Laboratory measurements of the cross sections for single, double, triple, and quadruple ionization of sulfur atoms by electron impact are presented for collision energies from threshold to 500 eV. The cross sections for single ionization of sulfur are measured relative to those of several elements whose absolute cross sections for single ionization are known. Cross sections for each multiple ionization process are then measured relative to those for single ionization. The configuration and operation of the apparatus for these measurements are described. The possible effects of excited sulfur reactants are examined, and the reported cross sections are felt to be characteristic of ground state sulfur atoms.

Formation and decay of KrCl/Kr(,2)Cl and (KrXe) + excited states have been studied by following the time dependence of the various fluorescence bands. For the KrCl/Kr(,2)Cl system the time dependence of the fluorescence from KrCl (B), KrCl(C), and Kr(,2)Cl was measured following pulsed 135 nm excitation of chlorine. The Cl* is found to react very rapidly with krypton to form KrCl in a vibrationally excited state. Above (TURN)75 torr the formation of the radiating states is found to be rate limited by vibrational relaxation (K(,vib) = 8.6(+OR-), 4 x k9('-11)). Following vibrational relaxation the B and C states are completely mixed and decay with a mixed state radiative lifetime of 5.4 (+OR-) .1 ns. The mixed state is strongly quenched by chlorine (K = 7.6 (+OR-) 5 x 10('-10)) and also by krypton in a three body process (K = 1.15 (+OR-) .03 x 10('-30)) which presumably leads to production of Kr(,2)Cl. However, the measured formation rate of Kr(,2)Cl is much slower than the three body quenching of KrCl is much slower than the three body quenching of KrCl and an intermediate ions lived state of Kr(,2)Cl is suggested. The Kr(,2)Cl emission, monitored at 325 nm, is strongly quenched by chlorine (K = 6.7 x 10('-10)) but only weakly, if at all, by drypton (K (') 1 (mu)sec). The time dependence of 4900 A emission in Kr-Xe mixtures was also measured following pulsed excitation by a low intensity high energy electron beam. The time dependence was obtained at pressures of 500 to 10000 torr and at temperatures ranging from 30 to 40 des C. The population of the upper ionic state of the transition is found to be governed by a dissociation recombination reaction which establishes an equilibrium between the heteronuclear molecular ion and its associated atomic ion. The measured dissociation and recombination rate constant for this reaction are 1.94 (+OR-) 12 x 10('-11) and 5 (+OR-) 2 x 10('-32) respectively. The radiative lifetime was determined to be (46 (+OR-) 4 ns). The temperature dependence of the dissocation rate constant yields an activation energy of 0.08 (+OR-) .02 ev.

Collisions of fast ions and neutral atoms with other atoms and molecules are important in a broad range of physical phenomena. A new technique, based on the use of a micro-channel plate position-sensitive detector, has been developed for studying some of these collision processes. Differential cross sections have been measured over a primary beam energy range of 500eV to 5000eV for the scattering of atomic hydrogen, helium, and oxygen on targets of N(,2), O(,2), H(,2), and He at laboratory angles between 0.08 degrees and 5 degrees. These differential cross sections provide useful information for atmospheric modelers as well as yielding new, fundamental atomic collision data. The results show good agreement with previous measurements when comparison is possible. Preliminary data for charge transfer reactions are included and interaction potentials obtained by inversion of the measured differential cross sections are compared with potentials available from the literature.

Cross sections for the collision of a highly excited atom with a polar molecule have been calculated using a classical trajectory Monte-Carlo approach. Many features of experiment have been observed and explained. For example, the tendency for the Rydberg atom to be further excited can be explained by the likelihood for multiple electron-molecule encounters. Furthermore, the probability of multiple encounters also casts doubts on the validity of the free electron model, a commonly used approach in Rydberg collision problems. Quantitatively, the state changing cross sections calculated in this study are typically smaller than experimental measurements. This may be due to the failure of classical mechanics to quantize the possible states of a physical system, especially since the total cross section agrees with experiment. It is thus hoped that a proposed semi-classical procedure, in which the molecule is treated quantum mechanically, will produce results in better agreement with experiment.

