Table of contents

Calculations are made of the s-, p- and d-wave contributions to the positronium formation cross section for positron - helium scattering in the Ore gap. The Kohn variational method is used with very flexible trial functions, and the results obtained are believed to have converged to within 20% of their exact values. In contrast to the findings of a recent analysis of the experimental results for positronium formation, that the p-wave contribution is dominant, the present results show that the d-wave contribution is the largest throughout most of the Ore gap. Nevertheless, the total positronium formation cross sections, calculated from the sum of the variational s-, p- and d-wave contributions and the results of the Born approximation for all higher partial waves, display a very similar energy dependence to that of the experimental results, but are approximately 30% smaller.

Electron-impact ionization of the ground state of helium is measured and calculated for the case of 64.6 eV incident electrons with coplanar outgoing 20 eV electrons. Various geometries are considered: symmetric, fixed and fixed . The method of calculation is the convergent close-coupling theory. This theory is able to reproduce the angular profiles in essentially all of these geometries, yet it yields cross sections which are a constant factor of approximately two lower than experiment.

We present results from R-matrix with pseudo-states (RMPS) and convergent close-coupling (CCC) calculations for electron-impact excitation from and direct ionization of the ground state of the ion. The CCC and RMPS results are in very good agreement with each other. The long-standing discrepancy between theory and experiment for the total ionization cross section is unresolved, with the RMPS calculations supporting the present and earlier CCC results of Bray. The present 2P excitation results are in good accord with those of Mitroy and coworkers, but the values for the 3S, 3P and 3D cross sections are substantially different. In our view, the excitation cross sections for the n = 3 and n = 4 states have been predicted accurately from threshold to 1 keV for the first time.

The use of B-spline basis sets in R-matrix theory for scattering processes has been investigated. In the present approach a B-spline basis is used for the description of the inner region, which is matched to the physical outgoing wavefunctions by the R-matrix. Using B-splines, continuum basis functions can be determined easily, while pseudostates can be included naturally. The accuracy for low-energy scattering processes is demonstrated by calculating inelastic scattering cross sections for colliding on H. Very good agreement with other calculations has been obtained. Further extensions of the codes to quasi two-electron systems and general atoms are discussed as well as the application to (multi) photoionization.

Using the Doppler-free resonantly enhanced two-photon technique, we have experimentally studied the Stark shift of Cs atom in a weak electric field and obtained the scalar and tensor polarizabilities of (n = 25 - 50) Rydberg states. The experimental results are compared with the calculated ones and the reason for the discrepancy between them is discussed.

The formation of electrons and ions in the collision between two indium atoms excited to the level is reported. The ionization cross sections and the recombination coefficient for the subsequent electron/ion recombination are measured.

We have characterized experimentally the trapping of resonance radiation in an effusive He beam produced by a needle injector. In order to take into account the influence of secondary effects, such as collisions, on the trapping process, the measurements were carried out by investigating the line intensity dependence on the atomic density of transitions , and in the visible and and in the vacuum ultraviolet region of the helium spectrum. The comparison between visible and vacuum ultraviolet spectra allowed us to characterize the excitation process and to evaluate the contribution of collisional effects. We have calculated the escape factor, g, as a function of the atomic density and of the distance from the needle injector, i.e. along the beam path. The results were also compared with the Holstein and Phelps approach describing the trapping process in terms of an effective radius, . Our results show that in the case of an effusive beam, the effective radius is of the order of the effusive beam radius, namely .

