Table of contents

To test the validity of classical trajectory and perturbative quantal methods for electron-impact ionization of H-like ions from excited states, we have performed advanced close-coupling calculations of ionization from excited states in H, Li²⁺ and B⁴⁺ using the R-matrix with pseudo states and the time-dependent close-coupling methods. Comparisons with our classical trajectory Monte Carlo (CTMC) and distorted-wave (DW) calculations show that the CTMC method is more accurate than the DW method for H, but does not improve with n and grows substantially worse with Z, while the DW method improves with Z and grows worse with n.

The advanced adiabatic approach describes all inelastic processes including ionization. It is based on the time–energy Fourier transform to obtain the asymptotic solution of the non-stationary Schrödinger equation. This transformation takes into account the time delay of the outgoing wave packet related to the ionization channel which is causally connected to the Hamiltonian at the moment of ionization. Another important aspect of the adiabatic approach is the discovery of hidden crossings in 1981. They arise whenever, in the classical description of the adiabatic state, the full-dimensional classical trajectory of the electron collapses into an unstable periodic orbit. As a result, a strong coupling between adiabatic states appears. The hidden crossings provide a complete description of non-adiabatic transitions and open the way to systematic applications. The advanced adiabatic approach together with the hidden crossings can compete with direct numerical calculations in respect to precision and, particularly, in respect to the required computer facilities. But what is much more important, it also provides clear insight into the mechanisms responsible for the processes studied. The application of the theory to various inelastic processes in atomic and molecular physics, such as charge transfer in He²⁺ + H(1s) collision, ionization of H(1s) by an electric pulse, transitions between rotational states of the H₂O molecule induced by an electric pulse etc, are discussed.

We studied the Penning ionization of the CHF₂Cl molecule with He and Ne metastable atoms (He* and Ne*). We measured the electron kinetic energy and the time-of-flight mass spectra; we also determined the branching ratio for the parent ion and charged CHF⁺₂, CHFCl⁺, HCF⁺/CF⁺ and Cl⁺ fragments. These data led us to discuss the dissociation channels for all the energetically-accessible electronic states of the ionized molecule. We evidenced a marked contrast in the fragment ion proportions for Ne*–CHF₂Cl and He*–CHF₂Cl systems, and related it to the difference in polarizability and internal energy of the He* and Ne* atoms.

The fragmentation of the H₃ 3s (N = 1, K = 0) molecule into three hydrogen atoms is investigated. The exchange properties of the protons and electron are considered and lead to selection rules both in the possible initial state as in the observed momentum distributions of the fragments. Model wavefunctions are used to calculate the fragmentation probabilities showing basically ring structure in the Dalitz plots.

We report on an experimental study of slow pulse propagation and polarization storage of light in an atomic vapour for pulse durations ranging from several µs to 100 µs, for non-zero magnetic fields, for temperatures ranging from 65 to 90 °C and for two different rubidium isotopes. The results confirm the presence of the electromagnetically induced transparency window as predicted by Lukin (1997 Phys. Rev. Lett.79 2959). We show that time-dependent optical rotation can mask the storage of light signal, and demonstrate storage of light for the ⁸⁵Rb isotope.

A comparative study of high harmonic generation (HHG) by atoms and ions with active p-electrons is carried out in the theoretical framework of the rescattering mechanism. The substate with m_ℓ = 0, i.e. zero orbital momentum projection along the electric vector of a linearly polarized laser wave, is found to give the major contribution to the HHG rate. Our calculations for HHG by an H atom in an excited 2p-state demonstrate that the rate for recombination into a final state with a different value of m_ℓ (=±1), is higher for lower harmonic orders N, while for higher N (beyond the plateau domain) the difference vanishes. For species with closed electron shells, the m_ℓ-changing transitions are forbidden by the Pauli exclusion principle. We report absolute HHG rates for halogen ions and noble gas atoms at various intensities. These results demonstrate that the Coulomb binding potential of the atoms considerably enhances both the ionization and recombination steps in the rescattering process. However, the weak binding energy of the anions allows lower orders of HHG to be efficiently produced at relatively low intensities, from which we conclude that observation of HHG by an anion is experimentally feasible.

The partial ion yield spectra of Xeⁿ⁺ ions (n = 1–8) were detected by an ion time-of-flight mass analyser in the photon energy range of 664–694 eV, and the branching ratios of ions following the 3d⁻¹_5/2 ionization and the 3d_5/2→ 6p resonance excitation were obtained. The branching ratios were found to be almost the same. Theoretical de-excitation pathways and branching ratios of ions following the 3d_5/2→ 6p resonance excitation were obtained using the single-configuration approximation and a semi-empirical model, and results were compared with the experiments and the previously published calculations for the 3d⁻¹ ionization.

