Table of contents

The independence of orthogonal operators is used to extend the Hamiltonian with hyperfine structure operators. The interaction strength of electrostatic, fine and hyperfine operators may then be determined in a single least-squares fit to the observed energies. To illustrate the framework, we studied the transition array of Bi VI. This spectrum exhibits a large hyperfine structure and is highly relativistic, but polarization effects are small. A Dirac - Fock calculation turns out to be successful and even shows the superiority of an extended nucleus over a point nucleus description.

We predict that ultra-broadband light consisting of over 150 distinct frequencies of comparable energy can be generated in air at atmospheric pressure by stimulated Raman scattering using resonant symmetric pumping. Nanosecond input pulses and the highest available intensities are found to be optimal. Gain suppression analysis, incorporating a finite Stokes shift, gives a qualitative explanation of the pumping requirements.

Angular distributions of valence photoelectrons showing effects due to higher-multipole photon interactions have been measured for the first time. Neon 2s and 2p photoemission exhibits effects beyond the dipole approximation throughout the 250 - 1200 eV photon-energy range studied. The results suggest that any photoemission experiment, on any sample, can be affected at relatively low photon energies, pointing to a general need for caution in interpreting angle-resolved-photoemission measurements.

Coincidence measurements have been performed of double photoionization in atomic Ca in the region of the Ca 3p - 3d resonance. The two electrons have been detected with equal and unequal energies and coincidences observed over a range of angles between the two electrons from approximately . Two pairs of lobes are observed in comparison with the single pair of lobes observed in the TDCS for He. Comparisons are made with the theoretical data of Ceraulo et al.

The C 1s and O 1s photoelectron spectra of the CO molecule have been measured with high resolution. The vibrational spacing of the state is significantly lower than in the state and in the molecular ground state. Compared to the ground-state equilibrium C - O distance of 1.128 Å, we extract from the analysis of the Franck - Condon factors bond lengths of 1.079(2) Å and 1.167(4) Å for the C 1s and O 1s core-ionized states, respectively.

A novel one-colour pump - probe scheme with 130 fs pulses is used to study the fragments arising from the multi-electron dissociative ionization of molecular chlorine. This approach allows us to demonstrate the coexistence of nascent fragments and those fragments which are further ionized by the probe pulse. A simple model, based on tunnel ionization theory, accounts well for the observed population transfer of the ionic fragments. In contrast, only nascent fragments are observed for intense 2 ps pulses. Our results show that the absence of post-dissociation ionization in most experiments is not due to the strong decrease of the ionization rates beyond the critical distance. A possible explanation is laser-field induced screening of the nuclei by electrons leading to a `retarded' Coulomb repulsion. The multiple ionization of the molecule followed by dissociation would then occur later in the pulse where the intensity increase is not sufficient to induce post-dissociation ionization.

The spatial distribution of chlorine ions in the focal spot of intense 130 fs laser pulses is studied. To this end a one-colour pump - probe experiment is performed, where the pump pulse produces multiply ionized molecules exploding into energetic fragments and the probe induces additional ionization of these fragments shortly after their creation. Our method is based on the simulation of the recorded experimental pump - probe spectra, using tunnel ionization theory to describe the further ionization of the fragments by the probe. The ionization process is shown to strongly depend on the spatial distribution of the molecular ions produced in the focus of the pump. Comparison of the simulated and experimental spectra strongly suggests a pyramid-like distribution within the laser focus, thereby allowing us to discard onion- or potato-like distributions as observed for atoms.

Single-pulse energy measurements of the Xe(L) emission induced by femtosecond multiphoton ultraviolet (248 nm) excitation of Xe clusters have been made with a transmission grating spectrograph equipped with a calibrated x-ray CCD camera. On the basis of the observed magnitude of the Xe(L) single-pulse yield and the morphology measured by Thomson scattering of the self-trapped channels guiding the incident 248 nm pulse, it is concluded that the coupling strength for x-ray production from the clusters cannot be accounted for by an interaction involving only independent single-electron processes. Analysis indicates that the coupling requires augmentation by a factor of . This conclusion is in good agreement with an independent spectral study involving the direct comparison of the multiphoton coupling strength with electron collisional data for Xe obtained from EBIT studies, an analysis which placed the corresponding factor of augmentation of the coupling in the interval . These findings are consistent with estimates of the magnitude of the strengthened coupling which could arise from the development of ordered multi-electron motions.

