Table of contents

It is shown that the strong coupling through the momentum-matching mechanism between bound and pseudocontinuum states belonging to different centres causes spurious oscillatory structures in the excitation and capture cross sections of two-centre close-coupling calculations. The discreteness of the energy eigenvalues of pseudocontinuum states enhances the coupling excessively when the relative velocity v between the projectile and the target matches the condition , where is the eigenenergy of the ith pseudocontinuum state and m is the electron mass. The spurious structure tends to be less prominent as the density of the pseudocontinuum states becomes higher since the couplings with adjacent pseudocontinuum states averages the coupling with the matched state. Large-scale two-centre calculations are carried out for collisions and satisfactorily smooth cross section curves are obtained for the n = 2 excitation and capture cross sections.

The electronic structure of scandium monohalides has been analysed by means of ligand field theory and locations of many electronic states of ScBr and ScI have been predicted.

We report the observation of quantum jumps in the fluorescence light of a single laser-cooled ion confined in an RF Paul trap. From about 3000 quantum jumps we determine the lifetime of the metastable level to be . Systematic errors have been reduced to a negligible level. Our result is compared to both existing theoretical and experimental data.

The production of and ion pairs in carbon monoxide at photon energies below the adiabatic double-ionization threshold of 41.25 eV has been probed in a threshold-photoelectron - photoelectron coincidence (TPEPECO) experiment using tunable VUV radiation and a sensitive electron spectrometer. The TPEPECO spectra provide evidence of production that does not involve creation and dissociation of a molecular dication, but instead results from complete dissociation of a molecular cation followed by autoionization of the atomic oxygen fragment. Furthermore, an electron - electron coincidence signal has been detected at photon energies as low as 36.5 eV, well below the previously measured onset for production.

Double ionization is considered within the context of the impact-parameter version of the continuum distorted-wave eikonal initial-state approximation. Starting from the independent electron model, the effective potential describing the ejected electron - residual target interaction is modified so that the single-ionization energy of each electron is half the double-ionization energy of helium. Further, the correlation of the ejected electrons is accounted for by explicitly including the Coulomb density-of-states factor for the electron - electron interaction in the calculation. Total cross sections for proton, alpha particle and impact on helium, over the energy range are presented. It is concluded that the inclusion of electron - electron correlation in the exit channel is crucial if an adequate account of the double-ionization process is to be obtained.

Within the classical trajectory Monte Carlo (CTMC) model, we calculate electron transfer and ionization cross sections for singly charged ions colliding with Rydberg atoms in the presence of a laboratory-strength magnetic field of 4 tesla. A new method for generating a stationary microcanonical ensemble for a quasi-integrable initial-state Hamiltonian is presented. The calculated cross sections show signatures of electron capture and ionization mechanisms for the field-free case, e.g. multiple swaps and saddle-point electrons; their structure as well as their magnitude, however, are strongly modified by the presence of the magnetic field.

Partitioning of excess energy between electrons departing at and after electron-impact ionization of helium has been measured. A recently developed scheme, using a magnetically collimated electron-impact spectrometer, pulsed incident electron beam and time-of-flight detection was used to detect the electrons departing in the forward and backward directions. The ratios of the forward and backward doubly differential cross sections (DDCS) for an incident electron energy 3 eV above threshold and secondary electron energies of 0.7, 1.5 and 2.9 eV were measured. The relative DDCS at and are also presented as a function of the secondary electron energy, for incident electron energies 3 and 5 eV above threshold. Our measurements show that the distribution of the secondary electrons is, in contrast to electrons detected at , not flat for electrons departing at and at the present low primary energies. The yield of the faster secondary electrons is forward peaked, the yield of the slower secondary electrons is backward peaked. The sum of the and DDCS is, however, flat within the experimental uncertainty limits.

In the light of new absolute measurements of triple-differential cross sections for electron-impact ionization of helium we re-examine the various forms of the Coulomb T-matrix in the plane-wave impulse approximation. At higher energies the regularized form is supported by the measurements of Pochat et al while the Ford form is supported by the older, conflicting, measurements of van Wingerden et al. A revised calculation of the second-order Faddeev - Watson approximation is presented and compared with the measurements of Pochat et al.