The present paper deals with a three-level model of interaction with three cross-polarized and co-propagating fields having the same frequency. The two pump waves have arbitrary intensities and the third one (the probe wave) is assumed to be non-saturating. Using the radiative renormalization method, density matrix equations are solved analytically. The forward degenerate four-wave mixing (FDFWM) signal is calculated under thermodynamic conditions where collisional and Doppler broadenings are comparable. The calculation also takes into account crossed-polarization effects. The FDFWM intensity is computed as a function of power densities and pressure. FDFWM experimental spectra of the - (0 - 0) band of OH obtained in a welding torch flame are compared to theoretical profiles. For incident laser intensities in the range of a few , a good agreement is obtained between calculated and experimental profiles. Discrepancies arise under stronger conditions of saturation, showing the limits of application of our model. However, because there is little virtue in using higher laser powers, the model should be extremely useful in practical applications. In particular, we demonstrate that the signal intensity is optimal and the pressure dependence is minimized when the saturation parameter is unity.

The prospects for producing orientation of the 1s level in highly charged ions using tilted foils is discussed. We analyse the transfer of orientation from the initial orbital angular momentum of an ion emerging from a foil to the ground-state spin via a combination of the spin - orbit interaction and cascade decays. An enhancement of the orientation can be obtained using a multi-tilted-foil technique similar to that used to orient nuclei.

Quantum close-coupled calculations of the collisional self-broadening of neon lines involving transitions whose upper level is either , , or and lower level is , , or are presented for temperatures between 70 and 400 K. These calculations are based on the quantum-mechanical impact theory of Baranger in which the interatomic interaction is represented by adiabatic molecular potentials calculated using model potentials for the electron - atom and atomic core - core interactions. We discuss the complications arising from the contributions of the highly excited states and give parametric fits to the width and shifts of all lines in the temperature range 70 - 400 K.

With wavefunctions from a kind of atomic potential model including the fine-structure (FS) interaction as a basis, we calculate the positions and widths of anticrossings due to the FS interaction for the case of K Rydberg states in a static electrical field by using the diagonalization method. The results are in good agreement with experimental data; and we confirm that the influence of the FS interaction is very small on anticrossing brought about by core interaction. The accuracy of the potential model wavefunction including the FS interaction is sensitively and numerically tested in the situation of external fields.

With a relativistic partial-wave method we calculated numerically the pair-production polarization correlations between incident photon and emitted positron in the field of atoms with atomic number Z=1, 50 and 92 for photons of energies and . In this intermediate-photon-energy region, our partial-wave results show that the Born approximation works fine for low Z elements, as expected; while the high-energy approximation is poor for the prediction of the polarization correlations, just as its prediction for the unpolarized positron energy spectrum. This indicates that the energy is not yet high enough for the cases we considered. Our partial-wave results also show that the polarization correlations are almost independent of the atomic-electron screening.

We report on the vibrational energy distribution in ions resulting from the associative ionization reaction between two Na(3p) atoms as determined via their wavelength-dependent photodissociation. Results at 10, 25, 85 and 140 meV mean collision energy were obtained for excitation of the sodium atoms with circularly polarized light. For 10 and 25 meV mean collision energy the results show a dominant population of the v = 3 state (55% and 33%, respectively). For 85 meV collision energy two maxima are found, located near v = 3 and 7, whereas for 140 meV the results show a roughly linear increase of the vibrational population with v within the wavelength range measured. These results are in agreement with previously published results for conditions where a comparison was possible. Measurements at 25 meV exciting with linearly polarized instead of circularly polarized light do not differ significantly from the results mentioned above. The measured vibrational state distributions give information on the shape, at small internuclear distances, of the diabatic molecular potential curve(s) involved in the associative ionization process.

Conflicting calibrations of the absolute photoionization cross sections of atomic oxygen in the vacuum and extreme ultraviolet are tested by a sum-rule analysis. It is shown that the scaling factor obtained by Samson and Pareek results in much closer adherence to three sum rules.