We demonstrate that the spectra of a probe beam probing an ensemble of two-level atoms damped by a broadband squeezed vacuum and driven by a weak laser may exhibit a large refractive index accompanied by zero absorption when the probe beam frequency is tuned to the laser frequency. This resonant effect is due to coherent population oscillations. The same theoretical model also exhibits a large refractive index with zero absorption at the Rabi sidebands and near resonance. These results may be verified by present experiments.

Using theoretical results from complex rotation calculations and data from experimental photoionization cross sections, the quantum defects, the widths, the oscillator strengths and the shape parameter of Rydberg series of autoionizing resonances in helium, excited with synchrotron radiation from the ground state, are reviewed and analysed systematically. The relation of these resonance properties to the propensity rules for radiative and non-radiative transitions in two-electron atoms is established.

The one-electron Fues potential model is used to describe the like ion interaction for the isoelectronic sequence up to Z = 26. Ab initio data of the first few excited energy levels are used to determine accurate potential functional parameters depending on all the quantum numbers of a given state. A 1/Z expansion of the parameters is given in order to cover the He sequence. Accurate bound energies and well defined wavefunctions of single excited states are given. Continuum wavefunctions and elastic scattering electron - hydrogen-like ion phase shifts are derived.

General methods for evaluating three-electron integrals in Hylleraas coordinates are given. Formulae are obtained for the matrix elements of various operators arising in Hylleraas-type variational calculations for states of arbitrary angular momentum. For the calculations of Breit interaction, a number of reduction relations are developed for the elimination of singularities in some singular integrals. A numerically stable scheme is presented for the case when one of the powers of is -2.

The state-specific expansion approach to the solution of the time-dependent Schrödinger equation was applied to the calculation of aspects of the multiphoton response of He to short laser pulses of and . Ionization rates and probabilities, saturation intensity and generalized cross sections are compared with recent experimental findings by Charalambidis et al. Above-threshold ionization spectra and the corresponding photoelectron angular distributions show a minor dependence on the temporal shape of the pulse.

More than 200 low-lying and moderately high-lying doubly excited states of He-isoelectronic systems (Z = 2 - 5) have been studied in the nonrelativistic Hohenberg - Kohn - Sham density-functional framework by incorporating the nonvariational work-function exchange potential suggested by Harbola and Sahni and a simple parametrized local Wigner-type correlation functional. In essence, a Kohn - Sham-type equation has been solved numerically. Our results on excited-state energies, excitation energies and differences between excitation energies are in good agreement with available experimental and theoretical results. In 50% of cases, the percentage deviation in the calculated excitation energy is less than unity. The estimates of the relatively small distances (0.02 - 0.1 au) of the He states below the corresponding N = 2, 3, 4 ionization thresholds are in error by 0.5 - 34.9%, indicating that the Wigner correlation functional needs to be improved for greater accuracy. The excited radial densities display expected structures. Some new states are reported.

Isotope shifts have been measured for isotopes on the ionic (312.7 nm) and (310.0 nm) lines by fast collinear ion - laser beam spectroscopy. The inconsistent reports of the isotope shifts are considered and a complete set of compatible optical and non-optical charge-radii measurements are derived. Simple macroscopic descriptions totally fail to explain the rapidly changing course of the nuclear shape and size.

Photoelectron angular distributions for the laser photodetachment process have been measured at six wavelengths in the region 457.9 - 647.1 nm. An intense 10 keV beam was produced in a caesium sputter-type ion source and mass-selected with a bending magnet. The mass-selected ion beam was subsequently crossed at with a linearly polarized, continuous photon beam. Angular distributions were obtained by measuring the laboratory-frame energy spectra of photodetached electrons as a function of the angle between the velocity vector of the ejected electrons and the polarization direction of the linearly polarized photon beam. The photoelectron angular distributions were used to determine asymmetry parameters. The present measurements are in excellent agreement with those of Hall and Siegel at 488.0 and 514.5 nm (1968 J. Chem. Phys. 48 943), and good agreement with a calculation by Cooper and Zare (1968 J. Chem. Phys. 48 942). The photodetachment asymmetry parameters for are compared with a recent measurement of the photodetachment asymmetry parameters for .

