Table of contents

We investigate the effect of the relativistic mass shift of the electron on adiabatic (high intensity, high frequency) stabilization by numerically integrating the relativistic Schrödinger equation in one dimension. We find that a relativistic treatment implies better stabilization than would be expected non-relativistically. We propose an explanation which involves consideration of the force acting on the electron within the Kramers - Henneberger frame.

The use of double translational energy spectroscopy has been successfully demonstrated to allow studies of state-selective electron capture with state-prepared primary ions. Excited product channels arising from one-electron capture by ground-state and metastable ions in collisions with argon have been separately identified.

First experimental results are presented from a new electrostatic spectrometer designed to study electron scattering from positive ions. Measurements of elastic scattering from have been carried out over the centre-of-mass energy interval 12 - 24 eV for fixed scattering angles of , and . Results are compared with the Rutherford formula and a phaseshift calculation based on a semiempirical potential.

Bremsstrahlung spectra of a 0.5 and 4 keV electron on an isolated La atom as well as on a solid La target are calculated for the photon energies above the 4d threshold, eV. The polarizational bremsstrahlung is taken into account. It is shown that in the case of a thick target the magnitude of the polarizational bremsstrahlung cross section is greatly influenced by self-absorption of radiation and inelastic electron scattering.

We present numerical ab initio calculations for a two-electron quantum system that is a one-dimensional analogue of the He atom in a strong laser field. Electron correlation occurring within this model can be treated exactly. We compare our calculations with a single-electron model that employs a proper effective potential for the `outer' electron. Evidence of fast correlated double ionization is presented and discussed.

With the aim of obtaining a practical approach to molecular photoionization continua, a new formulation is given for the random phase approximation (RPA). This development is based on the so-called algebraic-diagrammatic construction (ADC) used previously to derive higher-order approximations to the polarization and other propagators. In the ADC reformulation the RPA pseudo-eigenvalue equations split into two equivalent, generally Hermitian, secular problems of half dimension. Here the elements of the resulting ADC secular matrices are given in the form of `regular' perturbation expansions. Similar perturbation expansions result for the `effective' transition amplitudes required to calculate spectral intensities. Approximation schemes converging to the full RPA are obtained by truncating these perturbation expansions at successively higher-order n. For the lowest orders n = 1 and 2 the ADC(n)/RPA schemes are specified explicitly. In addition to the perturbation-theoretical approach, direct closed-form expressions are given for the ADC secular matrix and effective transition amplitudes relating these quantities to the RPA eigenvalues and eigenvectors. As shown by these relations, the ADC reformulation can be viewed as a specific form of quasi-degenerate perturbation theory (QDPT) applied to the RPA pseudo-eigenvalue problem. In the single-channel (SC) approximation the ADC secular equations reduce to a one-electron eigenvalue problem for an energy-independent non-local potential. A particularly useful method is the single-channel ADC(1) scheme combining the scattering solutions of the familiar frozen-core Hartree - Fock (FCHF) model with a simple first-order expression for the transition moments. Illustrative applications of the SC-ADC(1) method to the photoionization in and are reported. These calculations show that ground state correlation can quite substantially influence both the magnitude and the shape of the cross sections as a function of energy.

Precise electromagnetic measurements of the polarizability (dielectricity, refractivity) of a macroscopic sample of helium gas are re-examined in view of recently published ab initio calculations of the helium atomic polarizability, including relativistic effects. The collation yields a new estimate of the Boltzmann constant as well as a general assessment of the reliability of optical and electrical measurements of the polarization properties of helium gas.

Auger spectra of xenon in the energy regions of 0 to 100 eV (NOO, NNO, NNN, MMN, LLN) and 250 to 550 eV emitted upon creation of single vacancies in N, M, or L shell are calculated by straightforward construction of de-excitation trees of vacancy cascades. Strong dependence of the shapes of the cascade-affected spectra on the configuration of an initial-state ion is predicted. It is shown that to describe deep-initial-vacancy cascades with an acceptable degree of accuracy one has to include the branches with probabilities of up to about in which case the number of branches one has to consider may amount to about .

We report the observation of coherent control of the ionization rate in four-photon resonant, five-photon ionization schemes in Kr and Xe. The intermediate resonance is excited via different interfering channels involving four-photon absorption of the fundamental laser frequency ( ionization) as well as one third harmonic and one fundamental frequency photon absorption ( excitation), a particular aspect of the scheme being that interference occurs at a virtual level. By varying the relative phase of the two electromagnetic fields, a large modulation depth has been observed which is due to the chosen non-linear excitation scheme. In the Xe excitation experiment the interference between the different channels could be additionally probed through the modulation of the third-harmonic signal.