We have used angle-resolved photoelectron spectroscopy and improved R-matrix calculations to study the (n = 3 - 5) autoionizing resonances and the doubly excited resonance in the photon energy range between 44 and 48.5 eV. The photoelectrons were detected at different angles using three time-of-flight spectrometers. The measurements were obtained utilizing the 18 meV photon energy resolution provided by an undulator coupled with a spherical grating monochromator of the Advanced Light Source. Angular distribution parameters of these resonances have been parametrized using a model by Kabachnik and Sazhina and have been compared with our R-matrix calculations which are similar to those of Burke and Taylor. We find that the calculations agree quite well with the measurements. Our analysis of the cross section shape shows no strong n dependence of the q and parameter, while the width varies as the quantum defect theory predicts. Also, the differential cross section parameters agree well with previous experiments carried out for the lower resonances.

Using a crossed atomic and laser beams set-up with mass spectrometric ion detection we have studied the photoionization of laser-excited, aligned atoms from the threshold up to photoelectron energies of . Absolute cross sections are reported for parallel and perpendicular polarization directions of the linearly polarized exciting and ionizing CW lasers over the range . The cross sections exhibit (J = 2,3) autoionization resonances whose combined appearance varies with angle ; this behaviour is traced to the dependence of the partial cross sections for the final (ion + electron) channels with different total angular momentum, . Using theoretical formulae whose derivation is outlined we determine the energy dependence of these partial cross sections from least squares fits to the data measured at and . Experimental results are also presented for the asymmetry function, , which describes the linear dichroism for the photoionization of aligned atoms, and for the full dependence of the ionization cross section as obtained for 10 selected photoelectron energies in the range (0.017 - 0.846) eV. Many-electron calculations have been carried out which provide additional insight into the photoionization dynamics. In contrast to the situation for (3p, J = 3), photoionization of (4p, J = 3) is not well described by just two reduced matrix elements , . For the term dependence of the Hartree - Fock wavefunctions and correlation effects lead to substantial differences between the reduced matrix elements for the five d-wave channels associated with the formation. Satisfactory overall agreement is observed between the experimental and theoretical cross sections and their polarization dependence. The calculations also provide information on the weak g-wave contributions and on the cross section for leaving the ion in the excited level; the latter amounts to about 1% relative to formation in agreement with experimental and MQDT estimates.

A general theory is developed for the angular distribution of Auger electrons emitted by polyatomic molecules in the gas phase after ionization by electron impact. General formulae are derived which are useful as a basis for numerical calculations and for an analysis of experimental data. The Auger process is described using a two-step model. We suggest a parametrization of the molecular axis distribution function in terms of order parameters, which offers a convenient characterization of the anisotropy produced in the primary ionization. The process is discussed separately for non-degenerate and degenerate point groups. In the latter case the description of the coherences produced in the primary process requires additional parameters, which have not been taken into account in previous formulations for polyatomics but which are important for any numerical calculation of angular distributions. Consequences of molecular symmetries are pointed out for some examples.

We have demonstrated that a suitably magnetized surface can be used to retroreflect cold atoms for applications in atom optics. This has some advantages relative to evanescent wave mirrors because no light is involved. Multiple bounces of cold rubidium atoms have been observed for times up to 1 s in a trap formed by gravity and a 2 cm diameter spherical mirror made from a flexible computer disk (`floppy disk'). We have studied the dynamics of the atoms bouncing in this trap from several different heights up to 40.5 mm and we conclude that the atoms are reflected specularly and with reflectivity 1.01(3). The performance of this mirror is limited at present by collisions with the background gas and by unwanted harmonics in the magnetization of the surface. This is the first in a series of papers concerning the use of magnetized surfaces in atom optics.

A bound state and an autoionizing state of a complex atom, resonantly coupled by a multiphoton process in a strong laser field, can become degenerate at a certain laser frequency and intensity. These states are referred to as laser-induced degenerate states or LIDS. The ab initio, fully nonperturbative R-matrix Floquet method has been used to study the behaviour of LIDS coupled by three photons in argon. A simple single-photon model has been successfully applied to interpret the calculation and to predict a further degeneracy at a higher frequency and intensity.

