Table of contents

Rydberg states in sodium are excited using two identical broadband, dye laser pulses. Each 8 ns pulse has a bandwidth of approximately 45 cm^-1, and Rydberg states with several different principal quantum numbers are simultaneously excited. The number of Rydberg atoms created by each pulse pair is detected as a function of the delay between the two pulses. Strong modulations in the population in each Rydberg level and in the total Rydberg signal are observed. The oscillations are identical to those which would be expected if two transform-limited, 300 fs laser pulses had been used. We show explicitly, in the limit of weak pulses, that these 'bound-state interferograms' acquired using the time-domain, Ramsey separated-oscillatory fields method are only dependent on the excitation cross section and pulse frequency spectrum. Therefore, weak-field interferograms do not in themselves give information on the spatial-distribution of the electron wavefunction. In addition, the benefits of using strong or weak pulses for bound-state interferometry are discussed.

The effects of very strong, short-pulsed,high-frequency fields on one-dimensional hydrogen atoms is studied. The classical motion shows signs of incipient chaos with increasing field strength. We derive a sequence of maps which accurately describe the classical dynamics and quantize an approximation to these maps to describe the quantal behaviour of the system. The quantum effect of the incipient chaos is observable in a dramatic way. We also show that because the field frequency is high the approach of the quantum system to the classical limit is very slow.

Calculations incorporating the effects of electron correlation on the Auger energies and widths of the neon 1s²(2s2p⁵)^3,1P⁰3p²S states have given results in agreement with the recent measurements of Pahler et al. The Ne⁺ 1s²2s2p⁵¹P⁰3p²S autoionizing state is above the Ne²⁺ 1s²2s2p⁵³P⁰ threshold and, using the theoretical Auger electron energy, its decay width to this channel is computed to be 0.48 eV. At the Hartree-Fock level this quantity is equal to 0.280 eV. The partial width of the ¹P⁰3p ²S state to the Ne²⁺ 2s²2p⁴¹D channel is essentially zero while that to the Ne²⁺²s²2p^{4 1}S channel is 0.028 eV.

The 19-state and 29-state R-matrix methods of Fon et al. (1994) have been extended to calculate the differential cross section (DCS) and electron impact coherence parameters (EICP) associated with excitation of the 3 ¹D state of helium for an incident electron energy of 26.5 eV at electron scattering angles ranging from 0-180 degrees . The calculated EICP values are compared with the only experimental data reported recently by McLaughlin et al. (1994). The agreement is good.

The convergent close-coupling (CCC) method for the calculation of electron-helium scattering is used to generate singly differential ionization cross sections, These are obtained directly from cross sections corresponding to the positive-energy states (pseudostates) used in the close-coupling expansion of the total wavefunction. The method is applied to 100 eV electron impact ionization of the ground state of helium. Agreement with experiment is found to be very good indicating that the CCC method is able to obtain correct ionization cross sections even in the case where the two outgoing electrons have similar energy.

Elastic cross sections for positron scattering on noble gases (Ar, Kr) at energies of a few tens of eV exhibit structures which have been attributed to absorption effects. Recent, precise measurements for Ar do not confirm the existence of such effects for electron scattering. Here, we show that previous measurements indicate the existence of an absorption effect for e^-+H₂ scattering around 60 eV. The reason for this can probably be ascribed to the e^--H₂ total cross section partitioning scheme, which is more similar to the one for e⁺-Ar than to the one for e^--Ar.

The relativistic configuration interaction method has been used for the investigation of the Sr I spectrum which arises from excitation of one electron out of the 4p shell. Many-body effects are found to dominate the lower part of this spectrum through extensive configuration mixing. Oscillator strengths for the transitions between these levels and the Sr I ground state are used to obtain a convoluted Sr I absorption spectrum. Theoretical predictions for the structure and position of Sr II levels, serving as Sr I series limits in corresponding experimental investigations, are also provided. A statistical analysis was carried out on the theoretically predicted Sr I level energies and a Wigner distribution for them has been obtained. This is a consequence of the strong configuration mixing, which in turn is related to the contraction of the Sr 4d orbital. A value of 0.59 for the Brody parameter q, a measure of deviations from equal interlevel spacings and an indicator of the degree of level repulsion, has been determined.