By applying a recently published multistate approach (MSA) to the calculation of partial and total predissociation rates, a series of systematic ab initio computations of resonance positions and widths of rovibrational levels of the and electronic states up to the third dissociation continuum of the prototypical excimer and its isotopes , and have been carried out. The resulting resonance landscape, unique for each isotope, clearly displays all near degeneracies and crossings of levels, effects that account for the erratic phenomena observed in the spectra, such as anomalous line intensities, missing lines and band break-offs. Dipole transition probabilities for a number of bands have also been calculated, allowing predictions of observable emission lines to be made. The results demonstrate the validity of the MSA and show that this type of theoretical spectroscopy requires as input potential energies and coupling matrix elements of high accuracy.

Dispersed vacuum-ultraviolet fluorescence from neutral dissociation fragments in the wavelength range from 100 - 200 nm has been measured after photodissociation of CO as a function of the exciting photon energy between 18.5 and 29 eV. The decay channels of neutral dissociative excitation of the formed CO Rydberg states converging to the D and C states have been analysed. The vibrationally excited levels of the and Rydberg states preferentially dissociate into the fragments . Relative cross sections for the observed lines of and have been obtained. For the lines at 193, 167.5 and 156 nm estimations for absolute cross sections are presented. Evidence for superexcited states has been found in the exciting photon energy range between 24 and 27.5 eV.

Transitions between states with the same principal quantum number n in the collision of a Rydberg atom with an ion are described within the classical approximation for the Rydberg electron. The quantum electron state is represented by an ensemble of classical trajectories. Within the dipole approximation for the interaction the final value of the orbital momentum l is calculated analytically for each initial trajectory in the ensemble. Averaging over the ensemble gives the distribution function over l. For the initial s state of the Rydberg atom the distribution functions are found also for the pairs of variables, namely: {the angular momentum; its projection on the collision velocity}; {projections of the angular momentum and the Runge - Lenz vector on the collision velocity} and {projections of the angular momentum and the Runge - Lenz vector on the plane perpendicular to the collision velocity}. The l-distributions for the orbitally polarized initial p and d Rydberg states with definite projections of the initial angular momentum on the velocity axis are also obtained. The influence of the non-Coulomb core is taken into account by an approximation. The final l-distributions (calculated for the initial s-state) are shifted towards the large-l values as the relative collision velocity vn decreases. Good agreement is achieved with the recent experimental data for Na(28d) and Na(26d) Rydberg atoms.

The orientation of the excited state of , created by electron capture in collisions of ions with atomic H and D, is investigated theoretically. The collision dynamics are described using an adiabatic representation of the collision complex and calculations are carried out in the 0.5 to 10 keV energy range. Total and differential cross sections for electron capture are presented. The range of scattering angles for which there is a strong propensity for orientation of the state is determined. The influence of the short range adiabatic potentials on the orientation at small scattering angles is discussed.

The atomic physics required to study impurity influx from a localized surface is examined and then applied to chromium and molybdenum. To this end, we have calculated term energies and oscillator strengths for the and transitions in neutral Cr and Mo in a multiconfiguration Hartree - Fock approximation. We have calculated excitation cross sections for these dipole transitions and for the transitions, which couple the ground to the metastable system, in a close-coupling approximation using the non-relativistic R-matrix method. Pseudo-resonances were removed from the latter transitions using a novel transformation and reduction method which eliminates those linear combinations of (N + 1)-electron bound states that are not required by orthogonality. We have calculated the direct ionization cross sections from the ground and metastable terms in a distorted-wave approximation, and find significant near-threshold sensitivity to the effective potentials used. We have incorporated this primary atomic data into the Atomic Data and Analysis Structure (ADAS) and solved the quasi steady-state generalized collisional - radiative population rate equations. We present results for the number of ionization events per observed photon, the ratio, over a wide range of electron temperatures and densities. The ratio can be used to directly relate spectroscopic emissivity measurements to impurity flux from a localized surface such as the target plates in a magnetic fusion divertor.

Absolute stopping powers of the rare gases for ions were measured for energies from 50 - 200 keV. The measurements were made using a gas cell with a differential pumping system and a electrostatic energy analyser. The results were lower than earlier experiments for He and Ar. There are no previous measurements to compare with for Ne, Kr and Xe, and the results show that in this energy region the velocity proportionality of the stopping power does not hold.

