Table of contents

The photoelectron spectrum of He has been measured, with very high resolution, using synchrotron radiation at 96.5 eV photon energy. The final Rydberg states in the satellite spectrum can be followed up to n=11. The relative intensities have been determined and it is found that the intensities very accurately follow the n^-3 dependence for the higher Rydberg states. Also the intensity (per eV) of the shakeoff continuum near to the threshold, relative to the main 1s single-ionization channel has been measured to be 0.20(2)% of the main 1s peak per eV.

The method of continued fractions (MCF) of Horacek and Sasakawa (1983-5) is adapted to study low-energy electron scattering by atomic and ionic helium in the static-exchange approximation as well as positron scattering by atomic hydrogen in two- and three-channel coupling levels. This is the first real application of MCF to electron-ion and multi-channel scattering problems. For all the applications studied herein, the convergence of the iterative procedure is rapid and, in most cases, monotonic. Our study reinforces the potential of this method as an efficient tool for studying electron-atom scattering. Possible applications of MCF to treat electron-molecule scattering are also suggested.

Measurements of absolute partial cross sections, integrated over the angular range 90 degrees -180 degrees , are reported for the elastic scattering of electrons from Ar⁺ at impact energies 3.5-6.5 eV. These constitute the first measurements of backward scattering and for energies below the inelastic threshold in the elastic scattering of free electrons from positive ions. Comparison with the predictions of the Rutherford formula, and partial wave analysis using semi-empirical static potentials, suggests that low-energy, large-angle scattering from singly charged positive ions provides an excellent regime for the testing of dynamic effects in electron-ion collisions.

By a meticulous six-state close-coupling calculation of positron-hydrogen scattering using the Harris-Nesbet algebraic method, we have been able to locate below the n=2 hydrogen threshold three resonances instead of just one as Mitroy and Stelbovics (1994) recently reported. These resonances might have been identified roughly by Wakid (1975) and by Seiler et al. (1971) in a calculation employing a smaller coupling scheme.

Experimental data for positron-hydrogen scattering have been made available in the recent literature. We have, therefore, recalculated the modified Glauber (MG) total cross sections for positron-hydrogen scattering with the consideration of an improved calculation for the second-Born term of the MG amplitude. The results were found to agree very well with experimental data at energies above about 40 eV.

Recent experimental advances that allow study of electron-molecule collisions at electron energies extending down to a few mu eV are reviewed. The new capabilities they afford are illustrated by considering collisions with molecules that attach low-energy electrons and with targets that have permanent dipole moments. Two different experimental approaches are discussed. The first makes use of free electrons produced by near-threshold photoionization. The electron energy can be controlled by varying the photon energy and, with the use of lasers, photoelectron energy resolutions as low as approximately 50 mu eV can be achieved. Low-energy electron scattering processes can also be investigated using atoms in high Rydberg states. For sufficiently large values of principal quantum number n, the excited electron and core ion can be considered as independent particles and the atom viewed, in essence, as a microscopic low energy electron trap with the trapping potential provided by the core ion. Measurements with very high n atoms, n>or approximately=1000, permit study of electron-molecule interactions at electron energies down to approximately 4 mu eV. The use of Rydberg atoms with low-to-intermediate values of n to probe the dynamics of dissociative electron attachment is also discussed with emphasis on determination of the lifetime of the excited intermediate and on how the excess energy of reaction is distributed between internal and translational motions of the products. The creation of weakly bound negative ions in Rydberg atom collisions is described focusing on low-energy electron attachment to van der Waals clusters and the formation of dipole-bound negative ions.