We consider photon emission in collisions of electrons with atoms and clusters, under conditions in which the radiation of atomic electrons is significant or even dominates the total spectrum. This occurs, for example, when the frequency of the emitted photon is comparable with the energy of a giant collective or plasmon resonance in an atom or cluster. We consider the general results of the theory of polarizational bremsstrahlung, and we present the results of recent calculations of the cross sections of this process for a number of targets. The theoretical results are compared with the available experimental data.

Extensive new data on the odd-parity, single-colour, two-photon optogalvanic spectra of argon in the visible region is reported using an excimer-pumped dye laser in conjunction with an argon discharge cell. The experimental technique enables us to record a pure two-photon spectrum in a region which is dominated by one-photon transitions. A large number of two-photon transitions, originating from the two metastable states and to the excited states of configurations (J=0, 2) and (J = 0 - 4) of argon, has been recorded and arranged into 10 Rydberg series. The interchannel interactions among the and the , n d J = 2 channels and the and the J = 3 channels have been studied using the multichannel quantum defect theory (MQDT). In addition, higher members of the two-photon parity forbidden series have also been detected.

Results from systematic multiconfiguration Hartree - Fock studies of the - transition in the Be-like sequence in the range of Z = 4 - 10 are reported for several models. The inclusion of core - valence correlation is essential for obtaining good agreement in the length and velocity form of the oscillator strength, the agreement improving with Z and reaching one part in 4000 for neon. In the case of Be, extensive cancellation occurs in the calculation of the dipole matrix element and correlation in the core already plays a role at the 1% level of accuracy. The orbitals obtained from the core correlation model were used in a Breit - Pauli calculation mixing states. The effect on the allowed transition data is included. The present calculations are generally in better agreement with semi-empirical values predicted from experiment than from experiment itself.

Recently, Kiernan and co-workers observed many new resonances in the inelastic scattering region in a high-resolution synchrotron radiation experiment. They also reported the energy (64.569(10) eV) and width (0.1 eV) for the lowest resonance, . In this work, we carried out multichannel saddle-point complex-rotation calculations for the 15 resonances between the and thresholds. Relativistic corrections are included. Our results for the , 3s energy and width are 64.568 and 0.117 eV, thus confirming the identification of the experiment unambiguously. However, we found that the structure observed near 65.2 eV is also caused by a resonance at 65.152 eV with a width of 89.7 meV. Our results agree with experiment except for the state. We found the position of this state to be below the threshold.

We report here first measurements of the odd-parity J =1, 2 and J = 0, 1, 2, 3 autoionizing Rydberg levels of lead using a three-step laser excitation scheme in conjunction with an atomic beam apparatus. Four initial levels J = 1, 2 were chosen for the excitation of the final levels. The assignments have been made using the -coupling scheme for the observed levels. The Rydberg series , , and as well as are reported. All the observed series exhibit constant quantum defects with respect to the ionization potential value . The full widths at half the maximum of the observed levels are also reported.

Deep core excitation of a small molecule leads to ejection of several electrons and dissociation into numerous alternative sets of fragment ions. We describe an experiment which identifies complete such sets arising from decay of triatomic molecules (and measures their total charge) via triply coincident detection in a time-of-flight mass spectrometer. Careful data correction furthermore enables estimation of contributions from channels comprising one neutral fragment. and OCS molecules were photoexcited by monochromatic x-rays tuned through the entire region around the sulphur 1s threshold (2478 eV). For the first time energy dependent total photoion charge spectra were obtained for deep core excited molecules. They closely resemble those of argon at its K edge, demonstrating the similarity of electronic de-excitation. Electronic processes reflected in these spectra are ejection of excited valence or Rydberg electrons via shake-off and/or Auger decay, photoelectron recapture by post-collision interaction with a fast Auger electron, and pre-edge 1s ionization which extends to tens of eV below threshold. Results on the dissociative aspects are given in the following paper.

and OCS molecules were excited and fragmented by x-rays of variable energy around the sulphur 1s threshold (2478 eV). Coincident detection of three ionic fragments by a time-of-flight mass spectrometer enabled the determination of branching ratios for 18 dissociation channels with total ionic charges between 2 and 7. While the electronic de-excitation is similar in both molecules (see the preceding paper), the dissociation shows differences, notably a more asymmetric distribution of charge onto the fragments of OCS. Branching among channels of the same total charge is independent of photon energy although the dominant de-excitation cascades are not. This is interpreted as an indication of strong interchannel mixing. A simple, parameter-free Coulomb explosion model to simulate the dissociative kinematics is described. Detailed results based on this model give, contrary to opinions to be found in the literature, very good agreement with observed momentum correlations whereas maximum kinetic energies are somewhat overestimated for channels of higher total charge.

