Table of contents

Leading logarithmic corrections in the order to the Lamb shift of states and to the special difference of energies of states in the hydrogen atom are evaluated.

Multiphoton ionization of helium at high laser intensities has been investigated by direct numerical integration of the full time-dependent two-electron Schrödinger equation on a Cray T3D. At field intensities above , two-photon ionization occurs in a few field periods, and double ionization and high harmonic generation are prominent. We present calculations of double ionization yields, investigate the sensitivity of double ionization to the electron - electron electrostatic repulsion, and present evidence of unexpected non-exponential behaviour in ionization decay rates.

Examination of the intensity dependence in the range of the multiphoton-induced Xe(L) spectrum from Xe clusters has demonstrated (i) a very high sensitivity for the production of ions having multiple vacancies and and (ii) a reduction of the average charge state of the emitting ions over the range of intensities in which the multiple-vacancy excitations are diminished. These results give evidence for a dynamical enhancement of the coupling strength arising from ordered many-electron motions induced by the incident 248 nm wave and indicate that the enhanced interaction can lead to a regime of strong coupling in which multiple electron ejection from inner shells can occur with substantial probability. Furthermore, the results are found to be in agreement with previous analyses of multiple electron processes, which involved comparisons with experimental data stemming from both electron (EBIT) and ion - atom collisional studies. The specific comparison with known properties of ion - atom ( + Ne) charge exchange collisions indicates that the multi-electron channels can dominate the multiphoton interaction if a threshold intensity is exceeded. The experimental results give evidence that this threshold falls in the 0.4 - range at 248 nm for the production of 2p vacancies in Xe clusters. These findings provide an experimental basis for the general expectation that a strengthened multi-electron coupling will govern the production of ions with deeply bound (>10 - 20 keV) multiple-vacancy states in clusters of heavy atoms irradiated at 248 nm with intensities above .

We investigate systems of three identical bosons in the context of current experimental efforts to achieve Bose - Einstein condensation. Calculated adiabatic hyperspherical potential curves permit us to draw some general conclusions about the dependence of the three-body physics on the two-body scattering length. In particular, we find that the long-range three-body interaction is effectively repulsive for either sign of the scattering length. This also hints that a stable Bose - Einstein condensate might be formed for either sign.

Two independent calculations have been carried out to obtain accurate differential cross sections (DCS) and spin asymmetries for the elastic scattering of electrons from ground-state hydrogen over the energies of the lowest and resonances. They use the IERM method and the direct numerical approach of Wang and Callaway. The effects of the ionization continuum on DCS, resonant positions, and spin asymmetries are re-examined in light of the present calculations. The calculated elastic DCS are compared with the experiments of Williams and the calculation of McCarthy and Shang. A benchmark calculation for the phaseshifts has been tabulated to assist future experimental normalization procedures.

We present a combined experimental and theoretical study of 50 eV electron-impact ionization of helium. The absolute coplanar triple differential cross sections (TDCS) are measured for 4 and 10 eV slow electrons for fixed scattering angles , , and of the fast electron. The convergent close-coupling (CCC) theory is used to calculate these and is found to be in good quantitative agreement with experiment for the 4 eV case, but lower than the measurements for the 10 eV case. The shape of the CCC theory shows good agreement with previous relative coplanar symmetric TDCS measurements.

We report a strong prepulse dependence of the emission of X-rays from plasmas created by 150 fs Ti:sapphire laser pulses on solids. The laser pulses were focused onto plane samples of aluminium and vanadium with a main pulse power density of about . By splitting off a portion of the main pulse we were able to generate a prepulse whose delay and intensity were variable relative to the main pulse. With no prepulse, very weak X-ray radiation from and was observed, whereas a prepulse to main pulse ratio of more than 0.1% and the prepulse preceding the main pulse by more than 2 ns produced up to 50 times stronger X-ray output.

