Table of contents

Conditions are discussed for the binding by the polarization interaction alone of positronium to ions and positrons to atoms. For positronium with doubly charged ions and for positrons with the heavier alkalis it is argued that the long-range interactions may be sufficient to yield binding.

We have studied the ion kinetic energy spectra produced by fragmentation of into single ions in high-intensity femtosecond laser pulses at 780 nm. was observed to dissociate completely in the intensity range to producing ions from to with kinetic energies up to 1 keV. For peak intensities the explosion was found to be isotropic and the major features were reproduced by a model based on Coulomb explosion. At higher intensities, however, the kinetic energy distribution was found to be significantly anisotropic.

Using a simple exact analysis of the one-photon optical transition amplitudes for the emission of correlated electron pairs from randomly oriented targets it is shown that information on the phase differences of these amplitudes can be obtained by the variation of the helicity of the absorbed photon. Experiments performed with different polarization of the photon can be related to each other by analytical formulae that should be used as a consistency check. The usefulness of this approach is demonstrated by analysing recent experimental and theoretical data.

Recently Balashov, Martin and Crowe reported on measurements of the angular dependence of the resonant part of the transition matrix for auto-ionization from spherical symmetric states. When post-collisional interaction (PCI) is neglected, the resonant part of the transition matrix for this kind of collision is independent of the electron emission angle. On the other hand, when PCI is include in the theory, an angular dependence becomes evident. This is illustrated by calculating the effect of PCI on the resonant part of the T -matrix for auto-ionization from the spherically symmetric state. It is argued that it is the distance in momentum space rather than the real space distance that is decisive when PCI effects are important.

Absolute stopping powers of Ne, Kr and Xe for ions were measured for energies between 50 and 200 keV. The measurements were made using a gas cell with a differential pumping system and a electrostatic energy analyser. The author has previously shown stopping powers of He and Ar for the same ion of the same energy. We now have stopping powers of all the rare gases for the ion in this energy range. The results are compared with the stopping powers of the rare gases. All the measurements show that it might be difficult to hold the velocity proportionality in the studied energy region.

This article considers the scattering of particles off complex, central-field potentials. Starting from the Schrödinger radial-equation, we find that the partial-wave -element can be decomposed as a sum of two contributions, , due to the real and imaginary parts of the potential. We obtain element as the asymptotical limit of a single, ordinary, first-order, nonlinear, differential equation. This equation is only associated to the imaginary part of the potential. We apply the procedure, combined with the semiclassical uniform WKB method, to the scattering of and systems.

Electron energy loss spectra in the region below and above the Ne 1s ionization threshold have been measured at several scattering angles between and and at a fixed scattered electron energy of 1.6 keV, while the impact energy has been scanned from 2.46 to 2.52 keV. Singly and doubly excited core states, corresponding to and transitions, respectively, have been observed. The generalized oscillator strengths for both types of excitations have been derived from the angular variation of the double differential cross section and ab initiocalculations have been performed using two methods based on a configuration-interaction approach.-1

Absolute differential and total cross sections for single-electron loss and capture were measured from ions on Kr in the energy range of 1.0-5.0 keV. The reduced differential cross sections as a function of are shown to scale reasonably well for both products studied. For single-electron loss, our cross sections are found to be of the order of magnitude of , while for single-electron capture the magnitude is . The measured total cross sections are compared with theoretical models available in the literature.

The capture of slow antiprotons (energies less than 1.0 au, i.e. roughly 27 eV) by the rare gas atoms helium, neon and argon is considered. Appropriate to this low velocity, the capture cross sections are calculated using the adiabatic `hidden-crossing' theory in which the collision complex is viewed as a transient diatomic molecule with the positively-charged atomic ion and antiproton as nuclei. In addition to the total capture cross section, estimates of the percentage population of long-lived `circular' states is given. Our calculations suggest that a few per cent of captured antiprotons occupy these states in helium but in the case of argon or neon the probability of primary capture into such long-lived states is negligible. These results are at variance with previous calculations of antiproton capture cross sections.

