Table of contents

The authors have studied atomic hydrogen in intense, short, visible laser pulses (I<or approximately=1.2*10¹⁴ W cm^-2, tau approximately=0.5 ps, lambda approximately=608 nm). At high intensities (I>6*10¹³ W cm^-2) they observe a new series of higher order above-threshold ionization peaks in the photoelectron energy spectrum. They discuss some tentative explanations. Two models are described in some detail. One relies on ionization saturation, the second one may be named 'parking of atoms as Rydberg wavepackets'.

A rotational laboratory frame close-coupling approximation has been employed to investigate positron-CO scattering below the Ps formation threshold. In the calculation ten rotational states (j=0-9) are retained and a parameter-free positron correlation polarization potential following Jain is used. The present LFCCA predictions differ appreciably from the ANR predictions. The difference in results is due to the failure of the ANR theory to include the rotational motion of the target molecule dynamically. The result shows the necessity of using rotational LFCCA to investigate the scattering of molecules having a permanent dipole.

The rate of production of solar neutrinos is very sensitive to small variations in the radiative opacity of the material at the centre of the Sun. Relatively small changes to the predicted values of the solar radiative opacity are therefore of particular relevance to the solar neutrino problem. The author shows that the effect of partial electron degeneracy on the photoionization cross sections is to reduce the most recent values of the solar centre opacity by 2%.

Slater determinants and the group-theoretical basis of Racah are expressed in terms of quark-like states by means of explicit examples drawn from the atomic f shell. Four distinct quark-like objects (which, for convenience, the authors refer to simply as quarks) together with two parity labels are required. Racah's group SO(7), which acts on the seven states of an f electron, is augmented by SO(7)', the group acting on the seven states of an f quark. The assignment of group labels is illustrated by means of the commuting generators of these groups. The components of S_z and Q_z of the spin and the quasispin are constructed as operators acting on two quarks at a time, and their eigenvalues, M_s and 1/2(N-7), are used to identify states generated by the shift operator L_-. The complementary group to SO(7)' is exemplified by two ¹N terms of f⁶ and f¹⁰ spanning a two-dimensional irreducible representation of the permutation group for the four quarks. Connection is made with the quark model for the p and d shells.

The electron intracule (relative motion r=r₁-r₂) and extracule (centre of mass R=1/2(r₁+r₂)) pair densities for the first-row hydrides (LiH-FH) in their ground states have been calculated from both self-consistent-field (SCF) and correlated wavefunctions. For each molecule, the radial and the projected pair densities along (longitudinal) and perpendicular to (transverse) the molecular axis are displayed and analysed as well as the correlation shifts on these pair densities. The effects of electron correlation on the moments of the pair distributions are also discussed. For the cases studied, the pair densities show monotonic changes from LiH to FH, in contrast to the results reported by Sarasola et al. (1990) for the same SCF wavefunctions. This monotonic behaviour may be ascribed to two factors: the electronic structure of the heavier atoms and the chemical bonding in these hydrides.

The authors present an analysis of the effect of a stray electric field on 7snl and 10snl double-Rydberg spectra for n=12-15. To excite these states they use the Stark switching technique to populate high l, along with the isolated core excitation (ICE) method. The 7snl spectra show a single peak for each l excited, while, as observed for 8dnl states, the 10snl spectra show a multiple peak structure common to all l (for n>12). This structure is explained by taking into account a 1 V cm^-1 stray electric field which l mixes the 6snl levels from which the 10snl are excited. Application of a small, <or=10 V cm^-1, static electric field allows one to identify the different l components in the 10snl spectra. With the spectral components identified in this manner, the authors have measured the 7snl and 10snl quantum defects and have determined their l dependence.

The 6pnf J=2,3,4 autoionizing levels of barium are studied through a combination of the eigenchannel R-matrix and the multichannel quantum defect theory (MQDT) methods. The calculation reproduces well the spectra of 6pnf series observed in excitation from 5d6p levels and confirms that the 6pnf levels have very large autoionization rates. Calculated MQDT parameters are compared with those obtained empirically from experiment and used to analyse channel interactions.

Measurements of the self-induced pressure shift of the helium triplet line lambda 706.5 nm (1s2p³P_2.1,0-1s3s³S₁) from a gas discharge have been carried out at temperatures of 77 K and 300 K using a scanning Fabry-Perot interferometer. The shift coefficient is positive (a blue shift) and at 77 K it is consistent with the results of a theoretical calculation carried out by Leo et al. (1992). At 300 K, the measured shift parameter is much greater than the theoretical value. As expected, the shifts are significantly larger in both cases than those of the corresponding singlet line, for which the broadening and shift are dominated by the resonance interaction. The authors have also compared their experimental results with predictions based on the Lennard-Jones potential.