For pt.I see ibid., vol.28, p.357-72 (1995) X-ray emission transition energies and probabilities have been calculated for the 3 pi and 8 sigma core-excited resonant states of the nitrogen atoms in the N₂O molecule at the ab initio Delta SCF level. Initial- and final-state wavefunctions were generated using fully relaxed Hartree-Fock wavefunctions, and the transition probabilities were calculated with multicentre contributions. The amount of 2p electron population in the final-state valence-hole was a determining factor in producing changes in the X-ray spectra for the resonant states relative to the diagram states. Geometry optimizations were performed on the core-ionized and core-excited states to measure the effects of changes in the geometry of the initial states. Occupation of the 3 pi resonant orbital produced small changes in the initial-state geometry while occupation of the 8 sigma resonant orbital produced substantial changes in the initial-state geometry. It was determined that inclusion of multicentre contributions for each case is essential for analysing the calculated spectra. Experimental data are required to test the theoretical predictions.

High angular momentum Rydberg states of atoms in strong magnetic fields or in strong magnetic and weak crossed electric fields correspond to classical regular drift trajectories around the positive ion core. The motion can be adiabatically separated in a fast cyclotron motion, an oscillation in magnetic field direction, and a slow drift motion. Some of the drift trajectories display huge electric-dipole moments induced by the weak electric field. Corresponding atomic drift states with large electric dipole moments are theoretically predicted. In the experiment drift states are excited by an optical one-photon transition. The excitation of drift-state atoms is very efficient in the vicinity of anti-crossings between Rydberg states with large photoexcitation cross section and drift states. The drift-state atoms are separated from other Rydberg atoms via electric-dipole deflection in an inhomogeneous electric field. The size of the observed dipole moments agrees quite well with theoretical values.

We have measured the avoided crossings between the Cs (n+4)s states and the lowest three members of the adjacent n Stark manifold for 20<or=n<or=24 using a novel technique based on the differences in the pulsed field ionization of the states under study. The measured positions of the avoided crossings are in excellent agreement with values calculated by matrix diagonalization, and the experimentally determined widths of the avoided crossings, which range from 29 to 78 MHz, are in reasonable agreement with the calculated values.

The 2p-subshell photoabsorption spectrum of Mg-like silicon has been recorded for the first time using the dual laser plasma (DLP) technique. The spectrum is discussed within a framework of the evolution of 2p photoabsorption along the Mg-like isoelectronic sequence. Assignments of the Si²⁺ spectrum based on Hartree-Fock configuration interaction (HFCI) calculations are presented.

The cross section of double-electron ionization of two-electron atoms and ions in Compton scattering of high energy photons is calculated. It is demonstrated that its dependence on incoming photon frequency is the same as that for the single-electron ionization. The ratio of 'double-to-single' ionization in Compton scattering proved to be energy independent and almost coincides with the corresponding value for photoionization. For the He atom it is 1.68%. This surprising result deserves experimental verification.

Near the n=3 threshold of the residual ion O⁷⁺, the resonant photoionization of the multiply charged ion O⁶⁺ is considered by calculating the energies and the partial and total widths of the ¹P⁰ and ³P⁰ autoionizing states. These calculations are made in the diagonalization approximation in the LS coupling scheme. The results are compared with the available results obtained by authors using other theoretical approaches.

The vacuum ultraviolet photoionization spectrum of atomic bismuth, covering the region from ionization threshold to 740 AA, is reported. Comparison with prior photographic absorption studies from threshold to approximately 1320 AA (encompassing the ³P₀-³P₂ energies of Bi⁺) indicates saturation of intense autoionization features in the densitometer traces. Although fluorescence has been observed corresponding to many of these peaks, their widths imply that autoionization is dominant. Consequently, estimates of the oscillator strengths can be made from the photoionization spectrum. The peak widths and profiles differ from those in arsenic and antimony, probably due to significant departure from LS coupling. New series are observed, involving excitation of the inner valence 6s orbital, and converging to Bi⁺, ⁵S₂. They are interpreted with the aid of relativistic ab initio calculations.

We have studied the Auger decay of argon, photoexcited at and just below the 2p_3/2 inner-shell threshold. The results show how the post-collision phenomena, as seen above the threshold, slowly evolve into the resonance Auger effect, as observed below the threshold. Our experimental results suggest that also below the threshold some aspects of the existing PCI models have to be taken into account to explain the observations. In addition we propose a mechanism leading to possible interference effects in this case, but so far we have not found any experimental evidence.

