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In this thesis a general theory of electron detachment in slow collisions of negative ions with atoms is presented. The theory is based upon a semiclassical close-coupling framework, following the work of Taylor and Delos. The Schrodinger equation is reduced, under certain assumptions, to a non-denumerably infinite set of coupled equations. We develop a new method for solving these equations that is more general than the methods used by Taylor and Delos. A zero-order approximation of our solution is applied to the case of H('-)(D('-)) on Ne collisions, the results are compared with the experimental data, and we find good agreement between theory and experiment, particularly with regard to the isotope effect. A first-order approximation of the solution is proved to be very close to the exact solution, and it is applied to the case of H('-)(D('-)) on He collisions. We use quadratic and quartic approximations for the energy gap (DELTA)(t) to calculate, among other things, the survival probability and electron energy spectrum. There are some interesting results for the electron energy spectrum which have not yet been observed in experiments.

Absolute total cross sections for electron detachment, reactive scattering, charge transfer and dissociative charge transfer have been measured for collisions of hydrogen and halogen negative ions with various molecular targets. The reactants investigated involve H('-), D('-), F('-), Cl('-), Br('-) and I('-) ions as projectiles and H(,2), D(,2), HD, N(,2), CO, O(,2), CO(,2), CH(,4) and Cl(,2) molecules as targets. The energy range of these experiments extended from about 1 eV to about 300 eV in the lab.;The threshold behavior of the detachment cross sections for the reactants H('-)(D('-)) + H(,2),D(,2) and HD has been determined. The thresholds for detachment for both H('-) and D('-) ions are found to be larger than the electron affinity of hydrogen and isotopic substitution reveals that the detachment cross sections scale with relative collision energy at low collision energies and with relative collision velocity at high collision energies. Upper and lower bounds on detachment-rate constants which are based upon the measurements are presented.;Studies of the reactants H('-)(D('-)) and B(,2), CO, O(,2), CO(,2) and CH(,4) reveal that electron detachment is the dominant process for all the molecular targets except O(,2) for which charge transfer dominates. Isotope effects are observed in all the cross sections. The general features of the charge-transfer cross section for the O(,2) target are in agreement with the ideas of a simple two-state collision model. The cross sections for charge transfer (or dissociative charge transfer) are found to be small for all targets except O(,2).;In the case of the collisions of F('-) and Cl('-) with H(,2), D(,2) and HD, reactive scattering is found to be the dominant inelastic channel for F('-) projectile. Electron detachment of F('-) is found to occur by two distinct mechanisms. A striking difference in the detachment and reactive cross sections is observed when Cl('-) is substituted for F('-) in that the electron detachment cross section is generally larger than that for reactive scattering. Isotope effects are observed in all the cross sections for both F('-) and Cl('-).;Charge transfer and dissociative charge transfer cross sections are found to be the dominant channels for collisions of Cl('-), Br('-) and I('-) with Cl(,2). The electron detachment cross section for I('-) + Cl(,2) is found to be anomalously low. Some energy loss spectra are reported for I('-) + Cl(,2). They exhibit substantial inelastic scattering which is consistent with the calculated potentials of Cl(,2).

Measurements of the total cross section for electron detachment, (sigma)(,e)(E), are presented for low-energy (E(,1ab) < 300 eV) collisions of Na('-), K('-), and Cs('-) with atomic and molecular targets. For many of the atomic (rare-gas) targets, the energy dependence of (sigma)(,e)(E) is striking: virtually no detachment is observed until relatively high collision energies (50 eV in the center-of-mass frame) are reached, in contradistinction to what has been observed for similar collisions involving H('-). The thresholds for alkali anion detachment are approximately equal to the thresholds for excitation observed in collisions of neutral alkali atoms with these same targets. The similarity between the dynamics of the neutral system and that of the negative ion system, together with the observation (at greater energies) of detachment accompanied by excitation of the alkali parent, suggests that electron detachment may be mediated by a two-electron process in some cases. A simple curve-crossing mechanism adequately reproduces the observed (sigma)(,e)(E) for several of these rare-gas targets.;Measurements of both (sigma)(,e)(E) and the cross section for charge transfer (sigma)(,i)(E) have also been completed for H(,2), D(,2), N(,2), O(,2), CO, CO(,2), SO(,2), N(,2)O, CH(,4), and SF(,6) targets. Electron detachment is the dominant process for all of these targets except O(,2), SO(,2), and SF(,6), with thresholds on the order of a few eV. Structure in (sigma)(,e)(E) for the CO(,2) target has been attributed to charge transfer to a metastable state of CO(,2)('-)(('2)A(,1)). Similarly, in the case of N(,2)O, both (sigma)(,e)(E) and (sigma)(,i)(E) exhibit behavior which suggests that a temporary negative ion state is formed during the collision. In the case of the O(,2), SO(,2), and SF(,6) targets, charge transfer is observed to have particularly large cross sections (>100 (ANGSTROM)('2)) at low collision energies.