Multiconfiguration Dirac-Fock (MCDF) calculations have been made for the field isotope shift ratio of the 3p⁵(4p-5d) and 3p⁵(4s-4p) transitions in argon, in an attempt to account for a seeming anomaly in the existing experimental values.

The Stark shift of the (6s)^{2 1}S₀ to (6s6p)³P₁ transition in ytterbium was measured using a laser to excite an atomic beam first in a uniform electric field and subsequently in a field-free region. The laser frequency was scanned across the transition while fluorescence produced by the radiative decay of the excited state was detected. The frequency scan was calibrated using isotope and hyperfine splittings that have been measured previously. The Stark shift rate was found to be -15.419+or-0.048 kHz kV^-2 cm^-2 and the tensor polarizability of the (6s6p)³P₁ state was determined to be alpha₂=5.81+or-0.13 kHz kV^-2 cm^-2.

Spectra of C, N and O have been generated simultaneously by focusing 1 GW laser pulses on targets made of either ammonium hydrogen carbonate or ammonium oxalate. These materials, in the form of compressed powders, were found to give significantly smaller linewidths than more conventional targets such as boron nitride or aluminium oxide. The wavelengths of the 1s²¹S-1snp ¹P (n=2-4) resonance lines in N VI and O VII (17-30 AA) have been determined with uncertainties ranging from 0.2 to 0.7 mAA, by measurements in up to the 8th spectral order. Previous experimental results are confirmed, but the substantially increased accuracy of the present values reveals small but significant and systematic discrepancies with theoretical predictions for the 1s²¹S-1s2p ¹P transition in N VI and O VII.

Applying the momentum method, the Fokker-Planck equation is derived from the Boltzmann equation for the r^-4 potential between atoms and ions. Two earlier attempts are revealed to be limiting cases of our more general equation, namely for very large and very small mass ratio of the collision partners. The Fokker-Planck equation, after being completed with a decay term and a source term, is solved by the Green's function method. The solution turns out to be composed of modes in the velocity space which decay independently towards a stationary spectrum. The amplitudes of the latter depend on the source term which we assume to be shaped like a delta -function if laser pumping is accounted for. Only for very long relaxation times does the lowest mode prevail in the spectrum which then merges into the Maxwell distribution. Several new lines for ion-atom pairs as well as atom-atom pairs have been measured. They are presented along with the fits using the aforementioned formula. As expected, the agreement is satisfactory only in the first case. In the second case the Fokker-Planck approximation no longer applies.

The absolute photoabsorption cross section of atomic barium in the region of the 4d-giant resonance (80-140 eV) has been determined. The vapour density in the absorbing metal vapour column, required for the determination of the absolute cross sections was measured quasi-simultaneously with the absorption using a Mach-Zehnder interferometer. Our data show good agreement in terms of spectral signature with existing relative photoabsorption measurements. The measured absolute cross section corroborates the results of recent many-body calculations.

Using the (t,t') method as introduced by Peskin and Moiseyev (1993) the resonance quasivibrational energies and lifetimes of a time-dependent (not necessarily time-periodic) Hamiltonian are obtained by computational methods originally developed for time-independent Hamiltonians. As an illustrative numerical example the multiphoton resonances of the dissociating H₂⁺ are calculated using three Fourier basis functions. The calculation of resonances at high intensity fields by direct methods (i.e. diagonalization of the Floquet Hamiltonian matrix or a coupled-channel propagative approach) requires, however, the inclusion of 10 or more Floquet channels.

Using the (t,t') method the time-dependent Schrodinger equation for two electronic potential surfaces which are coupled via a time-dependent (not necessarily time-periodic) field is transformed from the length to momentum, reduced momentum and acceleration gauge representations. Combining the (t,t') method and complex scaling enables the calculation of partial widths by the asymptotic analysis of the tail of the resonance wavefunction obtained in the length gauge after being transformed to the acceleration gauge such that a free particle is obtained as R (the reaction coordinate) to infinity regardless of the shape of the laser pulse. As an illustrative numerical example the partial widths of H₂⁺ in high periodic fields are calculated.