New high resolution observations of the 4p inner shell and of the doubly excited states of rubidium covering the spectral region between 500 and 700 AA are reported. The spectra were recorded in the first order of a 3-meter spectrograph equipped with a 5000 line/mm holographic grating and synchrotron radiation emitted by the 500 MeV electron accelerator as the background source of radiation. More than 130 new levels of Rb I are reported which are interpreted as inner-shell transitions 4p⁶58²S_12,1/2 to 4p⁵5s(³P_2,1,0,¹P₁)ns,nd and the doubly excited transitions 4p⁶5s²S_1/2 to 4p⁵4d(³P_0.1.2)nl. All the observed levels lie well above the first ionization threshold and are arranged into Rydberg series converging onto seven conspicuous limits. The inter-channel interactions between the overlapping series have been parameterized using multichannel quantum defect theory.

We report an experimental study of a laser-induced resonance in the continuum of sodium, performed by measuring the modified atomic susceptibility. The sensitivity of the set-up has allowed the observation of the induced structure in conditions of relatively weak intensity of the dressing laser field, where the line shape is independent of the field amplitude and the comparison with theory is strict and direct. We have found a good agreement with theoretical predictions and measured the Fano parameter of the resonance.

The time-dependent dissociation probabilities, lineshapes of dissociation rates and branching ratios of photofragments between the neutral hydrogen and deuterium channels have been investigated for resonant two-photon dissociation of HD⁺ from the ground level in the presence of two laser fields of different intensities I₁, I₂ and frequencies omega ₁, omega ₂ using the resolvent operator formalism. The symmetry breaking due to non-adiabatic (NA) interactions causes splitting between the two asymptotic channels, unlike the Born-Oppenheimer (BO) approximation. Simultaneous dissociation to the continua of the two electronic states at two different energies occurs through the absorption of either omega ₁+ omega ₂ or 2 omega ₁ photons. Only the field of frequency omega ₁ was considered to resonantly couple the near-resonant level nu '=17, J'=1 or nu '=18, J'=1 to the ground level nu =0, J=0. Non-resonant discrete and continuum states significantly affect the dissociation process in the case of high intensity fields. The electric field gauge (E.d) form of interaction Hamiltonian has been used in the computation of various laser-dependent parameters. The results with NA coupling included are found to deviate appreciably from those obtained with the BO approximation.

One-electron capture involving either simple charge transfer or transfer ionization leading to the formation of Cu^q+ ions for q=1-5 has been studied for H⁺-Cu collisions in the energy range 70-720 keV amu^-1. Both one- and two-electron capture by He²⁺ ions leading to Cu^q+ ions for q=1-7 and q=2-5 in the ranges 35-360 keV amu^-1 and 35-150 keV amu^-1 respectively have been determined. A crossed beam coincidence counting technique has been used to measure cross sections for each of these processes corresponding to particular values of q. In the case of H⁺-Cu collisions, measured fractions of Cu^q+ ions formed in particular states of q have also been satisfactorily described in terms of an independent electron model with electron removal from either the 4s or 3d subshells. Comparisons between the present cross sections for H⁺ and He²⁺ impact have also been made with the corresponding values previously measured for iron.

The relativistic distorted-wave method has been used to calculate electron impact excitation of caesium to the (6p)²P_1/2,3/2, (5d)²D_3/2,5/2, (7s)²S_1/2 and (7p)²P_1/2,3/2 states with incident electron energies of 20-100 eV. The differential and integrated cross sections, differential and integrated Stokes parameters and generalized STU parameters are presented.

Metastable excitation cross section measurements are presented for the first time for deuterium hydride and deuterium. Narrow resonance features observed in the 11-13 eV energy range are compared with the previously observed resonance features in molecular hydrogen. A time-of-flight technique is used to isolate and measure the metastable molecules from the other products of the collision and this has allowed the detection of the v=0,1,2,3,4 vibrational levels in the c ³Π_u state. The positions of the resonance features are calibrated on an absolute scale and compared with earlier studies.