In last few years significant progress has been achieved both in the experimental technique and the theoretical methods for the determination of the energy levels of simple hydrogenic systems. We review recent two-photon spectroscopic measurements performed in Garching and the relevant theoretical predictions for the hydrogen energy levels. Good agreement is achieved when all theoretical contributions are included, showing the importance of recently calculated higher order corrections.

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A careful discussion of the interaction of the inner electron with the nucleus of the helium atom allows for the reduction to a one-dimensional Hamiltonian describing the frozen planetary atom configuration. This is obtained by cancelling the angular momentum of each electron, even though it is only their sum which is guaranteed to be constant. The true oscillatory nature of the angular momenta in the full two-dimensional Hamiltonian can be included in a time-periodic model that correctly accounts for the finite lifetime of the frozen planetary atom. This system can be further simplified by averaging over the motion of the inner electron so as to define an oscillatory Hamiltonian with a single degree of freedom.

The diagram and resonant x-ray emission spectra are calculated for each atomic species in the OCS molecule. In particular, the emission spectra due to the presence of the (O K), (C K), and (S L) core holes and a resonant electron are calculated. The SCF method is used in a relaxed orbital Hartree - Fock framework to determine the transition energies. Rates were calculated using a multi-centre model. The resonant profiles vary from the diagram analogues due to significant changes in the valence electron density distribution.

We performed a model experiment to study the motional Stark effect on visible lines of a fast helium beam in a magnetic field. Line shifts and the appearance of forbidden transitions were analysed.

Using the state-specific expansion approach to the solution of the time-dependent Schrödinger equation, we calculated a number of observables from the response of the He metastable state to strong as well as to weak laser pulses. The photon energies () were 1 eV, 2 eV, 2.47 eV and 3 eV, and the intensities (I) covered the range from to . For the weak intensities, the results are in full accord with the experimental findings of Haberland and co-workers. For the strong intensities, our predictions are of quantitative as well as of qualitative value. They permit conclusions about the fingerprints left on observable spectra by electron correlation, by Rydberg levels and by doubly excited states and about the extent to which high harmonics of short wavelength can be produced as a function of I and when the initial state lies above the ground state. Finally, we argue that calculations of harmonic spectra based on the use of the acceleration form of the dipole operator will produce, in general, unreliable results even when some correlations are accounted for.

Experimental studies of the characteristics of Xe(M) emission ( - 19 Å) produced by multiphoton excitation of Xe clusters indicate that the nonlinear interaction automatically acts as a template leading to the preparation of the maximally radiating configurations for that spectral range. The new mechanism deduced is a general multiphoton multi-electron process which dynamically combines rapid multiphoton ionization, 4f-orbital collapse, and correlated electron motion.

The one-Coulomb-centre problem is considered in the prolate spheroidal coordinate system. The asymptotic expansions for the separation constant and the Coulomb spheroidal quasiradial and quasiangular wavefunctions are derived when the distances between the foci of the spheroidal system R are large. The constructed wavefunctions are in excellent agreement with the exact wavefunctions in intra-atomic space, when the condition is fulfilled. The formulae obtained in the present paper can easily be generalized for the case of the two-Coulomb-centre problem.

The CDW-EIS theoretical method is applied to explain recent measurements of helium single ionization by proton and deuteron impact. This theory can cope with the proton angular distribution data for a wide range of projectile energy from 100 keV to 6 MeV . At intermediate energies the CDW-EIS is shown to considerably improve the first Born approximation. The theoretical angular differential cross sections consider the internuclear interaction required for large scattering angles. A seminumerical technique for the introduction of the internuclear interaction is developed.

Single and multiple ionization cross sections of CO have been measured as a function of the projectile charge. The experiment was carried out at fixed collision energies of 1 MeV using the coincidence time-of-flight technique. The cross sections increase rapidly for small q and more slowly for large q. The branching ratios of all the transient molecular ions and the kinetic energy distribution of the dissociating transient exhibit similar projectile charge dependence. These observed trends are explained quite satisfactorily using a simple model based on perturbation theory and the independent electron approximation. The model predicts that the average value of the projectile - electron interaction strength in collisions resulting in target ionization varies quickly for small q and more slowly for large q.