We present measurements of absolute cross sections for low- and intermediate-energy elastic electron scattering from , which have been measured independently on different experimental apparatus in two laboratories. The results are compared with a number of previous measurements and calculations. Whilst there are some interesting differences between recent experimental determinations, the largest discrepancies are observed between experiment and contemporary scattering theory at very low energies.

A previously detected negative-ion dipole-supported state of is identified as a virtual state whose energy is about eV at J=K=0 , where Jand Kare the rotational angular momentum and its component about the symmetry axis. For higher Jand low Kthe S -matrix poles in the complex energy plane move off the real axis and typically do not correspond to quasibound states, i.e. there is no time delay in scattering. If Kis close to J , the S -matrix poles correspond to quasibound states and may appear as Feshbach resonances.

Stark widths and shifts for twenty-four lines of Sn II and three lines of Sn I have been measured and calculated. For the 8s-6p, 7p-5d, 7d-6p, 8d-6p and 5f-5d transitions in Sn II and 5p6s- in Sn I they are the first experimental values known. Measurements are carried out by observing the emission lines from an optically thin laser produced plasma from a tin target in Ar atmosphere. The electron temperature was close to K and the electron density . Calculations are based on the semiempirical method proposed by Griem with atomic matrix elements obtained by using the Hartree-Fock approximation including relativistic corrections and - - configuration interactions.

The simple Thomas-Fermi model is widely used to give reasonably accurate approximations for the electron component of the free energy in the equation of state. The well known atomic scaling means that the model can be used to efficiently cover a wide range of materials. Unfortunately, the limited validity of these approximations normally requires a large data table if this approach is used repetitively. In this paper we examine the scope of the available approximations and show how an additional one can be constructed, such that the entire domain of density and temperature is nearly encompassed.

Auger decay from the C s core-excited state of CO has been studied by means of angular resolved vibrationally selective electron spectroscopy. It is observed that the so-called strict spectator model provides only a qualitative description of the C Auger decay. We interpret this as evidence for a strong valence character of the 3s orbital. Transitions assigned to the Auger shake-up are revealed. For transitions to the final state with s leading configuration, there are indications of angular asymmetry which depend upon the vibrational level of the final state.

By means of a time-of-flight technique, we have measured cross sections for production of and ions from a gas-phase C₆₀target bombarded by 0.4-5.0 keV electrons. The results were in fairly good agreement with other data at overlapping energies below 1 keV. Semiclassical calculations of the single-ionization cross sections were also performed up to 10 keV using the Deutsch-Märk formula proposed for the molecule. A fairly good agreement between the experimental and theoretical cross sections was obtained both in magnitude and in energy dependence. It was found that the double and triple ionization cross sections both decrease monotonically and exhibit no hump structure, indicating that the inner-shell ionization does not play an important role in electron-impact multiple ionization of C₆₀ .

We extract information about collisions of ultra-cold ground-state rubidium atoms from observations of a g-wave shape resonance in the system via time-independent and time-dependent photoassociation. The shape resonance arises from a quasi-bound state inside a centrifugal barrier that enhances the excitation to the bound electronically excited state by the photoassociation laser in the time-independent experiment. The shape resonance is sufficiently long-lived that its build-up through the barrier can be observed by first depleting it via a photoassociation laser pulse and then measuring the rate of photoassociation by a second laser pulse with a variable delay time. A combined method of analysis of the time-independent and time-dependent experiments is presented. We discuss the spectroscopy of states of two particles with spin trapped inside a centrifugal barrier, interacting via direct and indirect spin-spin interactions.

Within the framework of the two-step model we derive specific equations to characterize the intermediate singly ionized atomic state as well as the angular distribution and spin polarization of the Auger electrons where the incident photon beam can be arbitrarily polarized. Different experimental set-ups are discussed in detail. In particular, if the photon beam is 100% linearly polarized perpendicular to the reaction plane the Auger electrons become unpolarized and show an isotropic angular distribution within this plane. We introduce ionization parameters which allow for a factorization of the orientation and alignment parameters into the photoionization dynamics and the polarization state of the photons. Estimates are given for the case of the rare gases. The relative line intensities and a complete set of angular distribution and spin polarization parameters for the Xe Auger spectrum have been calculated. With some physical assumptions, predictions for all components of the spin polarization vector become possible. Our results are compared to recent experimental data where good agreement is obtained.