Photoabsorption studies, using the dual laser plasma technique, have been initiated along the zinc isonuclear sequence as far as in the energy region corresponding to 3p-subshell electron excitation. For neutral, singly and doubly ionized zinc the 3d-subshell remains closed and excitations of the type 3p - ns have been found to predominate. The symmetric nature of the resonance profiles is evidence of the weak interaction of these discrete states with the underlying continua. The relatively large widths of the resonances can be interpreted as being mostly due to the nonradiative decay of the 3p core hole through both super and normal Coster - Kronig decay processes. The spectral signature of the `transition metal-like' 3p - 3d resonance is dramatically different, the asymmetric nature of which reflects the increased interaction between direct 3d photoionization and the discrete excitation processes. The main resonances in the spectra of , and have been parametrized and compared with calculations based on both single-particle and collective models.

Oscillator strengths for the four lowest transitions of neutral nitrogen are calculated in a non-orthogonal framework which allows the 2s and 2p (and other) orbitals to be represented by radial functions which are different for different states even within the same LS-symmetry. Previous calculations of these transitions have met with difficulties in obtaining agreement between the length and velocity forms of the oscillator strength which have been overcome only by using several thousand configurations in the representation of the wavefunctions. In our work we have obtained comparable agreement with these extensive calculations and also with experiment by using a total of only 26 configurations within our non-orthogonal procedure.

The general cross section is expressed in terms of Stokes parameters of incoming and detected radiation. The dependence on the experimental arrangement is made explicit. Circular dichroism is considered for both randomly oriented and polarized target systems. A simple example illustrates how incident left- and right-handed circularly polarized x-rays can be used to obtain orientational information. We also examine the validity of the impulse (Compton) and optical (Raman) approximation for scattering by 1s electrons of hydrogen and hydrogen-like ions using an exact scaling law.

We report the theoretical calculation of the total and partial cross section and asymmetry parameter for photoionization up to the N = 3 threshold. Multichannel continuum wavefunctions are obtained via a least squares solution of the Schrödinger equation employing a basis set expansion based on a primitive B-spline radial basis and a general configuration interaction (CI) form. This allows the use of a mixed full-CI and close coupling expansion in a completely general approach to improve correlation description in the continuum channels. The present algorithm correctly describes the full range of phenomena encountered with remarkable accuracy both on nonresonant cross sections and resonance structures. An accuracy of better than 0.01 Mb is estimated on both the total and partial cross sections, except for the shape resonance. In addition, Fano resonance parameters compare very well with the best methods available based on Hylleraas-type wavefunctions and complex coordinate rotation.

The Auger zones, which characterize the participation of various levels of initial and final configurations in the Auger transitions between these configurations, are investigated. The expressions for the shift of the average energy of the Auger zone with respect to the average energy of the configuration are presented. The large shift of zones is related to the existence of two groups of levels for complex configurations. The concentration of transition rates in the high-energy part for some Auger spectra is predicted.

We calculate a probe response of a homogeneously broadened two-level transition in the presence of a strong pump. The derivation is carried out for a generalized interaction Hamiltonian, which gives a uniform account of both the Bloch - Siegert shift and dynamic Stark splitting. It is shown that the Bloch - Siegert shift can be viewed as an extreme situation of the dynamic Stark splitting, commonly known as light shift in the literature. Finally using a perturbative approach an analytical expression for the Bloch - Siegert shift is obtained, which is compared with the previous results.