In this paper we present the calculations of: (a) the single-ionization cross section of He by protons and anti-protons in the incident energy range of about 10 keV - 50 MeV, (b) the double-ionization cross sections by both projectiles in the energy rage of about 50 keV - 20 MeV and (c) the ratio of double- to single-ionization in the energy range of 50 keV - 20 MeV. The calculations have been performed in the two-coupled channel plane wave Born approximation, which is essentially the Born approximation that includes the final channel interaction between two electrons, in addition to the usual projectile - electron potential. The projectile has been represented by plane waves, and a correlated Hylleraas type of wavefunction has been used for the initial ground state. For the single-ionization case the final state electronic wavefunction is taken to be an anti-symmetrized product of a hydrogen-like (ls) and a continuum Coulomb wavefunction. For the double-ionization case, the final state wavefunctions of the two ejected electrons are taken to be Coulomb functions for calculating the matrix elements of the electron - projectile interaction, whereas plane waves are used for calculating the matrix elements of the electron - electron interaction. The calculated results are in reasonable agreement with the data in all three cases. The single-ionization cross section has also been calculated in the Lewis - Merzbacher approximation to determine the range of incident energy where it might represent a suitable approximation.

In this paper, we investigate the Penning ionization of the CO molecule by helium atoms in metastable states. The energy distribution spectrum of electrons related to the ion state shows a deviation of the vibrational population from the predicted Franck - Condon factors. The angular distribution measurements exhibit an intensity increasing towards the backward direction for the v = 0 level, while the corresponding angular distributions of the higher vibrational levels (v = 2, 3 and 4) are isotropic. To explain these results, an excitation transfer process between the colliding particles is suggested. This gives rise to a CO molecule either in a doubly excited Rydberg state converging to or in the quasi-bound state (shape resonance). Autoionization of the doubly excited state leads to the population of vibrational levels which are forbidden by the Franck - Condon principle in the case of a direct processes.

Using high resolution recoil-ion momentum spectroscopy we have measured differential cross sections of single-electron capture from He atoms by multiply charged boron ions, bare and hydrogen-like , at 0.5 - 1.0 MeV/u. From the measurements of the longitudinal momentum distribution of the recoil ions, the final states of the captured electron in and ions were analysed and the n = 1 ground states were clearly separated from the other excited states. Capture to the excited states gives the dominant contribution to the capture cross section. However, the fraction of captures to the ground state increases with the collision energy. Theoretical calculations based on the eikonal approximation well reproduces the experimental transverse recoil-ion momentum distribution of capture to both the ground and excited states.

Differential cross sections for electrons elastically scattered from mercury have been calculated over the incident electron energy range 9 - 25 eV. The applied theoretical method is based on the Dirac - Fock equation, where the exchange is calculated exactly. The scattering potential is the combination of the frozen-core Dirac - Fock potential and the polarization model potential with two parameters: the static polarizability and the cut-off parameter .

A general formalism is developed for a description of the angular distribution of electrons ejected after the electron-impact excitation/ionization of the laser-excited and polarized atoms. Properties of the angular distributions are analysed in a general case as well as in some particular geometries of the experiment. Within the two-step approximation the formalism is applied to the resonant single and double ionization with ejection of autoionization and Auger electrons. As an example the angular distributions of autoionization electrons from laser-excited sodium atoms are calculated in the plane-wave and distorted-wave Born approximations. Angular correlations between autoionization and scattered electrons, which can be measured in a coincidence (e,2e) experiment with polarized targets, are also analysed.