We discuss measurements of convoy electrons performed with incident (50, 150 keV) and (100, 200, 400 keV) ions, in coincidence with charge-analysed projectiles emerging from thin (150 - ) C and Cu foils. A strong dependence of convoy electron yields on the projectile charge, as well as cusp-electron spectra in coincidence with neutrals are observed and compared with the characteristics of presently available models for beam - foil convoy electron emission. Measurements performed with sputter-cleaned target surfaces confirm that the transmission through the surface potential is important in determining both the coincident convoy electron yields and the final charge distribution of the emerging projectiles.

We report experimental evidence of electron anticapture to the continuum at about in - Ar scattering at 1000 eV. This double-binary collision mechanism (the electron scattering analogue of the Thomas forward peak in ion - atom scattering) has been isolated by studying correlated projectile and ejected electrons travelling in the same direction following - Ar collisions. The cross section for this process rises as is approached, but drops precipitously at about . The dip is the signature of anticapture to the continuum for negative projectiles and corresponds to a zero Coulomb density of states factor at . In addition, a significant departure from the Rutherford formula is found for the one-electron differential cross section at large scattering angles.

A comprehensive theoretical study of NO Rydberg states and of Feshbach resonances associated with excited states of is made for an internuclear separation of 2.175 . Calculations are performed using the UK R-matrix molecular package. The lowest four target states (, , and ) are considered in the construction of the CI target wavefunction. Quantum defect theory is employed to analyse the multiple Rydberg series obtained. In the bound-states region of the NO spectrum, six Rydberg series are resolved and quantum defects are given up to n = 11. In the continuum spectrum, Rydberg series of resonances converging to the three excited states of considered are identified and compared with available experimental data. Some previous assignments for both bound and continuum states are reinterpreted and the dominant source of partial-wave mixing re-analysed. Data for the resonance widths are presented for the first time.

We present calculations of the rates of decay of an excited atom embedded in an absorbing dielectric. Decay can occur by spontaneous emission into transverse radiative modes of the electromagnetic field and by Joule heating via longitudinal coupling of the atom to the dielectric. The spontaneous emission (transverse) decay rate is modified in a dielectric, being the free-space rate multiplied by the real part of the refractive index at the transition frequency of the atom. There is a further modification due to the difference between the macroscopic dielectric field and the local field at the position of the atom. In addition there is a longitudinal decay rate which is proportional to the imaginary part of the dielectric constant and therefore vanishes in non-absorbing media. We derive expressions for each of these rates of decay and discuss the physical mechanisms leading to them.

-factors in the and series (n = 8,...,23) of ytterbium have been investigated by quantum-beat spectroscopy using two-photon excitation. The results show noticeable deviations from the predictions of Russell - Saunders coupling and the influence of a strong perturbation for n = 9 and n = 11. The data can be used to extend the MQDT model obtained from the values of the level energies.

X-ray line spectra from highly charged impurity ions have been taken with a high-resolution Bragg-crystal spectrometer on the Tore Supra (`TS') tokamak. By cross-checking the wavelengths of reference lines from the helium-like ions (2.6 Å) and (3.95 Å) we first demonstrate that it is possible to measure wavelengths with a precision, , of better than 50 000. We then determine the wavelengths of n = 3 to n = 2 transitions of neon-like in the 4 Å spectral range.

In this recent article, the printers inadvertently omitted the caption of table 2. Table 2 with its caption is shown below.

Table 2.  Relative intensities and total Auger rates for the KLL Auger transitions of sodium. The four initial states of the [ls^-13p] multiplet are shown in the head of the table. The Auger energies and rates are given in atomic units. Transition energies are listed with respect to the lowest ³P0 level of the initial state multiplet. ∼0 denotes an absolute rate < 10^-6. Note, that absolute rates for columns 3 - 6 must be multiplied by 10^-4. An Auger transition marked by ` - ' is not allowed by coupling rules. Occupied orbitals are displayed only as necessary to classify the state.

In equations (27), (28), (42), (46), (89) and (90), 2×2 matrices have been printed incorrectly as column vectors. Equation (27) should read:

Equation (28) should read:

Equation (42) should read:

Equation (46) should read:

Equation (89) should read:

Equation (90) should read:

Also, our paper cited in [23] will be published in J. Phys. A: Math. Gen. and not in J. Phys. G: Nucl. Part. Phys. as initially planned.