In the emission spectrum of the nitric oxide isotopomer eight bands: 0 - 3, 0 - 4, 1 - 4, 1 - 5, 1 - 6, 2 - 6, 2 - 7 and 3 - 4 comprising about 1160 lines belonging to the -band system have been remeasured and reanalysed. The rotational analysis of these bands and the calculation of the rovibronic structure constants have been performed via a nonlinear least-squares fits with the effective Hamiltonians of Brown et al. The merged analysis of the currently analysed bands and earlier observed vibrational - rotational 1 - 0, 2 - 1 and 3 - 0 bands made it possible to considerably enlarge, unify and specify with greater precision the information about the and states in . In particular, new constants of the rovibronic structure have been calculated for the and levels and equilibrium molecular constants as well as the Franck - Condon factors and r centroids for the bands in the molecule.

Using monochromatic synchrotron radiation, we have recorded high-resolution Br 3d photoelectron and photoabsorption spectra (PES and PAS, respectively) for the five molecules: HBr, DBr, and . The Br 3d PES of HBr, DBr and show five peaks due to the combined effects of ligand field (or molecular field) and spin - orbit splitting. The and PES are vibrationally broadened, but also show additional broadening from ligand field splitting. The Br 3d ligand field splittings (as quantified by the crystal field parameter) are similar to the I 4d values for the iodine analogues, and increase in the order of . The Br 3d binding energies also increase in the above order due to the increasing electronegativity of the substituents.

The very complex Br 3d photoabsorption spectra also show the same ligand field/spin - orbit splitting which split each transition into five transitions. These spectra are dominated by the peaks from the five transitions; but a myriad of other peaks from the (n = 4, 5), np (n = 6 - 9), ns (n = 6, 7) and nf (n = 4, 5) transitions are required to fit these spectra. Good fits to all the spectra have been obtained using ligand field splittings which are similar to the PES results, and quantum defects for s, p, d and f orbitals of , 1.6 - 2.0 and 0.8 - 1, respectively.

Dissociative recombination of vibrationally cold has been studied at the ion storage ring CRYRING, with an accurate measurement of the kinetic energy of the recombination fragments. When the incident electron energy exceeds , fragments are observed with a kinetic energy of only . The energetics suggest that this low-energy channel derives from either recombination of the state or recombination of the metastable state. High-quality ab initio calculations have been used to obtain the state vibrational relaxation time (10 ms), excitation energy (2.25 eV) and radiative lifetime (0.030 s). The dissociative recombination cross section has been obtained in a model calculation. The results imply that the state has too small a recombination cross section and too short a radiative lifetime to explain the experimental results, which were obtained after 15 s of storage in CRYRING. We conclude that the 120 meV channel is due to recombination of into the asymptotic limit. Possible mechanisms are discussed.

Vibrational levels and widths of the n = 3 Rydberg states of BeH and BeD have been calculated by a multistate complex scaling method. Large widths are obtained which are consistent with the absence of emission from these states.

The cross sections for the excitation energy transfer induced by collisions with He, Ar and Na ground-state atoms have been measured. The sodium atoms in the 4D state have been produced by two-photon excitation using cw laser radiation at 578.89 nm. Within a model which takes into account collisional population and depopulation of the 4F level as well as radiative relaxation (direct or cascade) of the 3D, 4P, 4D and 4F states ending in the 3P level, the rate for the collisional transfer turns out to be related to the ratio of the steady-state populations created in 3D and 4D states. They have been determined by measuring the fluorescence intensities of the and transitions. The measurements yield the following cross section values for those processes investigated: , and , at .

A merged-beam apparatus has been used to measure cross sections for mutual neutralization for centre-of-mass energies between 3 and 460 eV. Different ion sources and gas mixtures were used to vary the vibrational state population of the ions and the cross section appears to be insensitive to the initial vibrational state. The measurements differ significantly from the results of a recent calculation but, at interaction speeds greater than about , converge with results for the atomic ion pairs , , and .

The BBK model is modified by the introduction of effective Sommerfeld parameters for both symmetric and asymmetric geometries on an empirical basis, while still maintaining the philosophy that all three Coulomb interactions are included on an equal footing. The triple differential cross sections for electron-impact ionization of atomic helium at an incident energy of 40 eV in an asymmetric geometry are calculated. Results of this approach are compared with the absolute measurements and the only existing theoretical results of the convergent close-coupling method.

