Table of contents

A one-electron model for lithium is used to calculate the radiative lifetime of the resonance 2p level and the ground-state dipole polarizability. The model includes dipole, quadrupole and octupole K-shell excitations, asymptotic corrections of order r^-6 to the dipole operator and dynamical effects. The results are consistent with most other recent calculations and with a very recent measurement via photoassociative spectroscopy, but are not consistent with an earlier precise fast-beam laser measurement. The predicted hyperpolarizability of Li is in satisfactory accord with estimates from two extensive ab initio calculations.

New measurements of the zero-energy electron yield in the region of the photodouble ionization threshold in helium are presented, showing for the first time a Wannier-type cusp with a small asymmetry of the excitation/ionization wing amplitudes, similar to the electron impact results in the region of the first ionization threshold. The observed pressure dependence of the cusp amplitude asymmetry and other second-order effects are discussed in an attempt to explain the discrepancy with the earlier photoionization results of Hall et al. (1991).

Parity conserving weak-interaction corrections to energy levels of highly charged few-electron ions are considered in the framework of the standard model. The corresponding effective potentials in coordinate space are derived. Due to the Z³ dependence on the nuclear charge number Z and due to the strong enhancement of the Dirac wavefunctions at the nuclear origin these corrections are strongly amplified for heavy few-electron ions. Their contribution to the Lamb shift and to the hyperfine splitting are of the order of 10^-5 and 10^-7 eV, respectively. This is much smaller than the recent experimental accuracy for the Lamb shift but not too far away from the expected accuracy for future hyperfine structure measurements.

A two-step model of Coulomb explosions of diatoms in intense laser fields is presented. In this model the molecule loses several electrons when the atoms are at the equilibrium internuclear distance and then fast Coulomb explosions occur until the products reach a critical distance R_c approximately=9 Bohr, at which several additional electrons are lost due to a recently discovered maxima of ionization rates occurring at R=R_c. Then the subsequent Coulomb explosion for the higher-charged ions takes place. The total combined Coulomb explosion energy agrees well with experimental results, showing striking regularities. The origin and intensity dependence of unexpectedly high ionization rates of dissociating nuclei at preferential, large internuclear distances R=R_c is also discussed and an analytic expression for R_c is derived.

We use a novel multidetection coincidence spectrometer to study electron impact double ionization of atoms. New results in the e,(3-1)e mode have been obtained for the first time for helium, where it was not possible with previous systems, highlighting the high sensitivity of the new apparatus. The coincident angular distribution of the two slow ejected electrons has been measured at equal or unequal energy sharing of the available excess energy among the two electrons. The results are compared with preliminary (unpublished) calculations, and with similar double photoionization studies.

We have investigated the generation of the high order harmonics (q=5-15) of a 150 fs Ti:S laser operating at 780 nm in the intensity range 2.5*10¹³-10¹⁵ W cm^-2. We report, we believe for the first time, high order harmonic generation from gases of pure organic molecules (butane C₄H₁₀ and 1,3-butadiene C₄H₆). By comparison with xenon under nominally the same irradiance and gas density conditions, we infer that the conversion efficiency in the molecular systems is approximately an order of magnitude less than that from xenon. We report broad similarity between butane and butadiene under these conditions, with butane being more efficient at higher irradiances. In contrast, at the lowest irradiances investigated (2.5*10¹³ W cm^-2) harmonic generation in butadiene becomes relatively more efficient, when compared to butane, which we tentatively attribute to the presence of the pi delocalized electrons in this molecule.

A new spectroscopic method for the determination of the "bulk-electron temperature" of non-Maxwellian plasmas containing highly charged ions is proposed. Even for extremely high fractions (up to some 10%) where the energetic electrons carry most of the energy the new approach permits one to diagnose the "bulk temperature". An analytical model is derived which approximates the numerical calculations very well for a wide range of plasma parameters and elements (Z_n=6-42). The method is applied to an experimental soft X-ray H- and He-like argon spectrum from a high density plasma.

A consistent analytical approach for calculation of the quasi-classical radial dipole matrix elements in the momentum and coordinate representations is presented. Very simple but relatively precise expressions for the matrix elements are derived in both representations. All analytical expressions contain only one special function-the Anger function and its derivative. They generalize and increase the accuracy of some known quasi-classical expressions. The small difference between the two forms of the expressions for the dipole matrix elements indicates the applicability of the simple expressions given by the consistent quasi-classical approach even for low atomic states.