About 70 odd-parity high-lying levels have been observed as a result of two-step laser excitation and electric field ionization. The energies are determined from the registered dependence of the observed ion signal on the wavelength of the exciting laser radiation by adding the known first step frequency. The maximum experimental error is . The investigation also includes the estimation of the radiative lifetime of some Rydberg states. The ion signal as a function of the ionizing electric field delay is considered to represent the radiative decay curve and is used for estimating the radiative lifetime. New results on the highly excited levels have been obtained.

About 40 single laser-induced transitions to Ce intermediate even-parity states by one-step laser excitation and photoionization have also been observed, some of which have not been seen previously.

We present an integrable method with B-spline basis to describe the electronic continuum of two-electron diatomic molecules. The method is applied to obtain (i) energy positions and widths of the lowest doubly-excited states of the molecule, and (ii) photoionization cross sections of in the fixed-nuclei approximation. Our results are compared with existing theoretical and experimental works. We show the existence of avoided crossings between two series of doubly-excited states, and , converging to the ionization threshold of .

Antiproton impact ionization near threshold is analysed from the point of view of an unstable dynamical equilibrium in the slow three-body break-up configuration. The instability is shown to be displaced from the Coulomb saddle - the centre of charge of the antiproton and the electron - and hence with the proton on the line between the negative pair but about six times closer to the antiproton than to the electron. The Wannier threshold law is derived, and possible negative-ion probes of these underlying Coulomb instabilities considered.

The experimental results on the right - left scattering asymmetry of helicopter-polarized atoms, either in state or colliding with Ar, are presented and qualitatively interpreted in the framework of the sudden-locking model. It is shown that this model provides a simple description of key features of the cross section difference and the relation between it and the total cross section for scattering of unpolarized atoms.

We have applied the first-order non-degenerate adiabatic (FONDA) theory to electron - scattering with exchange treated rigorously. The pure rotational body-frame and the laboratory-frame FONDA cross sections are calculated and compared with our body-frame fixed-nuclei (BF-FN) and adiabatic-nuclei (AN) results, respectively. For consistency, all the scattering calculations performed are based on identical potential models and values of input-scattering parameters. These comparisons reveal significant differences between the pure rotational FONDA and BF-FN and AN results near threshold energy range. For instance, the laboratory-frame FONDA (LF-FONDA) and AN cross sections differ by 54, and 117% for to j = 2 and to j = 4 excitations, respectively at 0.01 eV. The body-frame FONDA (BF-FONDA) and BF-FN results are also different by 53, 81, 10 and 96% at 0.0027 eV in the , , and symmetries, respectively. These differences stem from the breakdown of the target-state degeneracy assumption near threshold energy range. We also compare our cross sections for the - system with previously published results. We conclude that stable rotational excitation cross sections require significantly higher values of convergence parameters than previously used.

Absolute cross section measurements for electron-impact single ionization of (q = 4 - 8) ions and for double ionization of and are reported. The animated crossed-beams method has been applied. The measurements cover the energy range from threshold to several keV. The and ion beams are found to contain an important population of ions formed in metastable states. For single ionization, results are in overall good agreement with direct ionization theoretical predictions, when available. Excitation - autoionization is observed in the ionization threshold region for and in the K - L transition energy region for and . Double ionization of and is found to be dominated by K-shell ionization followed by autoionization.

Absolute cross sections for the double ionization of and by energy-resolved electrons have been measured from their threshold energies to 130 and 180 eV, respectively. The double ionization cross section of displays a broad rise beginning at about 80 eV which reaches a peak at around 125 eV. This peak can be attributed to a giant resonance as a result of reactions involving the excitation or ionization of a 4d electron followed by autoionization. Many steps and resonant peaks are observed, some of which can be exclusively assigned to resonant recombination followed by triple autoionization (RETA). There is no evidence of a giant resonance in the double ionization of , but a wealth of structure, due to autoionizing processes, is displayed in the cross section. The cross section of is concave at threshold.