The combined R-matrix, complex-coordinate rotation method is extended, by incorporating a frame transformation, to allow the calculation of the near-threshold spectrum of a simple molecule, para-H₂, in the presence of a laboratory strength magnetic field. The below-threshold photoabsorption spectrum obtained is in very good agreement with that from a previous calculation using a different method and the role of excited rotational states of the parent molecular ion in the above-threshold spectrum is identified.

Results are presented for the angular distributions of electrons from spatially aligned core excited CO using an angle-resolved photoelectron-photoion coincidence technique in the regions of the C and O 1s edges and sigma * resonances. Coincidence measurements were recorded between ions ejected at 0 degrees and 90 degrees with respect to the electric field of the linearly polarized radiation and for photoelectrons ranging from 0 degrees to 90 degrees with respect to the molecular axis. The results show a strong departure from angular distribution measurements made more conventionally; in particular, multiple outgoing partial waves of high angular momentum are seen to make a significant contribution.

Cross sections for the ionization-excitation of helium following electron and H⁺, H₂⁺ and H₃⁺ ion impact have been measured for He II (np) ²P⁰ states with n=2-5. These measurements were performed for the velocity range from 3.8-8.5 au for electrons, 1.4-7.5 au for protons and 1.4-4.0 au for H₂⁺ and H₃⁺ ions. The cross sections have been obtained by measuring the extreme ultraviolet (EUV) emission from the decay of the excited target atoms at 90 degrees to the projectile beam. We have compared our results to the current limited literature for He II (2p) ²P⁰ cross sections with good agreement observed. We have also examined the cross section ratio of ionization-excitation to excitation, sigma ⁺*((np)²P⁰)/ sigma *((1snp)¹P⁰), for electron and proton impact in comparison with the ratio of double to single ionization, sigma ⁺⁺/ sigma ⁺, as a limiting value for large n. Finally, cross section results from H_n⁺ impact, where n=1-3, are discussed in terms of a semiempirical scaling relation for molecular hydrogen projectiles.

A crossed beam study of the rearrangement reaction H^-+D₂( upsilon =0) to HD( upsilon ')+D^- in the collision energy range E_rel=0.3-3 eV is reported. From the vibrationally state resolved energy and angular spectra of the D^- product ions, we derive state-specific differential cross sections in the CM system. Absolute units are obtained by using elastic H^-+He scattering as a reference system. The measurements allow detailed conclusions concerning the reaction mechanism. By integrating over angles and summing over partial cross sections (for individual upsilon ' states of the HD product molecule), we determine the absolute total cross section of the reaction. The reaction has a threshold at E_rel=0.42+or-0.12 eV. The cross section rises to a maximum of 2*10^-16 cm² at 1.5 eV and then rapidly decreases again. The present results resolve a discrepancy between two previous measurements bearing on the determination of the total cross section.

We have recorded photon spectra during collisions between N⁵⁺ ions and lithium atoms in the range 200-600 nm at different projectile energies (40-120 keV). The spectral resolution allowed the determination of capture cross sections of nl subshells with principal quantum numbers n=6, 7 and 8. The experimental results are compared to calculations using the three-body classical trajectory Monte Carlo method (CTMC). On the whole, a good agreement between experimental and CTMC calculated results is obtained. The influence of the projectile core electrons is examined comparing CTMC calculations for the B⁵⁺-Li collision system to those for N⁵⁺-Li. A small but not negligible core-electron effect is observed which manifests itself mainly in the population of the N⁴⁺(ns) subshells.

A procedure is described for developing numerical solutions to the monopole model of the e^--H scattering problem. Chebyshev collocation is used to generate an approximate solution and a K-matrix that includes contributions from the continuum states. A variational method is described which is used to calculate a correction to the K-matrix thus computed which is then found to be accurately symmetric. Illustrative calculations of excitation and ionization cross sections are presented.