The cross sections of charge transfer and electron detachment in collisions of H$\sp{-}$ and D$\sp{-}$ with Na, K and Cs have been measured for collision energies ranging from 3 to 300 eV. Both charge transfer and electron detachment are significant electron-loss mechanisms for H$\sp{-}$(D$\sp{-}$); both processes exhibit velocity-dependent isotope effects for H$\sp{-}$ and D$\sp{-}$. $\sigma\sb{\rm cg}$(E) displays high energetic thresholds for Na and K (about 20 eV for H$\sp{-}$ + Na and 40 eV for H$\sp{-}$ + K) yet no obvious one for Cs. $\sigma\sb{\rm e}$(E) does not depend on the target as much as $\sigma\sb{\rm ct}$(E) and displays near zero-energy thresholds. The relative importance of charge transfer as an electron-loss mechanism decreases as the mass of the alkali-metal increases.

A quantitative theory of oscillatory spectra for atoms in a magnetic field is developed. When an atom is placed in a magnetic field, and absorption spectrum into states close to the ionization threshold is measured, it is found that the absorption as a function of energy is a superposition of many sinusoidal oscillations. Such interesting and surprising phenomenon are fully explained and described by the theory.;The theory is based on three approximations: (1) Near the atomic nucleus, the diamagnetic field is negligible. (2) Far from the nucleus, the wave propagates semiclassically. (3) Waves returning to the nucleus are similar to (cylindrically-modified) Coulomb-Scattering waves. Using these approximations, together with the simple physical picture of absorption process, formula is derived for the transition rate as a function of final states energy.;The main result is that the transition rate is equal to the sum of two very different kinds of contributions. The first is the averaged transition rate in the absence of the magnetic field, which is a smooth function of energy; the second is itself a sum over many oscillations. Each oscillation is closely associated with a band of wave, initially going out from the nucleus, propagating along a family of trajectories, and finally returning to the vicinity of the nucleus. Because in the center of the family of trajectories is a closed orbit going from the nucleus and returning to the nucleus, we say "a closed orbit makes an oscillatory contribution to the absorption spectrum".;Formulas and algorithms are derived and specified for the calculations of the spectrum from the initial quantum state, dipole polarization, and the properties of the closed classical orbits. Good agreements with experiments were found. Very detailed interpretations are obtained.

Total electron detachment and charge transfer cross sections, $\sigma\sb{\rm e}$(E) and $\sigma\sb{\rm ct}$(E), have been measured for collisions of the negative ions O$\sp{-}$, S$\sp{-}$, F$\sp{-}$, Cl$\sp{-}$, Br$\sp{-}$, I$\sp{-}$, Na$\sp{-}$, and K$\sp{-}$ with atomic hydrogen for laboratory energies ranging from 2 to 500 eV. For the systems F$\sp{-}$, Cl$\sp{-}$, Br$\sp{-}$, O$\sp{-}$ and S$\sp{-}$ + H, $\sigma\sb{\rm e}$(E) displays no barrier for associative detachment; the results are found to be adequately described by simple curve-crossing models based upon available intermolecular potentials, or by classical orbiting models which assume that the anion interacts with the H atom via an attractive potential of the form 1/R$\sp4.$ Analysis of $\sigma\sb{\rm e}$(E) for the system S$\sp{-}$ + H required the cross section for $\rm S\sp{-} + H\sb2\to e +\...$ to be experimentally determined, and these results resolved an apparent discrepancy in previous measurements. The measured detachment cross section for the Cl$\sp{-}$+ H is also found to be in agreement with a calculation for that system based on the effective range potential model. Unlike the other halogen anion-hydrogen systems, $\sigma\sb{\rm e}$(E) for I$\sp{-}$ + H is found to increase with increasing energy over the higher collision energies investigated. The cross section for charge transfer in collisions of O$\sp{-}$ S$\sp{-}$, F$\sp{-}$, Cl$\sp{-}$, Br$\sp{-}$ and I$\sp{-}$ with atomic hydrogen is found to be less than 1 A$\sp2$ over the entire range of laboratory energies investigated. A reasonable extrapolation of $\sigma\sb{\rm ct}$(E) for collisions of O$\sp{-}$ + H is found to agree with a previous measurement at a higher collision energy. For the collision systems K$\sp{-}$ and Na$\sp{-}$ + H, $\sigma\sb{\rm ct}$(E) is found to be much smaller than $\sigma\sb{\rm e}$(E). The measured detachment cross section for Na$\sp{-}$ + H is described using available potential energy curves and by assigning the anion state an average lifetime in the unstable region. A perturbed stationary state calculation of $\sigma\sb{\rm ct}$(E) for the reactant Na$\sp{-}$ is performed, and this calculation underestimates the observed cross section for charge transfer at low collision energies.