Imperfect atomic state preparation in an optically pumped beam resonator results in over-Poissonian fluctuations of the fluorescence emission of residual unpumped atoms. This excess noise is induced by the frequency noise of the pump laser. We evaluate it theoretically for a three-level atomic system interacting with a noisy laser field, and show that it is predominant within a large range of pumping efficiencies and laser linewidths for usual atomic fluxes. Experimentally, extra noise is detected from ratios of unpumped atoms as low as 0.01% of the atomic flux equal to . This noise source debases the short-term stability of an atomic clock in a flop-in configuration if the proportion of unpumped atoms exceeds 1% of the global atomic flux. In a flop-out configuration this excess noise would decrease the clock stability from a proportion of unpumped atoms of 0.01%.

We report on the observation of a field phase dependent autoionization rate of calcium in the region of the doubly excited state. Excitation of the autoionizing state occurs from the atomic ground state through two phase related and hence interfering channels, namely a three photon channel and a single photon channel , being the third harmonic of . The autoionization rate exhibits a sinusoidal modulation as a function of the relative phase of the two excitation fields. Both ionizing fields are not focused in the interaction region, thus demonstrating the possibility of phase control in a large interaction volume and free of phase shift effects associated with focused geometries.

The line profile of several Cu autoionization levels have been measured experimentally by the optogalvanic technique. The optogalvanic signals related to the autoionization states have a distinct signature, namely a fast initial response which is the result of an essentially immediate ionization of these levels. As a result, the initial response overlaps the temporal profile of the laser. This is followed by a usual optogalvanic signal (OGS) reflecting the relaxation of the plasma to its equilibrium conditions on plasma timescale. A tunable pulsed dye laser is used to obtain OGS and to scan the resonance profile of several Cu autoionization levels. The transitions show the characteristic asymmetric Fano line profiles. The detection of fast autoionization processes as well as measurements of saturation broadening in some of these levels are demonstrated.

The coherence length concept and path integrals are applied for the quantum electrodynamical consideration of multiple scattering effects on high-energy radiation in a medium (the Landau-Pomeranchuk-Migdal effect). The multiple scattering of a fast charged particle in matter is treated as a Brownian motion process. A new general equation for the average probability density of polarized radiation, depending on the energy and direction of the emitted photon, is obtained. We have also performed a numerical analysis and compared angular distributions of high-energy radiation in oriented and non-oriented crystals of tungsten as an example.

We have studied the laser intensity effect on the vibrational branching ratio (VBR) in -photon resonance-enhanced two-photon ionization of molecules, considering the electronic autoionization through the lowest autoionizing (AI) state of symmetry. In this ab initiocalculation we have studied resonant two-photon transitions from the ground state to the doubly excited autoionizing state of symmetry as well as to the electronic continuum ( : ) of molecules via the resonant intermediate levels. Transitions to two different continuum energies (-0.4500 au and -0.5853 au) have been considered; the first one is 1.36 eV above the dissociation threshold (-0.50 au) of the ion and the second one is 0.048 eV above the vibrational level of the ion in the ground state . Parallel transitions through other intermediate near-resonant rovibrational levels v= 0-3 and 5-20 with the allowed values of total angular momentum jof the state have been included in the calculation. Besides the effect of interference of direct ionization with autoionization channels, we have considered here (i) the effects of mutual interference among parallel autoionization channels and that among parallel ionization channels via the near-resonant and resonant rovibrational levels of the state and (ii) the effect of the two-photon coupling between intermediate rovibrational levels via the autoionizing state and the continuum in order to study the laser intensity dependence of vibrational distribution of ions in the ground state. It has been shown that these two effects (channel interference effect and two-photon coupling effect) lead to non-Franck-Condon type VBR and the values differ from those obtained by considering the direct ionization channels alone. The laser intensity dependence of VBR is also different from that obtained by neglecting the autoionization channels. For transitions to two different continuum energies these effects become prominent at two different laser intensities. Furthermore, it has been found that the transitions through different intermediate resonant rotational levels of the state give rise to different vibrational branching ratios.

