrogen has been studied by means of photon emission spectroscopy. State selective electron capture cross sections into the degenerate He⁺(4l) states have been determined by deconvoluting the intensity distribution of the He II (n=4 to n=3) emission along the total ion-beam path inside the collision chamber. They are compared with theoretical results, especially with the large-basis set extended atomic orbital calculations of Fritsch (1988). Generally good agreement with these extended calculations is found. However, the structures in the theoretical cross sections for capture into the non-dominantly populated n=4 states are not observed experimentally.

The authors complete a preliminary study of slow C³⁺+H collisions, performed with an effective two-electron molecular approach by considerably enlarging the number of molecular states of the basis. The paper reports a detailed study of the characteristics of singlet and triplet correlation diagrams, as well as non-adiabatic couplings and their bearing on the transition mechanisms.

For pt.I see ibid., vol.24, no.18, p.4049-60 (1991). The authors report a treatment of slow C³⁺+H collisions for impact energies E=0.04-9 keV amu^-1, using an expansion in terms of 22 molecular states modified with a common translation factor. Calculated partial and total cross sections for charge exchange are in good agreement with measured ones. Cross sections and values for the alignment parameter for C³⁺ excitation are predicted. A detailed study of the collision dynamics is presented.

Theoretical and experimental isotope shifts for Ca I and Sr I levels are presented. The isotope shifts of the levels 4s6s ¹S₀, 4p²¹S₀ and 4p²¹D₂ of calcium have been measured between the isotopes 40, 42, 43, 44 and 48 in two-photon transitions from the Ca ground state using two lasers with different wavelengths. Results of ab initio mass and field shift calculations are presented for these levels as well as for a number of other strongly perturbed low-lying levels in calcium and strontium using the multiconfiguration Hartree-Fock method. The 'gradient' and the alternative 'Slater' forms of the specific mass shift operator have been tested; the latter is found to be unreliable with the MCHF expansions which are limited to valence correlation effects.