Using the R-matrix Floquet approach we have calculated single- and two-photon ionization cross sections for Sr within a model potential approach. The frequency ranges investigated range from 0.21 to 0.27 au and from 0.105 to 0.158 au. The results are in reasonable agreement with previous experimental and theoretical work. The two-photon cross sections are dominated by the 5s6p ¹P^o resonance. For the m = 0 level of this resonance, we obtain photoionization cross sections ranging typically between 5 and 20 Mb. A comparison with results for Ca shows good similarity for single-photon ionization, but significantly less agreement for two-photon ionization.

This paper presents an experimental and theoretical analysis of quantum interference between Rydberg wave packets in Na. Pairs of phase-locked wave packets manipulate the total orbital angular momentum of Na Rydberg atoms. Initially, the wave packet is composed of a superposition of s and d Rydberg series. Exploitation of the difference between the quantum defects of the two series allows one to predict the phase of the second wave packet required to engineer specific angular momentum compositions within the resultant wave packet. Experimentally, this final quantum state distribution is analysed in the frequency domain using state-selective field ionization and in the time domain using the optical Ramsey method. The theoretical calculations show how the phase difference between pairs of optical pulses is linked to the corresponding Rydberg frequency spectrum, therefore enabling the control of the quantum state composition of the wave packets. Finally, it is shown that by intuitively chirping one of the laser pulses it is possible to compensate for the dispersion of the wave packet and improve the effectiveness of the angular momentum control.

We ionized He, Ne and Ar in the tunnelling regime with a linearly polarized laser pulse and measured the ion-recoil momentum distribution on an axis perpendicular to laser polarization for two different wavelengths (λ = 800 nm and λ = 1800 nm). We observed a significantly narrower distribution for 800 nm than for 1800 nm. Classical simulations of the electron wave packet's evolution under the combined influence of the ion's Coulomb potential and the laser field show that the narrowing is caused by Coulomb focusing of the electron wave packet during recollisions. The narrowing is sensitive on the longitudinal momentum of the wave packet after tunnelling, which suggests a way of measuring it. Measurements at 800 nm in circular polarization–for which no recollision occurs–do not exhibit this narrowing of the perpendicular momentum distribution. Simulations for circular polarization show another aspect of Coulomb focusing: the Coulomb potential also affects the wave packet's transverse momentum as it leaves the saddle point immediately after tunnelling.

We report here an analysis of the B¹Π_u → X¹Σ⁺_g band system of the sodium dimer. The body of data includes laser-induced emission of Na₂ consisting of discrete molecular fluorescence (107 fluorescence series) excited by visible lines from an argon ion laser and rotational relaxation lines for strong transitions. In order to enlarge the data set of transitions within this band system, which cover both X¹Σ⁺_g and B¹Π_u states up to dissociation, the results were combined with spectroscopic experimental data available in the literature. The obtained Dunham coefficients were used to derive the corresponding molecular potentials using basically the RKR method. Hybrid potential energy curves for the X¹Σ⁺_g and B¹Π_u states have been checked by solving numerically the radial Schrödinger equation observing a good agreement with the experimental data.

In this work we calculate the outer valence ionization spectra of the CO, H₂S and TlH molecules by application of the recently developed Dirac–Hartree–Fock one-particle propagator technique. The molecules hereby serve as representatives of light and heavy systems and the influence of the Gaunt term on the spectral structure is investigated. The Gaunt term is a first correction to the non-covariant Coulomb repulsion term and has a considerable effect on the electronic ground state energy and also affects properties mainly determined by the inner-core region of the wavefunction where the electrons are fast. In order to reveal a possible influence of the Gaunt correction on the valence photoelectron (PE) spectra of heavy systems a consistent theoretical description of electron correlation and relativity is necessary. It was observed that the Gaunt correction plays only a minor role for the valence PE spectra even in heavy systems such as TlH and can safely be neglected.

Absolute cross-section measurements for K-shell photoionization of Be-like C²⁺ ions have been performed in the photon energy range 292–325 eV. These measurements have been made using the photon–ion merged-beam endstation at the Advanced Light Source, Lawrence Berkeley National Laboratory. Absolute measurements compared with theoretical results from the R-matrix method indicate that the primary C²⁺ ion beam consisted of 62% ground-state (1s²2s^{2 1}S) and 38% metastable state (1s²2s2p ³P^o) ions. Reasonable agreement is seen between theory and experiment for absolute photoionization cross sections, resonance energies and autoionization linewidths of K-shell-vacancy Auger states.

