Table of contents

Translational energy spectroscopy has been used to study one-electron capture by 0.5-2.0 keV amu^-1 He²⁺ ions in collision with H₂. The main excited state product channels have been identified and the relative cross sections determined. These measurements provide the first direct evidence of the relative importance of dissociative excitation channels in electron capture (with He⁺ formed in the ground state and H atoms mainly in the n=2 states) which dominate the entire energy range. Non-dissociative electron capture into the n=2 and n=3 states of He⁺, which is also observed, increases from 1% of the total electron capture cross section at 0.5 keV amu^-1 to about 25% at 2 keV amu^-1.

The electron affinities of the ground states of Sr and Ba have been calculated using a finite-element multiconfiguration Hartree-Fock method. The valence correlation value for the electron affinity of Sr is 135 meV. Core-valence correlation reduces the electron affinity by 121 meV. By adding relativistic and core-core correlation contributions, the final binding energies of 16 and -6 meV are obtained for Sr^-(2)P₍₁2)/ and Sr^-(2)P₍₃2)/, respectively. For Ba, the valence limit is 279 meV, and the core-valence correlation correction is -144 meV. The final binding energies of 113 and 56 meV are obtained for Ba^-(2)P₍₁2)/ and Ba^-(2)P₍₃2)/ by adding relativistic and core-core correlation corrections to the core-valence correlation value.

A practical polarization propagator method devised for the treatment of valence electron excitations in atoms and molecules is presented. This method, referred to as (second-order) algebraic-diagrammatic construction (ADC(2)), allows for a theoretical description of single and double excitations consistently through second and first order, respectively, of perturbation theory. The computational scheme is essentially an eigenvalue problem of a Hermitian secular matrix defined with respect to the space of singly and doubly excited configurations. The configuration space is smaller (more compact) than that of comparable configuration interaction (CI) expansions and the method leads to size-consistent results. The performance of the ADC(2) method is tested in exemplary applications to Ne, Ar and CO, where detailed comparison can be made with experiment and previous theoretical results. While the accuracy of the absolute excitation energies is only moderate, a very satisfactory description is obtained for the relative energies and, in particular, for the spectral intensities. Aspects related to the Thomas-Reiche-Kuhn sum rule and the equivalence of the dipole-length and dipole-velocity forms of the transition moments are discussed. Due to the relatively small computational expense and the possibility of a direct ADC(2) formulation this method should prove particularly useful in applications to large molecules.

The electronic and geometrical structure of neutral and cationic Hg₂ and Hg₃ molecules are calculated using the all-electron Dirac-Fock-Slater SCF method, with relativistic numerical atomic basis functions. An improved calculation of the direct Coulomb potential has been taken into account in order to get a numerically accurate potential energy surface. The binding, ionization and excitation energies have been compared with experimental results as well as other theoretical results.

A new analytical expression of a previously proposed Klein-Gordon dipole matrix elements in the quasiclassical approach (including quantum defects) is presented. The intermediate state method in the semiclassical Coulomb approximation is used to derive the Klein-Gordon dipole radial integrals corresponding to single-electron nlj to n'l'j' transitions with arbitrary quantum numbers in non-hydrogenic ions. This last approach is extended to the second-order Dirac-Coulomb equation. Similar expressions are obtained in the two electromagnetic field gauges which in the non-relativistic limit give the length and velocity forms of the transition operator. A computational procedure for the evaluation of the Dirac formulae by the use of recursion relations, expressed in terms of Anger's functions, is also described. Numerical applications of the above-mentioned WKB methods, starting from the well known Schrodinger dipole matrix elements, are carried out for the calculation of the lowest 2s_1/2-2p_1/2,3/2 transitions in the lithium isoelectronic sequence for atomic number Z=3-92. Oscillator strengths for Rydberg 2s_1/2-np_1/2(Ca¹⁷⁺, n=8-16 and Zr³⁷⁺, n=3-20) and ns_1/2-20p_1/2,3/2(Ca¹⁷⁺, Zr³⁷⁺, W⁷¹⁺, U⁸⁹⁺, n=19,20) transitions are also reported. The values obtained are compared and discussed with available experimental and other theoretical treatments.