Recently we made calculations of differential and integral cross sections for the X¹A₁ to ³A₁(3a₁ to 3sa₁) transition in H₂O in the energy range of 12-30 eV, where the distorted-wave approximation was applied for the first time to study the electronic excitation of a nonlinear polyatomic target by electron impact. In the present work we make an extension of this calculation for that transition to the energy range 40-150 eV. Calculations of cross sections for the X¹A₁ to ³A₁(3a₁ to 3pa₁) transition in the energy range 14-150 eV are also reported. The present study is the first theoretical investigation of electron-impact excitation of the channel 3a₁ to 3pa₁ in this nonlinear molecule.

We present analytical and numerical results for the T matrix and cross sections (differential and total) for scattering in a strong low-frequency laser field. Presented results, based on recently derived expressions for the off- and on-shell low-frequency T matrices are beyond the first Born approximation results of Daniele et al. (1986) for scattering on the Yukawa potential in a laser field. An analytical expression for the total cross section is obtained using the (1,1) Pade approximant to the off-shell low-frequency T matrix. This result explicitly shows departure from the sum rule and it is analysed as function of the laser field intensity and polarization angle. If the laser field is so strong that the laser field induced scattering particle impulse eA₀ is comparable with the initial impulse p_i this departure has a maximum. For an ultra-strong laser field (eA₀>>p_i) the total cross section is small in comparison with the field-free section, i.e. the influence of the scattering potential is weak. The results for different values of the screening radius are also presented and it is observed that the departure from the sum rule is larger for long-range potentials (when the interplay between the potential and the intense radiation field is important). The departure is strong for small values of the polarization angle ( Theta _i<20 degrees ) and weak for 45 degrees < Theta _i<135.

A beam of unpolarized electrons is crossed at right angles with a beam of partially polarized sodium or potassium atoms (degree of polarization P_A approximately=21%) and resonance lines from the excited ²P_{3/2, 1/2} states are observed in a direction perpendicular to both the atom and electron beams for electron energies from threshold to 200 eV. The polarization of the emitted photons is measured in the form of the Stokes parameters P₁, P₂ and P₃. The present measurements for sodium atoms are in better agreement with the theoretical values than the previously published experimental results. No other experimental or theoretical results are available for potassium atoms.

The equations describing the evolution during the linear-emission period of the electric field of superradiance formed by transitions with partially or totally resolved hyperfine structure are obtained in a semiclassical approximation in the case of arbitrary values of electronic angular momenta of the levels and nuclear spin. These equations are used to analyse the polarization properties of superradiance from the levels initially excited from the ground state by the ultra-short linearly polarized laser pulses with small area.

It is shown that any pulse with a time-asymmetric envelope and a constant non-zero detuning excites a two-level quantum system with a probability of nearly one-half provided the pulse is very asymmetric, adiabatic in the slowly rising part of its envelope, sudden in the steep part and the peak Rabi frequency is much greater than the detuning. Three particular cases are considered analytically: the asymmetric hyperbolic-secant, the Bambini-Berman and the exponential pulse, which illustrate the general conclusions. The results are applied to a special three-level 'vee' system whose two transitions are driven by two asymmetric laser pulses of the same Rabi frequencies and the same detunings. Analysis shows that if such a system is prepared initially in its ground state, effective population depletion of this state can be achieved, the results being quite insensitive to variations in the Rabi frequency and the detuning.

We present time-resolved measurements and calculations of Li- and He-like satellites in a 2000 AA aluminium plasma created with a 12 ps KrF laser at intensities of up to 10¹⁶ W cm². The evolution of the plasma at early times following the laser beam is inferred from the ratio of two satellite lines which have the advantage of small optical thicknesses compared to resonance lines. Hydrodynamic simulations and time-dependent non-local thermodynamic equilibrium (non-LTE) calculations of the satellite intensity ratios compare favourably with experimental results for the incident laser intensities used.