The photodetachment process from the state is studied in detail. Cross sections and angular distributions are calculated with photon energy from threshold up to 7.5 eV, which covers the energy region below the n= 4 threshold where nis the principal quantum number of the outermost electron of the Be targets. The cross section of the photodetachment to the channels shows a resonance at 3.944 63 eV, immediately below the ) threshold.

Excitation of helium atoms by 50-500 keV proton impact to singly excited states was investigated experimentally. By measuring the intensities of He I spectral lines as functions of an axial electric field applied to the collision volume, we analysed the electric charge distributions of collisionally excited states with principal quantum numbers n = 4, 5and 7. The experimental results reflect the transition from intermediate- to high-energy processes. States with large electric dipole moments were found in the lower part of the investigated energy range indicating a quasi-molecular evolution of the collision system. States with dominant components and accordingly very small electric dipole moments were found for 400 and 500 keV proton impact, signifying that the high-energy limit is reached.

The collisional system is studied as a test case to assess the efficiency of a full quantum time-dependent method to calculate charge-exchange cross sections. The molecular diabatic electronic channels are calculated at the CASSCF level from the adiabatic potential energy curves and the radial coupling matrix. The elements of the collision matrix are extracted, for each value of the total angular momentum K , by Fourier transforming the post-collisional amplitudes of the wavepackets which are prepared in the entry channel and propagated on the coupled effective channels for each value of K . The results are compared with previous calculations of cross sections obtained by solving the stationary scattering equations and with recent experimental results. The close-coupling wavepacket method also provides a time-dependent picture of the collision dynamics for any Kvalue. The kinematic isotopic effect is also examined.

The adiabatic energies and non-adiabatic corrections of the bound and lowest-lying quasi-bound S states of the light Coulombic three-particle systems, the negative positronium and muonium ions and the neutral He atom, are evaluated using the hyperspherical harmonics formalism. The calculations are based on the adiabatic separation of the hyperradial and the hyperangular degrees of freedom. The non-adiabatic corrections are determined using high-order Rayleigh-Schrödinger perturbation theory. Although the adiabatic perturbation series do not always converge, they can be accurately summed for isolated states applying Wynn's algorithm. The perturbation treatment exhibits high numerical stability with respect to the number of basis functions and it is therefore possible to perform these calculations employing very large basis set expansions. The perturbation series pertaining to the well-isolated ground states are summable even in the diabatic representation.

Cross sections for electron-impact ionization of and ions have been calculated in the distorted-wave approximation. In the present calculations both direct-ionization and excitation-autoionization processes are included. For ions, the cross sections are calculated for ionization from both the ground configuration and the excited configuration. Excitation-autoionization substantially contributes to the ionization of the excited configuration and is approximately a factor of four above direct ionization at energies below the ground-state threshold. For ionization from the ground configurations of both and ions, the direct process dominates the total ionization cross section, and contributions of excitation-autoionization are very small. The present results are in reasonable agreement with the experimental measurements of Thompson and Gregory (Thompson J S and Gregory D C 1994 A 501377). Remaining discrepancies between the experiment and theory are discussed.

The Schwinger variational iterative method combined with the distorted-wave approximation is applied to the calculation of differential, integral and momentum transfer cross sections for elastic electron- scattering in the 20-800 eV energy range. In this study, a complex optical potential consisting of static, exchange, correlation-polarization plus absorption contributions, derived from a fully molecular wavefunction, is used for the electron-molecule interaction. It is shown that the introduction of absorption effects influences significantly the calculated differential cross sections in the intermediate and high incident energy range. Particularly, in the 50-300 eV range, the used model absorption potential has improved significantly the agreement between calculated results and the available experimental data.