Absolute photoionization cross section profiles and asymmetry parameters of Ne, Ar and Kr have been calculated at the time-dependent local density approximation level. We employed a very accurate B-spline finite basis set and the modified Sternheimer approach, which is a first-order perturbative scheme particularly suited to finite basis set calculations. The gradient-dependent van Leeuwen and Baerends (VLB) exchange-correlation potential has been used, since it has the correct Coulombic behaviour at large distances which is a necessary condition for the existence of the Rydberg states. A simple modification of the boundary conditions considerably improves the number of terms of the Rydberg series. The resonance parameters obtained by fitting the calculated Fano profiles with a suitable analytic expression are compared with the experimental parameters.

A many-body perturbation theory calculation for the cross section of helium for double ionization due to photoabsorption is presented for the photon energy range from the threshold up to 14 keV. The energy and angular distributions of the photoelectrons are also discussed for relatively high photon energies. Comparisons with the results of experiments and other calculations are made.

The angular correlation between the photoelectron and subsequent polarized fluorescent radiation measured in coincidence is analysed theoretically for atomic photoionization. Using the density matrix and statistical tensor formalism the general expression for the correlation function is obtained for an arbitrary polarized incident photon beam. The particular geometry of a recent experiment (Beyer et al., 1995) is considered and the parameters which determine the angular correlation are expressed in terms of photoionization amplitudes. It is demonstrated that the photoelectron- (polarized) fluorescence coincidence measurements reveal strong circular and linear dichroism. As an example the photoionization of Ca atoms near the 4p ²P_3/2 threshold is calculated within the framework of multichannel quantum defect theory.

Velocity-selective coherent population trapping is a technique for laser-cooling atoms to below the recoil limit where the momenta of the atoms and photons in the light field are equal. We have studied this mechanism using an efficient simulation model, which both simplifies the calculations and gives a physical insight into the dynamics of the anomalous diffusion process. Our calculations have been carried out for transitions in which the total atomic angular momentum is F=1 to 1 in both one and two dimensions and F=2 to 2 in one dimension. In the latter case, no exact dark state exists, and the momentum distribution tends towards a Lorentzian in the steady state.

We present a new method for the calculation of above-threshold ionization (ATI) cross sections based on a description of the atomic structure through a discretized basis. This new approach handles the divergent terms within the discretized continuum by assigning a width epsilon to the nearest discretized continuum level and then extrapolating to the value of the amplitude as epsilon goes to zero. The method is in principle applicable to any discretized basis and to atoms with more than one electron. We demonstrate its usefulness through the calculation of up to four photons absorbed above threshold in hydrogen and up to two photons above threshold in helium.

We develop a time-dependent theory of resonant multiphoton ionization of an atom by a laser pulse, whose envelope is taken to be a hyperbolic secant in time. The choice of this shape enables us to solve the time-dependent Schrodinger equation analytically for an n-state system, of which up to (n-1) levels can be in resonance with the ground state for par of the time. Using the expressions deduced for excited-state amplitudes, a closed-form representation for continuum amplitudes is derived, from which the photoelectron energy spectrum is calculated. Stark shifts of the bound states explicitly retained in the eigenfunction expansion due to coupling to omitted levels are included perturbatively. The theory is applied in this paper to the two-photon ionization of caesium by weak fields. A new kind of structure is predicted. The results are compared to the spectrum generated by a weak rectangular pulse.

For pt.I see ibid., vol.28, p.5057-75 (1995). In the previous paper, a non-perturbative time-dependent theory of resonant multiphoton ionization by both smooth and rectangular pulses was derived, and as a first step, applied to the two-photon ionization of caesium in weak fields. In the present paper, we continue the analysis of the same system, extending the weak-field results and considering the case of stronger fields. At higher intensities the spectra exhibit energy modulations of a different character from those predicted for low power. Here, for stronger fields, the structure arises from Rabi oscillations, which, for the smooth pulses assumed here, are triggered by time-dependent Stark shifts. In order to obtain a better understanding of the energy oscillations, we also look at the photoelectron energy spectrum produced by a rectangular pulse, a problem which is solved exactly. We show that there are other sources of strong-field structure.