The effects of laser bandwidth and Doppler broadening on the population dynamics of the ladder configuration which consists of three levels of Gd such as ⁹D₅⁰ (999.121 cm^-1), 5d6s6p ⁹F₄ (17973.611 cm^-1) and 5d6s7s ⁹D₄⁰ (33534.095 cm^-1) are studied numerically using the Bloch equation. In particular, for the examination of the bandwidth effect we choose a finite-bandwidth quasi-monochromatic laser and describe its field as a set of phase-independent modes. The remarkable fact is noted that the laser bandwidth of 1 GHz causes radiation-atom interactions to be very strongly incoherent, so the three-level populations are the same throughout almost all the interaction-time region.

Two-photon ionization of helium and beryllium in the region of (2,n) and (3,n) autoionizing series is investigated in the perturbative regime. The problem involves above threshold ionization as well as multichannel coupling for series lying above the second ionization threshold. Our method is based on the Feshbach formalism in the context of L²-basis functions. It allows us to parametrize the ionization cross section in the region of resonances by generalizing the usual Fano and Starace forms to multiphoton processes. The resonance parameters are given for (2,n) and (3,n) series of helium and (3,n) series of beryllium lying below the Be⁺(3s) threshold. Symmetries ¹S^e and ¹D^e are considered.

Absolute photoionization cross sections from the laser-excited Sr I (5s5p) ¹P^o₁ state are reported showing a giant resonance with a peak cross section of 5600 Mb. The absolute calibration was obtained with a saturation method as published by Burkhardt et al. In the present study, however, the spatial distribution of the photons in the laser pulse was measured with a CCD camera and applied in the evaluation procedure as a Gauss profile. The uncertainties in the absolute values are estimated to be less than 20%. Based on a 12-state non-relativistic close-coupling R-matrix calculation, the giant resonance is assigned as predominantly (5p²) ¹D₂ with a strong admixture of (4d²) ¹D₂. While an ab initio calculation including only discrete physical target states was unable to reproduce the resonance structure correctly, reasonable agreement between experiment and theory was obtained by a semi-empirical simulation of additional short-range correlation and relaxation effects.

The 3p⁵ns'( 1/2 )_0.1 autoionizing resonances of argon have been investigated using a two-step excitation from the 3p⁵4s(3/2)_1.2levels (1s₄ and 1s₅ in Paschen's notation) populated in a discharge. Most of the levels of J=0 had never been observed previously. The absorption profiles have been recorded by means of a sensitive optogalvanic detection. Energy position and width data have been obtained for n ranging from 11 to 34. Our experimental values allow a precise redetermination of the second ionization limit I_1/2 and show that the width Gamma ₀ of the J=0 level is larger than the width Gamma ₁ of the corresponding J=1 level. We find I_1/2=128 541.41+or-0.07 cm^-1and Gamma ₀/ Gamma ₁=1.6+or-0.2, respectively.

We present a new adiabatic approach for non-perturbative treatment of multiphoton above-threshold detachment (ATD) in intense laser pulses. The electron energy distributions for the pulse detachment are expressed via the detachment transition amplitudes in the monochromatic fields, and the latter are calculated with the help of the adiabatic theory based on the smallness of the laser frequency compared with the electron affinity or ionization potential. The theory is applied to the first study of the pulse shape effects on the multiphoton ATD of the negative ion H^- by 10.6 mu m radiation. Two pulse shapes are considered: a Gaussian pulse and a square pulse with smooth edges. We present the angle-resolved as well as angle-integrated ATD electron energy distributions. They contain oscillatory satellite structures to the main peaks due to interference of the electrons detached on the rising and falling edges of the pulse. Simple analytical formulae describing these subpeak structures are also presented.

The inner-shell photoabsorption spectrum of gas-phase HBr below the Br-3d ionization thresholds was measured with high spectral resolution. Ligand-field splitting of the Br-3d levels could be clearly resolved in photoabsorption, in agreement with observations made in recent photoemission work. The good signal-to-noise ratio of the present photoabsorption spectrum allowed to follow the ligand-field splitting of 3d-core-excited np Rydberg states up to high quantum numbers (n=9), and to assign some additional weak transitions that could not be resolved before.