We have recorded very-high-resolution photoelectron spectra of the inner-valence structure of , excited with synchrotron radiation, and have resolved fine structure never previously observed. Recently presented resonant Auger decay spectra (Sundin et al1997 Phys. Rev.A 56480, 1997 J. Phys. B: At. Mol. Opt. Phys.304267) have established the energy of two-hole one-particle Rydberg states, and a comparison with these has been made to explain some details of the spectra.

We perform angular non-critical phase-matching third-harmonic generation (THG) experiments in (KTP). From these experiments, we determine the magnitude and sign of the two phase-matched nonlinear coefficients of by a complete analysis of phase-matching properties of the cascaded quadratic and pure cubic processes, the resolution of the coupled propagation equations and by the calculation of the quadratic and cubic dielectric susceptibility tensor by an extended bond charge model; we find: and . We also demonstrate that the two studied THG are mainly governed by the cubic interaction.

The classical trajectory Monte Carlo method has been used to calculate the final product principal quantum state -distributions in electron capture collisions involving a proton incident on a Rydberg hydrogen atom in an state. The generalized eccentricity, defined by orienting the classical eccentricity of the electron's orbit relative to the incident ion, was varied to show how the final state -levels depended on this quantity over the range of reduced collision speeds -2.4. Plots of the final product -distributions at low reduced velocities show a resonance peak near at all generalized eccentricities. At speeds of -2.2, two peaks arise in the final product -distributions from collisions with circular targets. In these cases, one peak is centred on the resonance point of , while the second peak migrates through higher -levels as increases and is believed to possibly be a new collision mechanism. This high -level peak reaches a maximum around before disappearing completely from the final product -distributions by , leaving only the resonant peak near .

We have studied the excitation of an atomic hydrogen by , and ions in the range of intermediate and high impact velocities, v= 1-7 au. The theoretical method is based on a one-centre close-coupling expansion of the time-dependent wavefunction, which includes a discretized representation of the ionization continuum. The calculated cross sections are in very good agreement with the available experimental measurements up to n= 6. In contrast, some discrepancies are observed for dipole-forbidden transitions when comparing with previous theoretical data. The results show the validity of the Janev-Presnyakov scaling law for both optically-allowed and forbidden excitation transitions.

Differential cross sections for the elastic scattering of electrons from the ions , , , and have been calculated employing model- and pseudopotential and quantum-defect methods. Comparison with recent experiments showed satisfactory agreement for and but poorer agreement for and targets. Generally, model-potential results were in better agreement with experiment than those obtained using pseudopotentials.

Single-channel quantum defect theory (QDT) is used to approximate the Born-Oppenheimer electronic wavefunction of Rydberg diatomic states. The development is based on an extension of quantum-mechanical and semiclassical approaches for the estimation of molecular transition dipole matrix elements as a function of internuclear distance R . The theory is tested by a calculation of the singlet and triplet transition moments of the molecule involving Rydberg states with n= 2-4; l= 0-2. For small and moderate internuclear distance the agreement with ab initiodata is found to be generally good, and in some cases identical results are obtained. The disagreement is attributed to a breakdown of the one-channel QDT representation of the interacting electronic states for large Rvalues. The predicted moments for - and - transitions remove existing discrepancies between theoretical and accurate experimental lifetimes for and states.

Electric-dipole intercombination and forbidden transitions have been optically observed with ions circulating in a storage ring, and atomic lifetimes determined. For the level in the Be-like ion , the measured lifetime of ms corresponds to a transition probability of . This result ties in with experimental work on the neighbouring ion and with recent calculations. For the corresponding level in the Mg-like ion , a lifetime of s (transition rate ) has been measured, corroborating earlier experiments but not the more recent calculations. The transition probability of the M1/E2 transition between the fine-structure levels of the ground state in F-like has been determined as , in agreement with the results of semi-empirically corrected multi-configuration Dirac-Fock calculations.