A theory for the many-electron multiphoton process is presented. It is shown that after single-electron excitation into some level in the continuum (ATI) an inelastic collision of the excited electron with the parent atomic particle can result in an excitation of the ion. It may be the continuum state excitation giving the doubly charged ion or the discrete state which also greatly enhances the doubly charged ion production. The probability of these phenomena greatly exceeds that of the direct ionization of a single-charged ion. The single-electron ATI makes possible the two-electron process even in a moderate field. The example of two-electron excitations of an He atom in a 780 nm laser field with intensity above ~10¹⁴ W cm^-2 is discussed.

We have measured absolute cross sections of multiple ionization in coincidence with final charge states of projectiles for collisions of C³⁺ ions with Ar atoms at 2 MeV. At this collision energy, the recoil ions with low charge states were mainly produced by pure ionization while capture and loss ionization were important processes for the production of recoil ions with higher charge states. In an analysis of the data, an independent-electron model was applied. The results indicate that the ionization probability of the target electron depends on the impact parameter in ionization processes, such as pure-, capture-, and loss-ionization. The obtained values were compared with calculations by a classical-trajectory Monte Carlo (nCTMC). The experimental cross sections are in good agreement with the calculated values for pure, single-capture and single-loss ionization.

Electron capture from the K-shell of a negative ion by protons is considered by accounting for the interplay between the final channel-distorted projectile and off-shell electronic scattering motions using the distorted strong-potential Born approximation. The distorted motion accounts for the ionized state of the target core and the short-range shielding of the K-shell by the passive outer-shell electrons. Total cross sections versus incident energy are compared for the negative-ion, neutral, and singly-ionized forms of the second- and third-row elements C, O, F and Si, S, Cl. Target core neutrality and diffuseness of the negative-ion screening consistently lower the cross section by amounts ranging from 22 to 37% while the positive-ion results are larger by only 1 to 15%. Previous comparison with experiment and coupled-Sturmian-pseudostate results for neutrals has shown very good agreement.

The driving terms in three-body theories of elastic and inelastic scattering of a charged particle off a bound state of two other charged particles contain the fully off-shell two-body Coulomb T-matrix describing the intermediate-state Coulomb scattering of the projectile with each of the charged target particles. Up to now the latter has usually been replaced by the Coulomb potential, either when using the multiple-scattering approach or when solving three body integral equations. General properties of the exact and the approximate on-shell driving terms are discussed, and the accuracy of this approximation is investigated numerically, both for atomic and nuclear processes including bound-state excitation, for energies below and above the corresponding three-body dissociation threshold, over the whole range of scattering angles.

Superelastic electron scattering involving the de-excitation of laser-excited ¹³⁸Ba (...6s6p¹P₁) to the lower-lying (...6s5d¹D₂) metastable level has been investigated. These measurements have been used to extract the electron impact coherence parameters which describe excitation out of an isotropic (...6s5d¹D₂) level to the higher-lying (...6s6p¹P₁) state. Studies have been carried out for an electron impact energy of 20 eV and a range of scattering angles up to 45 degrees . This investigation represents a significant extension of coherence parameter measurements to inelastic transitions between excited atomic levels.

New measurements of absolute differential cross sections (DCSS) for vibrational excitation of O₂, with improved sensitivity and resolution, are presented. The 90 degrees DCSS are given as a function of energy up to 16 eV for elastic scattering and vibrational excitation of up to v=7. A weak continuous, probably non-resonant, background scattering is found in the v=1 cross section in addition to the well known sharp ² Pi _g resonances dominating vibrational excitation in the energy range up to 2.5 eV. The sharp ² Pi _g resonances interfere coherently with the background scattering both in the v=1 and in the elastic channels. New absolute values for the energy-integrated DCSS for ² Pi _g resonances are given. They exhibit oscillatory Franck-Condon factors in the v=4-7 exit channels. The broad resonance peaking at 9 eV, dominating vibrational DCSS at higher energies, causes excitation of high vibrational levels (up to the dissociation limit) of the X ³ Sigma _g^- ground state, but not of the a¹ Delta _g and b¹ Sigma _g⁺ electronically excited states, supporting its assignment as the 4 Sigma _u^- resonance, the selectivity being caused by spin selection rules. The excitation of high vs indicates a relatively narrow autodetachment width for this resonance. The angular dependence at 9 eV is, however, not a simple p_sigma wave as would be expected for a simple sigma _u* shape resonance. Preliminary experiments with a free jet cooled sample, concerned with rotational broadening of the ² Pi _g resonance, are reported.