The influence of angle-dependent post-collision interaction on the lineshape of the N₅-O₂₃O₂₃¹S₀ Auger transition in xenon has been measured for the angles theta ₁₂=55' and theta ₁₂=165' between the Auger electron and its coincident photoelectron. Good agreement is found with the theoretical predictions which confirms the present angle-dependent PCI model in this angular range, but raises the question of how to derive correctly the non-coincident PCI formulation.

The strong radiative single photon coupling between two excited states of sodium atoms (3p and 4d) has been studied at high intensities, exceeding 10⁹ W cm^-2. These excited states are probed by Raman-like resonant five-photon ionization processes. Using two tunable lasers the authors have observed that the doubly resonant process does not occur. In the model picture of photon-dressed atomic levels, this situation can be described as an avoided crossing, with a large minimum separation of 30 cm^-1.

Fluorescence excited in CaF by fixed-frequency lines of a krypton-ion laser has been recorded at high resolution by Fourier transform spectrometry. The main aim-to characterize the lowest ² Delta state-was realized, following the analysis of lines of transitions between levels of C² Pi , excited by the laser, and B'² Delta . The spectra also provide some new information about C² Pi , and the results confirm recent work which shows that the accepted vibrational numbering in D² Sigma ⁺ must be increased by one. Constants, cm^-1, for B'² Delta are as follows: T_e, 21573.93; A_e, 12.34; omega _e, 528.57; B_e, 0.3295₀.

In the framework of a multiple scattering model the authors investigate the ionization of atomic hydrogen by the impact of an arbitrary structureless charged projectile. The dependence of the energy and angular distributions of the ionized electrons on the mass and charge of the projectile is analysed. It turns out that differences between cross sections for particle and antiparticle impact have the same origin for all particles investigated here. For small momentum transfer and for a given projectile charge the dependence of the cross section on the projectile mass in general follows the scaling law given by the Bethe-Born theory. In some cases for large momentum transfer the scaling law does not hold. The dependence of the height and position of the binary and recoil peaks on the projectile charge is directly traced to final-state interactions.

In atomic collision experiments with laser-excited target atoms it is often desirable to maximize the fraction f* of excited atoms. In the case of Na atoms pumped by a single mode CW dye laser radiation several effects including hyperfine pumping limit f* to about 10%. Using two side-bands of laser radiation, produced by means of an electro-optical modulator (phase modulation of laser radiation by a LiTaO₃ crystal in a microwave field) transitions from both hyperfine ground states of Na can be pumped simultaneously. In this way a significant increase of the fraction of excited Na atoms in the 3²P states to typically 30% could be achieved. The advantage of using an electro-optical modulator is demonstrated for the case of state-selective single electron capture studies of the He²⁺-Na*(3p) collision system.

Fon and Lim (1990) have reported the 11-state and the 19-state R-matrix calculations of 1¹S-2³S differential cross sections at low energies. The discrepancy between the R-matrix calculations at scattering angle 90º and the experiments remains at 20%. Another attempt has been made here to reduce the discrepancy. Sawey et al. (1990) reported a 29-state R-matrix calculation on the collision strengths for the 1¹S-2^3,1S excitations and an error in their representation of the 19-state collision strengths, which were wrongly shown to be significantly higher than the 29-state results, is corrected. In this paper, the same 29-state R-matrix calculation of Sawey et al. is extended to obtain 1¹S-2^3,1S excitation cross sections at 500 energies ranging from the 2³S threshold to 24.2 eV for scattering angles 30, 55, 60, 90, 120, 125 and 140 degrees. The convergence of the R-matrix calculations in shape and magnitude of the cross sections is re-examined.

Stokes parameter measurements have been made of the decay radiation in an electron-photon polarization correlation study of the helium 3¹D state. Small electron scattering angles q_e between -40' and +20' were studied for an incident electron energy of 40 eV. The first efforts to simultaneously determine all three Stokes parameters from the analysis of the variation in the coincidence rate as a retarder is rotated in front of a fixed polarizer in the photon analyser are reported. Measurements of the linear polarization Stokes parameter P₂ and the circular polarization P₃ agree well with previous sequentially acquired, discrete angle measurements. However, a significant disagreement with previous results was observed for the linear polarization parameter P₁.

The authors measured the total ionization asymmetry A in dependence on the incident electron energy E. Here 'total' refers to integration over all emission angles and energy partitions of the outgoing electrons. From a threshold value of A=0.125 the A(E) curve rises smoothly toward a broad maximum of A_max=0.31 at E_max=8.3 eV. The fall-off towards higher energies is quite similar to that of the other one-electron atoms. However, it shows a structure which can be explained by contributions from autoionizing P states. Bartschat (1990) has made a theoretical estimate of A(E) for various atoms. At low energies the agreement with the authors' data is satisfactory, at higher energies the experimental A values are considerably smaller than the theoretical ones. In the threshold region measurements were performed with small electron energy width (0.1 eV). No structure in the A(E) curve was found. The slope at threshold was determined as dA/dE=(0.136+or-0.005) eV^-1.