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We use the two-electron oscillator (with Coulomb interaction between the electrons) as a test system for the calculation of singlet - triplet splittings of excited states. Apart from exact solutions, we obtain values from simple and weighted (Ziegler, Rauk and Baerends (ZRB) and von Barth) total energy differences. We also calculated differences of Kohn - Sham (KS) eigenvalues (both exact and LDA values) and included corrections to these values proposed by Petersilka, Gossman and Gross (PGG) and by Görling (G). It turns out that ZRB, PGG and G are virtually equivalent for low-energy excitations, whereas for higher excitations, PGG and G underestimate the necessary correction to the Kohn - Sham energy differences considerably. ZRB, on the other hand, is reasonable for a wide range of excitation energies and much better than simple total energy differences. The best results are obtained from a new ad hoc formula using non-integer occupation numbers in a way different from Gunnarson and Lundqvist.

We report the first observation of linear dichroism in the photo-induced Auger electron emission from aligned atoms. The linear dichroism in the Cr Auger spectrum shows striking similarity to the corresponding linear dichroism in the 3p photoelectron spectrum. The theoretical analysis presented here proves that the linear dichroism in the Auger spectrum depends on the same dynamical parameters as the linear dichroism in direct photoionization.

The multiphoton ionization of was studied using linearly polarized femtosecond laser pulses from a Ti:sapphire laser. Comparing the ion signal versus intensity curve of the singly charged molecular ion with that of an Ar atom with a similar ionization potential, it was found that the rate of ionization of the molecule is much lower than that of the atom. The observation is explained based on the fact that due to the two-centre nature of the potential which produces the barrier for tunnelling, and the orientation of the molecular axis with respect to the laser polarization, the effective barrier is equivalent to that produced by an atom with an effective charge of less than unity. Accordingly, a model is developed which agrees very well with the experimental observation.

In this work we report the first measurements of the interaction of 60 fs laser pulses at 750 nm with vibrationally excited . Comparison with ground state is made and enhanced kinetic energies for and observed. These observations are consistent with unusually large bends in the molecule, induced by the laser field.

We study the dynamics of initially localized states in biased dissipative TLSs evolving under a time periodic force simulating monochromatic light. Accurate numerical calculations are presented using an iterative procedure to evaluate realtime path integrals. In the high-temperature and moderate to high friction regime, the system exhibits exponential decay. In this regime the decay constant approaches a plateau value that depends only on the intensity of the driving field. Simple analytical calculations using a Landau-Zener approach predict correctly the value of the decay rate. Bias does not affect significantly the plateau value of the decay constant provided that the asymmetry is small relative to the amplitude of the driving field. In the low-friction regime, phase relations are important and there is nonmonotonic variation of the delocalization rate. Weak friction broadens the window for extending the lifetime of localized states. At very strong friction the decay rate diminishes monotonically.

The angular asymmetry parameter of the photon-induced fluorescence radiation of fine-structure resolved satellite lines was measured from threshold up to 38.4 eV. It is well known that the cross sections of these valence shell satellites are dominated by the decay of aligned doubly excited states leading to an extremely complicated energy dependence. In contrast, we observe that the parameter becomes less structured with increasing total angular momentum J of the photoion. The measurements can be interpreted by a partial-wave analysis of the photofragmentation products. For the population of the ion state only photoelectrons are emitted on all resonances. For the population of the and the ion states the emission of the parity-unfavoured photoelectrons dominates.

The first ab initio calculations of the refined ion energy structure through consideration of the interelectron interactions and the photodetachment cross section (partial and total) in the vicinity of the shell threshold are reported. The resonance behaviour is found to be associated with the excitation and autodetachment of the quasibound `' state to 3p, 3d and 4s channels.

In this paper and in a subsequent one, we present relative and absolute measurements of photodetachment cross sections of negative gold ions in an RF trap. Several laser devices needed to be used. Absolute measurements are possible only when saturation occurs for a reasonable focusing of the laser and are deduced from the variations of photoelectron yield versus laser pulse energy. We describe the principle behind our measurements and we report on the results obtained in the energy range from threshold up to about above threshold. In this region, we essentially observe a slow departure from Wigner's law (always less than 50%).