We present new R-matrix calculations for ionization processes involving the beryllium K-shell, namely electron impact ionization of and single and double photoionization of Be. These are modelled near inner-shell thresholds in a unified way, coupling direct and indirect processes, enabling comparison of the respective autoionization features. Below the K edge, we predict that direct double photoionization contributes 4% to the total cross section background for photoionizing Be; the onset of K-shell excitation - autoionization (EA), an indirect mechanism, increases the background by a factor 20. By contrast, we find that EA in ionization increases its background by only 4%; this is less than previous estimates, but in line with experiment. We also study high-energy behaviour for photoionizing excited states, e.g. , where we find a background dominated by ionization. This process is normally omitted in R-matrix schemes used in opacity and recombination calculations, leading to orders-of-magnitude underestimation at high energies of excited-state photoionization, which also exhibits a large core resonance above the K edge.

An experimental study of caesium energy pooling collisions, , at thermal energies, has been carried out in a capillary cell using diode laser excitation. Use of the capillary cell minimizes the effects of radiation trapping, but nonetheless, such effects still play a significant role in the analysis. Consequently, a rate equation model, which treats simultaneous effects of saturation, optical pumping, and radiation trapping, has been developed and is used to determine the atom density under these experimental conditions. The excited atom densities are combined with measured fluorescence ratios to determine rate coefficients for the caesium energy pooling process. Our values for these rate coefficients are in agreement, within combined error bars, with values we have recently obtained under very different experimental conditions.

By means of ion translational energy spectroscopy, single-electron capture (SEC) by ground state and metastable on and CO has been studied at impact energies . For SEC from the main channels involve transitions with the final vibrational state distributions being found in reasonably good agreement with multichannel Landau - Zener calculations.

The previously used additivity rule has been modified by considering the geometric shielding effect of molecules. Total cross sections (TCSs) for electron scattering from hydrocarbon molecules and are calculated in 10 - 1000 eV employing this semi-empirical approach. The atoms are presented by spherical complex optical potential which is composed of static, exchange, polarization, and absorption terms. Compared with our earlier results, the modified additivity rule for the TCS on complex polyatomic molecules is simple but more effective and it yields better accord with experiments in the intermediate and high energy region.

Two ionization models containing realistic descriptions of the final state of the system, but employing only plane and Coulomb waves, are applied to positron ionization of molecular hydrogen. In addition, spherically averaged wavefunctions are used to simplify the complexity of the molecular geometry. The theoretical cross sections are compared with the measurements of Fromme et al and Moxom et al. The agreement between the experiment of Moxom et al and the CPE model with a reasonably accurate representation is similar to the agreement shown by Campeanu et al for positron ionization of noble gases. These results suggest that relatively accurate spherical representations of the molecular wavefunction can be used successfully in distorted-wave calculations of positron-impact ionization of molecular targets.

We study the electron response of a cluster excited by strong femtosecond laser pulses by means of a time-dependent density-functional method. We investigate the full electronic dipolar response and multiphoton ionization processes. A strong correlation between induced electronic dipole oscillations and electron emission is observed, leading to a pronounced resonant enhancement of ionization when using lasers operating at the frequency of the Mie plasmon. We also examine the probabilities for producing differently charged ionic states of the system.

X-ray spectra in the range 7 - 10 Å from highly charged tungsten (Z = 74) ions were observed in the tokamak ASDEX Upgrade. Lines emitted from to were identified by comparison with ab initio calculations and with the expected ionization equilibrium charge-state distribution. Most of the features in the observed spectra coincide with predicted lines. A spectral line of As I-like could be identified as the predominant one for plasmas with 2.5 - 3 keV central electron temperature. The calculations predict clearly separated spectral lines for Kr I-like W at about 18.4 Å and strong lines between 5.6 and 6.0 Å for all charge states under consideration that can be used for monitoring W densities in the soft x-ray region. Tungsten concentrations in the range of were extracted from the emissivities of spectral lines from and .

The value of the 5s cross section in table 1 (first raw) should be changed from 5.15 × 10^-20 to 1.5 × 10^-18 cm².

We have discovered a few typographical errors. Equation (70) should read:

In equations (74) and (76), and in the paragraph immediately following equation (79), θ_sf should be replaced by θ.