[en] This dissertation reports experimental measurements of partial decay cross sections in the H_- photodetachment spectrum. Observed decays of the ¹P₀ H^-**(n) doubly-excitedresonances to the H(N=2) continuum are reported for n=2,3, and 4 from 1990 runs in which the author participated. A recent analysis of 1989 data revealing effects of static electric fields on the partial decay spectrum above 13.5 eV is also presented. The experiments were performed at the High Resolution Atomic Beam Facility. the Los Alamos Meson Physics Facility, with a relativistic H^-beam (β=0.842)intersecting a ND:YAG laser. Variation of the intersection angle amounts to Doppler-shifting the photon energy, allowing continuous tuning of the laser energy as viewed from the moving ions' frame

[en] When static electric fields (F≤90 kV/cm) were applied to the H^- photodetachment interaction region, new structure and lowered thresholds for production of neutral hydrogen were observed. Relative partial cross sections were measured by detection of excited states of the fragment neutral hydrogen atom H(N=4, 5, or 6) resulting from laser interaction with relativistic H^- ions. Downward shifts in the onset of excited hydrogen production are observed to increase with field strength, and agree with a recent hyperspherical coordinate interpretation of Zhou and Lin [Phys. Rev. Lett. 69, 3294 (1992)]. Field-induced window-type resonance structure is observed both below and above the zero-field threshold (ZFT) energy. Quenching of high-lying autoionizing states was also monitored, providing evidence of field-induced tunneling by resonances lying just below the associated ZFT

[en] Foil stripping of H^- directly to H⁺ is being considered for proton injection in the next generation of high-current proton storage rings. This technique can result in significant losses because excited states of HO, which are also produced in the foil, are field stripped in the downstream bending magnets. Without due care in the injection system design, many of the resulting protons will be outside the acceptance of the storage ring and will be quickly lost. We measured the production of such H⁰ excited states at the LAMPF High Resolution Atomic Beam Facility. An 800-MeV H^- beam was passed through carbon foils of thicknesses 70, 100, 200, and 300 μg/cm², and the excited states were analyzed by a special magnet downstream of the foil. The magnet had a linear field gradient so that the trajectories of the outgoing protons could be used to reconstruct the field values at which the various H⁰ stripped. We found that about 1% of the H⁰ emerge in excited states which can be stripped to protons by ring-bending magnets

[en] Two resonances in the photodetachment spectrum of H^-, one just below the n=3 threshold and the other just below the n=4 threshold, were observed decaying into H⁰(n'=2)+e. The n=3 resonance shows a preference for the n'=2 channel when compared with total-detachment measurements. Fano-line-shape fits give central energies of 12.652±0.003 eV for H^-**(3) and 13.338±0.004 eV for H^-**(4). These values are in good agreement with current theoretical calculations

[en] Full text: The study of doubly-excited states of helium has long been used as a sensitive test of the understanding of electron correlations. Synchrotron radiation provides a high resolution source of photons in the energy range required for excitation of these states, which is inaccessible using laser sources. To date, studies of the influence of a high electric field on the behaviour of doubly excited states have been limited, and a new apparatus has been constructed which allows fields of up to 90 kV/cm to be applied to the interaction region where the states are formed. Photons from beamline 10.0.1 at the Advanced Light Source, Berkeley, have been used to study doubly-excited autoionising states of helium in a high electric field. The states appear as resonances in the detected ion yield, metastable yield and photon decay spectra. Previous experiments at the Photon Factory, Japan, has shown the Stark shifting and mixing of the resonances observed in the ion yield spectrum. The evolution of the states as a function of the field strength was, however, quite different to that predicted by a recent theoretical calculation. Application of novel techniques, taking advantage of the time structure of the synchrotron radiation, has allowed the separation of the photon decay and metastable yield channels. The doubly excited resonances have thus been able to be observed in 3 different decay channels, allowing further insight into the structure of these states. Complex X-ray form factors are used in crystallography, material science, medical diagnosis refractive index studies and XAFS. We introduce the X-ray Extended-Range Technique for accurate measurements of the mass attenuation coefficient and the imaginary component of the atomic form factor. This new technique achieves accuracies of 0.3% for attenuation coefficients of silicon single crystals in the range from 5 keV to 20 keV. This compares to accuracies of order 10% using atomic vapours, and to discrepancies between measurements using earlier experimental techniques of order 10%. Discrepancies of order 5% between current theory and experiments can now be addressed. The new result challenges available theoretical calculations and suggesting that new methods of computation may be required to approach the precision and accuracy of the experimental data

[en] The relative photodetachment cross section for decay into the H(N=2) channel by the ¹P degree shape resonance in H^- was measured, as well as that for decay into all channels. The branching ratio σ(N=2)/σ(total) was computed for a series of energies between 10.95 and 11.3 eV after normalizing the cross sections to theoretical peak amplitudes. The maximum branching ratio (∼0.8) appears at an energy about 20 meV higher than the central energy of the resonance. Results are compared with recent theoretical calculations

[en] We have measured the cross section for the detachment of three electrons from the Cl^- ion following the absorption of a single photon. The triple-photodetachment cross section is compared to our earlier measurement of the cross section for double photodetachment from the same ion. Possible direct and indirect mechanisms responsible for multiple-electron detachment are discussed. The direct process of ejection of three valence electrons seems to be the most likely process. This conjecture needs theoretical verification

[en] Cross-sections for inner-shell photoionization processes in beryllium-like carbon ions are determined both theoretically and experimentally for photon impact energies in the region of the carbon K edge. Measurements of absolute cross-sections, resonance energies, and lifetime/linewidths measured using the merged-beam technique at the Advanced Light Source are in good agreement with R-matrix calculations

[en] The correlated process involving the photodetachment of two electrons from the Cl^- ion has been investigated over the photon energy range 20-45 eV. In the experiment, a beam of photons from the Advanced Light Source (ALS) was collinearly merged with a counterpropagating beam of Cl^- ions from a sputter ion source. The Cl⁺ ions produced in the interaction region were detected, and the normalized signal was used to monitor the relative cross section for the reaction. An absolute scale for the cross section was established by measuring the spatial overlap of the two beams and by determining the efficiency for collection and detection of the Cl⁺ ions. The overall magnitude and shape of the measured cross section for this process agrees well with an R-matrix calculation. The calculation identifies the dominant mechanism leading to the production of the Cl⁺ ion as being a direct nonresonant process involving the ejection of a pair of electrons from the valence shell. Less important is the indirect nonresonant process that involves the production and decay of core-excited and doubly excited states of the Cl atom in an intermediate step. Direct and indirect resonant mechanisms involving the excitation of a single 3s core electron or more than one valence electron of the Cl^- ion were found to be insignificant in the energy range studied

[en] 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 (1s²2s²¹S) 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