Absolute differential cross sections are reported for electron capture and loss by 1–5 keV H atoms incident on CH₄ for laboratory scattering angles up to 1.62°, and for charge transfer of 0.5–5 keV H⁺ with CH₄ for scattering angles up to 2.09°. Electron-loss collisions are seen to result in comparatively large scattering angles and a very clear similarity exists between the present differential cross sections and those reported for other molecular targets. The present charge-transfer differential cross sections are consistent with that of Gao et al (1990 Phys. Rev. A 41 5929–33) but not with the calculations of Kimura et al (1995 Phys. Rev. A 52 1196–205). Prior experimental studies of electron-loss and charge-transfer are generally in good accord with the integral values reported here as are the calculations of Kusakabe et al (2000 Phys. Rev. A 62 062715).

Single-electron capture and ionization in He²⁺+Na collisions at energies around the matching velocity (2–13 keV amu⁻¹) have been studied both experimentally and theoretically. State-selective cross section for capture into the n = 2, 3, 4 and n ⩾ 5, and the ionization cross section as well as differential cross sections for capture into n = 3 and 4 were obtained by the MOTRIMS method and compared with CTMC calculations. Good agreement was found between experiment and CTMC, especially concerning capture into high n-shells (n = 4, n ⩾ 5) and the ionization cross sections. For capture into the subdominant n = 2 shell close coupling calculations show better agreement. The differential cross sections for capture into n = 3 and 4 show a different energy dependency.

(e, 2e) studies have been conducted ionizing H₂ from the X¹Σ⁺_g state to the X²Σ⁺_g H⁺₂ ground state. The electron energy ranged from 10 eV through to 40 eV above the ionization threshold for experiments performed in a coplanar symmetric geometry, with both scattered and ejected electrons sharing equal energy. Results are also presented for asymmetric scattering where one of the electrons carried 80 eV energy away from the interaction in a forward direction at an angle of 35° to the incident electron beam. These experiments were performed to test the hypothesis that the two nuclei in the molecule act as a 'double scattering' centre, with the possibility of producing Young's double-slit-type interference in the resulting differential cross section. New features are found in the ionization cross section for H₂ in a coplanar symmetric geometry; however it is unclear at present if these are due to Young's double-slit interference.

Electron-impact n-fold (n= 2–5) ionization of Sc⁺ ions has been studied covering an energy range from threshold to 1000 eV. An electron-ion crossed-beams set-up was employed for the measurement of absolute cross sections as well as for high-resolution energy-scans by which fine details in the energy dependence of the cross section could be uncovered. Contributions of indirect resonant and non-resonant processes to the partial ionization cross sections such as excitation or ionization of an inner shell electron with subsequent autoionization processes have been observed.

Electron-impact excitation collision strengths of the Fe-peak element Fe IV are calculated in the close-coupling approximation using the R-matrix suite of codes PRMAT designed for parallel processors. One hundred and eight LS-coupled states arising from the 3d⁵, 3d⁴4s and 3d⁴4p configurations of Fe IV, are retained in the present calculations. Detailed multi-configuration interaction target wavefunctions are used with the aid of 3p² → 3d² electron promotions and a correlation orbital in the present calculations. Effective collision strengths for optically forbidden transitions, which are extremely important in the analysis of lines in the Fe IV spectra, are obtained by averaging the electron collision strengths for a wide range of incident electron energies, over a Maxwellian distribution of velocities. Results are presented for electron temperatures (T_e in Kelvin) in the range 3.3 ⩽ Log T_e⩽ 6.0 applicable to many laboratory and astrophysical plasmas for transitions within the 3d⁵ manifold. The present results compared to previous investigations provide improved results for important lines in the Fe IV spectrum.

The outer valence shell photoelectron spectrum of CF₃SF₅ has been studied experimentally and theoretically. Synchrotron radiation has been used to record angle-resolved outer valence shell photoelectron spectra of CF₃SF₅ in the photon energy range 18–60 eV. These spectra have allowed photoelectron asymmetry parameters and branching ratios to be derived. The Outer Valence Green's Function approach has been employed to calculate the molecular orbital configuration and associated binding energies. A charge distribution analysis has also been obtained. Assignments have been proposed for the peaks observed in the photoelectron spectrum. The absolute photoabsorption cross section of CF₃SF₅ has been measured from threshold to 40 eV, and strongly resembles that of SF₆. Assignments, involving intravalence transitions, have been proposed for some of the principal features appearing in the photoabsorption spectrum of CF₃SF₅.