The efficiency of the two photoionization schemes

and

has been studied by a spectral simulation approach for the isotope-selective excitation of the isotope. Isotopic selectivities have been computed for (555.648 nm), (581.027 nm) and (398.8 nm) transitions for various laser bandwidths of the excitation lasers. The degree of enrichment of the low-abundant isotope has been computed using the obtained isotopic selectivity values. Results obtained for scheme 1 are in good agreement with the results reported earlier. The abundance of isotope has been computed for various laser linewidths of the excitation laser and Doppler widths for scheme 2. Comparison of the two photoionization schemes for isotope-selective photoionization of the isotope has been made. The present computational method has been examined for computation of degree of enrichment for the photoionization schemes under consideration.

The influence of autodetaching states of upon the total and partial detachment rates of its ground state are considered. The R-matrix Floquet method is used, in which the coupling of the ion with the laser field is described non-perturbatively. The laser field is varied over a range of intensities, while frequency ranges are chosen such that the lowest three even-parity autodetaching states of the ion are excited through a two-photon transition from the ground state. Total and partial photodetachment rates are found to be highly sensitive to the laser intensity when the intensity is close to that associated with laser-induced degenerate states of the ion. A model is presented to support this interpretation.

Energy distributions of electrons ejected into the forward direction in 15-214 keV collisions were measured in coincidence with charge-state analysed outgoing projectiles. All spectra show a cusp-shaped peak centred at an electron velocity equal to that of the projectile. The peaks observed in coincidence with are associated with the electron-projectile Coulomb final-state interaction. On the other hand, the physical origin of the cusp-like structure observed in coincidence with neutral outgoing projectiles is still unknown. Here we present some new evidence that seems to indicate that this peak can be attributed to the dipole interaction between the emitted electron and an excited state of the outgoing projectile. The fractions of the cusp yields show that the electron transfer into the continuum process associated with emerging becomes more important for decreasing projectile energies.

In view of recent experiments it can be suggested that the two-step process is not negligible even at high impact energy. An ab initio calculation has been performed on the basis of the second Born approximation using different correlated wavefunctions for the initial bound state. This calculation also shows that the contribution of the two-step process must be included at any impact energy.

Using a unified theoretical treatment of different ionization processes occurring in photon and electron impact on atoms and ions, we examine lithium near its `hollow-atom' threshold. By introducing a set of `core base states' for the target, and optimizing the collisional representation within an R-matrix model, we combine ab initio, outer- and inner-shell effects, excitation and ionization channels, direct and indirect ionization processes, resonant and non-resonant behaviour, for photon and electron impact processes. We use the method to interpret experimental data on electron impact ionization of ground state and photoionization of Li, and look at double photoionization of Li and electron impact ionization out of metastable . We examine and quantify hollow-atom interference effects and resonances, tabulating all 480 states for , and show the chaotic behaviour of overlapping series.

The momentum-transfer cross section for positronium-He scattering has been determined by analysing the thermalization process of orthopositronium in the gas. The momentum distribution of the positronium is observed by using the one-dimensional angular correlation of the annihilation radiation method. Silica aerogel is used to form a sufficient amount of positronium in a restricted region necessary for the high-resolution measurements. The average energies of orthopositronium for mean lifetimes ranging from 3.2 to 86 ns are determined by applying static magnetic fields. A thermalization model which includes the momentum-transfer cross section as an adjustable parameter is fitted to the average energy. The cross section thus obtained is for positronium in the energy range below 0.3 eV.

In a positive column plasma of glow discharge in neon the population of metastable atoms ( configuration) is substantial. Linearly polarized pulsed laser light excited these atoms to one of the configuration levels, and the direct fluorescence was observed with its polarized components resolved. From the temporal development of the intensities of these components, the disalignment (depolarization) rate due to atom (neon or helium) collisions was determined. From its atom-density dependence the rate coefficient was determined, and from the temperature (42-650 K) dependence of the rate coefficient, the disalignment collision cross section was estimated as a function of energy in the energy range of the order of 10 meV. The cross section of the (Paschen notation) atoms has positive dependence on energy, while that of the atoms has a negative dependence. The cross sections thus determined serve as a quantitative test of the molecular potentials to the order of .

In order to confirm that the temperature of the metastable atoms in the plasma is equal to the temperature of the surrounding wall, an additional experiment was performed; the line profile of the CW laser-induced fluorescence was observed for a plasma at liquid nitrogen temperature, and the metastable atom temperature was found to be 78 K.