Information on the structure of the ions of neon, argon, krypton and xenon dimers has been obtained using threshold photoelectron spectroscopy and synchrotron radiation under conditions of high resolution. Vibrational structure has been well resolved for the ground state of all these species thus allowing accurate values for the spectroscopic constants to be derived. Structure corresponding to excited states of the dimer ions dissociating to the ²P_3/2,1/2 levels of the atomic ion have also been identified and their dissociation energies determined.

Absolute emission cross sections for the excitation atomic and ionic lines of nitrogen are measured in the VUV region of 50 to 130 nm in the He⁺ projectile energy range 1-10 keV. Excitation of the intense lines are caused by formation of core-excited intermediate molecular states of N₂⁺* in the charge exchange processes. In particular the most intense line N I (120.0 nm) can be excited by formation of a 2s sigma _g hole in the ground state of the N₂ molecule. This intermediate one-hole molecular state of N⁺₂ can be formed by configuration interaction with ² Sigma ⁺_g core-excited molecular Rydberg states.

Electron scattering from hydrogen-like He⁺, Fe²⁵⁺ and U⁹¹⁺ ions has been studied using the R-matrix method with non-relativistic and relativistic approximations. The case of transitions 1s-2s and 1s-2p as well as transitions between fine structure n=2 levels was considered. Collision strengths have been calculated for the energy range from the n=2 excitation threshold to the ionization threshold. Effective collision strengths obtained by integrating collision strengths over Maxwellian distribution of electron velocities were compared with results of other calculations.

The chiral effects, electron-optic activity and electron-optic dichroism, are discussed for the forward scattering of polarized electrons by zero-spin molecules. Four coupled first-order differential equations are derived for the change in beam intensity and polarization as an electron beam passes through an ensemble of molecules; the relation between the solution of these equations and the chiral effects is fully discussed. General results are derived for molecules with the same fixed orientation in space and also for randomly oriented molecules. The four coupled equations are solved numerically for a model achiral oriented molecule showing that the effects can be quite appreciable.

In the study of low-energy electron and positron scattering from atoms one of the most important interactions which must be taken into account is that due to the polarization of the target by the incoming particle. We present here a generalization of the polarized-orbital method, put forward originally by Stone and Reitz (1963), for the determination of polarization potentials to describe this interaction for both elastic scattering as well as excitation processes when the dipole polarizability of the atomic system is relatively large.

Experimental and theoretical small-angle electron differential cross sections (DCSS) for excitation of H 2 ²P, He 2 ¹P and Li 2 ²P states from their respective ground states are investigated through the generalized oscillator strength (GOS) and compared with the GOS from the recent universal function in an attempt to delineate the behaviour of dynamic effects as a function of impact energy, E. The results cover the impact energy range from near threshold to the strong Born approximation regime. We find the surprising result that for the Li 2 ²S-²P transition the dynamic effects first decrease as E increases from 10 to about 20 eV where they reach a minimum, manifested through reasonable agreement between measurement and the universal curve over a wide range of the momentum transfer squared, K² values, 0.01<or=K²<or=0.1 au. Thereafter, for a given K², away from the asymptotic region, their importance increases until the Born approximation regime is reached. In contrast, dynamic effects in H 2 ²P and He 2 ¹P persist over a wide range of E values all the way to the Born approximation limit where it agrees with the universal function GOS, indicative of their diminished significance to zero.

We report extensive new data on the single-colour two-photon optogalvanic spectra of neon in the visible region. The experimental method enables us to record a pure two-photon spectrum in a region which is full of one-photon transitions. A large number of two-photon transitions originating from two metastable 3s(3/2)₂, 3s'( 1/2 )₀ states and one non-metastable 3s(3/2)_I state of the 2p⁵3s configuration of neon have been recorded and arranged into 19 Rydberg series. The interchannel interactions among the overlapping Rydberg series have been studied using the multichannel quantum defect theory.