We report angle resolved measurements of the excited Na atoms from the process with non-resonant light. The experiment provides a method to probe the NaKr collision pair by a photon. The Na() angular distribution shows an oscillatory structure, which can be used for a test of the NaKr potential curve system. The signal depends strongly upon the polarization of the photon. The effect provides information about the geometric properties of the collision pair.

A comprehensive analysis of the stopping power of antiprotons and negative muons in He and gas targets for projectile velocities (equivalent antiproton energies) ranging from about 0.1 to 10 au (0.25 keV to 2.5 MeV) is performed. Recent experimental data are contrasted with theoretical results obtained from different approaches. The electronic stopping power is evaluated within the coupled-state atomic-orbital method and the distorted-wave Born approximation as well as, for low projectile velocities, within a generalized adiabatic-ionization model that takes into account collisional-broadening effects. The departure of the antiproton stopping power from the proton stopping power (`Barkas effect'), observed for intermediate projectile velocities, is discussed. The contribution to the stopping power arising from energy transfer to the translational degrees of freedom of the target system (`nuclear stopping') is evaluated. Our analysis results in a good understanding of the stopping mechanisms of negative heavy particles in gases, in particular in He. Discrepancies between theory and experiment in the case are attributed to effects of the molecular structure of the target.

The effect of target polarization by the projectile along with the electron - electron angular correlation in it on the angular distribution of triple differential cross section for the ionization of helium close to threshold is studied. The target polarization is estimated variationally and the cross section is calculated in the distorted-wave Born approximation with suitable effective charges for the asymptotic description of the final state at 26.6 eV incident energy with equal energy sharing. The target polarization/correlation is found to lead to a substantial change in the cross section. The agreement with experimental data in kinematical arrangement with is very good.

An absolute integral cross section measurement for the electron-impact excitation of the CO(a) state is presented, which has been normalized to the theoretical work of Morgan and Tennyson. A comparison is made with previous experimental work and current theory, and a new determination for the magnitude of the cascade contribution into the a state has been made.

Earlier ab initio calculations for electron scattering by HBr are analysed to obtain the behaviour of the S-matrix poles in the complex momentum plane as a function of the internuclear distance. This analysis shows the existence of a pole corresponding to a bound state at large internuclear distances as well as a pole corresponding to a shape resonance at small internuclear distances. It is found that the bound-state pole does not evolve into the shape resonance pole as the internuclear distance decreases but instead splits into two mirror image poles which move onto unphysical Riemann sheets of the complex momentum plane confirming model-potential calculations by Herzenberg and Saha. The physical implications of the mirror image poles and the shape resonance pole are also discussed.

We have used several non-perturbative Floquet methods to calculate the differential and total cross sections for electron scattering from argon in a laser field. One method utilizes the mixed-gauge R-matrix technique to calculate the scattering cross section. A diabatic approximation adopted from molecular applications is also applied to this low-frequency limit. Results are compared with the well known Kroll - Watson approximation. We find that all three theoretical treatments agree well with each other but do not agree with some experimental results. Possible explanations for the discrepancy between the calculations and experiments are examined.

We have investigated the radiative electron capture process in collisions of electrons with metal clusters, at an electron energy sufficient for excitation of the giant collective resonance in the target cluster. We have demonstrated that accounting for the dynamical polarizability of the cluster plays an essential role giving rise to a new kind of giant resonance in the cross section for radiative capture.

In figure 2 of this article, the quantum calculations correspond to the statistical average of the results for all initial states within a given n level. The correct figure containing quantum calculations for l=2, m=0 initial states is shown below:

Figure: Ionization probability of H(n,l = 2, m = 0) atoms as a function of the scaled momentum transfer: classical results (solid lines) and quantum mechanical results (dotted lines).