A systematic study of the photoabsorption spectra of highly excited hydrogen and alkali atoms in electric fields is presented, based on the semiclassical closed-orbit theory. In most respects, hydrogen and alkali atoms behave similarly, because the excited alkali atoms have a single electron outside of a small ionic core, and the core only produces small shifts of energy levels and small phase shifts of scattered wave functions.;For hydrogen, the classical motion of the excited electron is regular and closed orbits can be enumerated. Above the zero-field ionization threshold, the system is rather simple. There is only one closed orbit, called the parallel orbit, which goes out from the Coulomb center along the electric field and later returns to the center. This orbit is unstable. Nevertheless, the orbit and its repetitions produce recurrences in time, that lead to oscillations in the absorption spectrum. Comparisons between theory and experiments show good agreement.;Below threshold, the parallel orbit becomes stable and, as the energy decreases, many other orbits bifurcate out of it. These closed orbits form orderly patterns, and the associated recurrences are most clear if the absorption spectrum is measured using a scaled-variables method and its Fourier transform, the recurrence spectrum, is computed. Bifurcations are readily observable in such spectra because they create new recurrences, and because at a bifurcation, observed recurrences are especially strong. We predicted the sequence of bifurcations, and the energies at which each would occur, in a paper published early in 1994. Recently, experimental measurements carried out at M.I.T. have confirmed these predictions.;Near a bifurcation, the original form of closed-orbit theory diverges, since a bifurcation is correlated with a focus of classical orbits. An improved closed-orbit theory is derived by using the uniform semiclassical approximation, and by extending the wave function from the real three dimensional space into a four dimensional space. In this extended space, the orbits of the electron near the nucleus are straight lines. These lines are arranged so that they form a cusped caustic, and furthermore they form cylindrical foci in two independent planes in the four-dimensional space. We derive a formula for the wave function associated with this cylindrically focused cusp, and make a new prediction of the behavior of the recurrence spectrum near a bifurcation. These predictions are compared with new experimental results. We find that the improved form of closed orbit theory accurately accounts for experimental measurements both globally and locally.

Absolute yields of negative ions and secondary electrons resulting from collisions of positive sodium ions (Na{dollar}\sp{lcub}+{rcub}){dollar} with polycrystalline aluminum (Al) and molybdenum (Mo) surfaces and the Mo (100) surface have been measured as a function of the oxygen (O) coverage, from none up to several monolayers, for impact energies, E {dollar}<{dollar} 500 eV. Negative oxygen ions (O{dollar}\sp{lcub}-{rcub}){dollar} are found to be the dominant sputtered negative ions and for the three surfaces at all O coverages and Na{dollar}\sp{lcub}+{rcub}{dollar} impact energies. The O{dollar}\sp{lcub}-{rcub}{dollar} and secondary electron yields share a common impact energy threshold at {dollar}\rm E\sb{lcub}th{rcub}\approx 50{dollar} eV, and both have a strong dependence on the oxygen coverage of the surface.;The kinetic energy distributions of the secondary electrons and sputtered O{dollar}\sp{lcub}-{rcub}{dollar} were determined as functions of O coverages and impact energies. The O{dollar}\sp{lcub}-{rcub}{dollar} distributions are characterized by a narrow, low energy peak (at {dollar}\sim{dollar}1-2 eV) followed by a low level, high energy tail. The secondary electrons have a narrow (FWHM {dollar}\sim{dollar} 1-2 eV) kinetic energy distribution, centered essentially at the same most probable kinetic energy as the ions. The shapes of the respective distributions and the most probable kinetic energies are essentially invariant with the impact energy, O coverage and the metal surface.;The results were analyzed in terms of conventional collision cascade model, but the calculation could not be fitted to the experimental results. An electronic excitation mechanism is proposed to augment the collision cascade and to provide a mechanism for secondary electron emission. In the model, adsorbed O, which resides on the surface essentially as O{dollar}\sp{lcub}-{rcub},{dollar} is collisionally excited into an (MO{dollar}\sp{lcub}-{rcub})\sp*{dollar} repulsive state, and as the O{dollar}\sp{lcub}-{rcub}{dollar} exits the surface along the surface potential energy curve, it can decay by emission of an electron to the metal or to the vacuum, or it can survive as an ion. The parameters of this model can be adjusted such that the calculated kinetic energy distribution, together with that of the collision cascade, can reasonably reproduce the experimental observations for the ions and provide a reasonable fit to the corresponding electron kinetic energy distributions as well.

We present the technique and results of our study of the redistribution of Rydberg states in barium by a half cycle pulse (HCP). A survey of previous research concerning the interaction of half cycle pulses with Rydberg atoms reveals a lack of experimental data in the area of state redistribution. This is primarily due to an inability to identify the redistributed states accurately using the currently available tool: selective field ionization (SFI). We present the limitations of SFI as a state identification tool. We then present a survey of isolated core excitation as a state identification tool, along with its limitations in dealing with a mixture of states. Combining SFI and ICE overcomes these limitations and allows for the accurate characterization of a mixture of Rydberg states, as found in HCP redistribution. We present our results for the HCP redistribution of a series of ns and nd initial states, for n from 30 to 40, at various HCP intensities. These results primarily involved single photon transitions, confining us to the linear regime. Analyzing the results of these redistributions with respect to the energy difference between initial and final states yielded information about the frequency structure, after transmission to the interaction region, of the HCP. This led us to postulate on the effects of diffraction on the HCP. We conclude with several suggestions for the improvement of the experimental system, as well as a direction for future research.