A systematic theoretical detail is presented for the angular correlation of Balmer-alpha and the cascade Lyman-alpha photons detected in coincidence with the scattered electrons after the excitation of the states of atomic hydrogen. The results are reported for the Stokes parameters and other correlation parameters for Balmer-alpha and cascade Lyman-alpha radiations. In order to calculate the scattering amplitude which relate these parameters a distorted wave approximation theory with different choices of distortion potential has been used. Theoretical results are obtained and compared with the available experimental measurements at 54.4 and 100 eV incident electron energies.

We present a combined experimental and theoretical study of double differential cross sections (DDCS) for electron-impact ionization of helium. Relative DDCS have been measured in a crossed-beam apparatus for primary energies 200, 100, 64.6, 50, 40, 32.6 and 28.6 eV. These are put on the absolute scale using the convergent close-coupling (CCC) calculations at 100 eV. The absolute data are found to be in good agreement with the CCC calculations except for the cases where the secondary electron energy is close to the half excess energy. In these cases, particularly at smaller energies, the CCC theory still predicts similar profiles, but gives substantially smaller magnitudes than the present measurements, which are consistent with the earlier measurements of Pichou et al.

Using the extended Su - Schrieffer - Heeger (SSH) model and the sum-over-state (SOS) method, we have calculated the third-order nonlinear polarizability and their dispersion spectra for fullerene isomers. Our numerical calculation results indicate that both symmetry and shape of the five isomers have great effect on their nonlinear optical properties.

This paper presents, for the first time, a systematic study of an optogalvanic method for photoionization spectroscopy. The method is particularly attractive for refractory and complex atoms, such as lanthanides and actinides. The relevant characteristics of the hollow cathode discharges used for this study are discussed in detail, along with the experimental protocol for this spectroscopic method. The rapid optogalvanic effect, which results solely from photoionization, is also described. Finally, we present as an example of the application of this method, a table containing some of the recorded uranium photoionization lines in the range, along with typical samples of the uranium single-colour photoionization spectrum recorded using the rapid optogalvanic technique. A brief discussion of the sensitivity of the rapid optogalvanic effect is also presented. It appears that the rapid optogalvanic effect is very effective in the detection of highly excited levels. This technique permitted the observation of many new single-colour resonant ionization uranium lines.

A novel Fourier-transform spectroscopic method using two-photon polarization to determine the spectral distribution of the two photons emitted in the spontaneous decay of metastable atomic hydrogen is described. The method uses birefringent retardation plates and takes advantage of the subtle interplay between the spectral properties and the entangled polarization properties of the radiation emitted in the decay. Assuming the validity of the theoretical spectral distribution, it is shown that the experimental results agree well with theory. On the other hand, success in solving the inverse problem of determining the spectral distribution from the experimental results is limited by the small number of experimental points. However, making reasonable assumptions it is deduced that the observed spectrum is characterized by a broadband signal of width and centre angular frequency in good agreement with the predictions of and , respectively, obtained from the theoretical spectral distribution modified to take account of the absorption of the two-photon radiation in air. The values of 1.5 fs for the coherence time and 440 nm for the coherence length for single photons of the two-photon pair which are obtained from the measured bandwidth imply that, in the ideal case, these values are determined by the essentially zero lifetime of the virtual intermediate state of the decay process rather than the long lifetime of the metastable state which, it is suggested, determines the coherence time and coherence length appropriate to certain types of fourth-order interference experiments.

The rate coefficients of the chemi-ionization collisions between and atoms are determined by optical absorption and probe measurements in decaying plasma. The corresponding cross section values at room temperature are as follows: Å, Å and Å. These values are compared with the close-collision cross sections and the cross sections calculated by the optical model of interaction.

High harmonic conversion efficiency has been experimentally compared in the atomic gas Ar and the diatomic molecular gases and . It is shown that, in the saturation region, if the atomic and diatomic molecular gases have similar polarizabilities, the one with the higher ionization potential has the higher harmonic efficiency. If the atomic and molecular gases have similar polarizabilities and ionization potentials, the atomic gas will have a higher harmonic efficiency than the molecular one.

The optical emission spectra (2000 - 15 000) Å of a plasma produced an a Tl arc lamp have been recorded and analysed; using the series and we have obtained that the electron density is of the order of and the excitation temperature is K. Relative transition probabilities for 26 lines from excited levels near the ionization limit of Tl I have been determined from line intensities.