Employing the 'crossed-beams' technique, electron-impact ionization cross sections for the single-ionization of W^q+ ions in charge states q=1...10 have been measured. The cross sections for the ionization of ions in charge states q>or=5 show significant contributions below the ground-state threshold caused by the ionization of ions in excited, long-lived metastable states in the parent ion beam. In the case of W⁶⁺, the measured cross section is completely dominated by metastable contributions. For ions in charge states q=3...7, excitation-autoionization from the ground state as well as from excited, metastable states contributes strongly to the cross sections. This results in a strong underestimation of the measured cross sections by the semiempirical Lotz formula in the energy range between the ionization threshold and the cross section maximum.

Differential cross sections for simultaneous electron-photon excitation of He 2³S and 2¹S are calculated for impact energies near threshold. An extension of Kroll and Watson's theory (1973) to inelastic collisions is used. Laser-off differential cross sections are calculated using a (29-state) R-matrix approach. Important modifications in the differential cross section are produced when the laser is turned on.

We consider the reflection of electrons by an infinitely high potential wall in the presence of a bichromatic laser field having two components of frequency omega and 2 omega . Both components are out of phase by an angle phi . We evaluate the induced and inverse bremsstrahlung for this elementary scattering process and we discuss the coherent phase-control of the non-linear currents j(n; phi ) of electron reflection. If E₀ is the initial electron energy and h(cross) omega the photon energy, two specific cases will be of interest: (a) E₀>>h(cross) omega and (b) E₀>or approximately=h(cross) omega , where in the latter case off-shell effects are particularly relevant. We shall relate our findings to the corresponding scattering of electrons by a hard sphere in a bichromatic field.

A recent experiment demonstrating a gain length approximately=6 in a C VI laser at 182 AA has revived interest in recombination as an approach to soft X-ray lasing. A single 30 J beam of a 2ps CPA Nd-glass laser was used to generate gains of 12.5 cm^-1 in a 5 mm carbon fibre. We extend earlier work on scaling laws to investigate the parameter space of the experiment, perform detailed simulations with a 1D Lagrangian code and consider some aspects of the refraction properties of the system. We investigate the extension of the scheme to longer lengths and to the shorter wavelength O VIII laser. Both of these extensions appear to create some difficulties which predominantly arise from the limitations on the target length imposed by the short time-scales of the gain. It appears, however, that in the case of carbon saturation should be attainable without the use of travelling wave pumping. We investigate the role of impurities in oxygen targets (which are unavoidably present) and conclude that the presence of impurities is generally detrimental, but need not be significantly damaging.

In the multiphoton regime, the correlation between high-harmonic generation and multiphoton ionization is investigated when the laser polarization changes from linear to elliptical. The ellipticity dependence of high harmonics and ion signal is measured experimentally. Both high harmonics and ion signal decrease with increased ellipticity at fixed laser intensity, but the decrease in ion signal is much slower compared to that of the harmonics. This phenomenon is due to the fact that high-harmonic generation is more strongly affected by the selection rules than ionization, when polarization changes.

Time- and two-dimensional spatially-resolved monochromatic imaging is performed on Ne-like germanium laser-produced line plasmas to study the behaviour of the inhomogeneities along the line created by a nonuniform irradiation. The imaging is done at the wavelengths of the 2s2p⁶3d-2s²2p⁶ (J=2-0) E2 quadrupole line ( lambda =8.031 A) and the 2s2p⁶3p-2s²2p⁶ (J=1-0) 3A dipole line ( lambda =8.371 A) which originate respectively from the upper and lower states of the core-excited 3d-3p X-ray laser line at 199 A. We compare the E2 and 3A results to LASNEX hydrodynamics and associated atomic physics, and investigate the problem of extracting the 3d-3p local gain dynamics along the inhomogeneous line plasma from spectroscopic imaging of the plasma. The effect of the irradiation uniformity conditions on the integrated lasing intensity is discussed.