Optical pumping is a very useful technique for the production of spin polarized particles. This thesis describes laser induced optical pumping in helium, argon, and neon metastable atoms. The laser used to optically pump helium is an (F(,2)+)* color center laser which was developed during the course of this work. Argon and neon were optically pumped with commercially available dye lasers. In the first experiment the color center laser was used to spin polarize a Doppler broadened ensemble of He 2('3)S metastable atoms contained in a flowing helium afterglow. The extracted chemi-ionized electron polarization was as high as 80%, indicating almost complete polarization of the 2('3)S atoms. A P('2)I quality factor of 8 x 10('-6) was obtained. The next set of experiments involved the optical pumping of metastable atoms contained in collimated beams. Although substantial polarizations were achieved, problems were encountered with laser frequency jitter as none of the lasers were frequency stabilized. In particular, the polarizations obtained in both helium and argon were unstable and not even a nominal polarization could be maintained consistently over any reasonable time interval. Stable optical pumping in neon was obtained with the DCM dye laser however, due to frequency jitter which essentially broadened the narrow spectral components in the laser lineshape. In an effort to improve on the limited performance obtained in the optical pumping of both helium and argon, a frequency modulation technique has been developed. By decomposing the very narrow spectral components present in typical laser radiation into a large number of closely spaced sidebands, this technique effectively fills in the gaps in the laser linewidth. This broad continuous linewidth enhances the ability of unstabilized lasers to optically pump very narrow atomic transitions by reducing the effects of laser detuning. In an initial experiment almost complete polarization of a beam of argon metastable atoms was obtained using a frequency modulated LD 700 dye laser. The polarization was stable on a time scale of several hours--more than adequate for proposed spin dependent studies.

A frequency stabilized ring dye laser, with a linewidth of (TURN)250 kHz, has been utilized to selectively excite Rb(N('2)S, N('2)D) states in the range N = 20 - 100 by a two photon process. The highly excited atoms were detected by the technique of electric field ionization. Excited states with (VBAR)M(,L)(VBAR) < 3 have been observed to ionize along predominantly adiabatic paths and states with (VBAR)M(,L)(VBAR) (GREATERTHEQ) 3 have been observed to ionize along predominantly diabatic paths for the range of N investigated. The electric field dependence of the ionization threshold has been determined to be in good agreement with that calculated from a simple model based on the energies and ionization rates of hydrogen. Collisions between highly excited rubidium atoms and neutral xenon atoms, which result in a change in the N and L of the excited state of the rubidium atom, have been investigated. The N of the final state has been shown to be limited to those states which lie closest in energy to the initial state, in good agreement with theory. The distribution of L of the final states has been shown to be completely statistical or biased towards production of high L. The distribution of final L is definitely not biased toward production of states with low values of L.

State-changing in collisions of Rb(nS) - HF are studied using a crossed atomic beam/modulated CW laser technique and selective field ionization for values of principal quantum number n where 40 (LESSTHEQ) n (LESSTHEQ) 49. Collisions between a rotating polar molecule and a Rydberg atom often result in a near-resonant interchange of the rotational energy of the molecule, and a corresponding change in the electronic energy of the atom. A critical parameter is the energy imbalance of the reaction, defined as the difference between the atom's initial and final electronic energy and the molecule's initial and final rotational energy. The molecule used in this experiment, hydrogen fluoride, has a large rotational constant which permits study of n-changing rate constants for transitions from an initially populated s-state to a single n-manifold of high-L states. Because only one final manifold is populated, the reaction involves a single energy imbalance, and thus proves a useful test of theoretical models. n-Changing rate constants for collisions with energy imbalances ranging from 0.057 cm('-1) to 3.447 cm('-1), and ionization and L-mixing rate constants are compared to those predicted by an impulse approximation model and a semiclassical approach.

Absolute differential cross sections for charge-transfer collisions of H$\sp{+}$ and He$\sp{+}$ with a variety of rare-gas- and molecular-gas-targets are presented. The cross sections are measured within the energy range 0.25-5.0 keV, and between angles of 0.01$\sp\circ$ and 1.0$\sp\circ$ (for He$\sp{+}$ - He scattering at 0.25 and 0.5 keV, the angular range is extended to 7.8$\sp\circ$). A position-sensitive detector is used to detect both the primary and scattered particles. The data typically exhibit striking oscillations. Theoretical calculations for some of the reactions yield good agreement with the measured cross sections. The integrated cross sections match the total cross sections in the literature very well.

This thesis reports measurements of absolute cross sections, differential in angle, for elastic scattering of keV-energy atomic H and He projectiles from He, Ne, Ar, Kr, Xe, H$\sb 2$, and N$\sb 2$ at laboratory scattering angles between 0.02$\sp\circ$ and 0.5$\sp\circ$. It also reports measurements of absolute cross sections, differential in angle, for elastic scattering of keV-energy H$\sp +$ and He$\sp {+}$ projectiles from He, Ne, Ar, Kr, and Xe and H$\sb 2$ at laboratory scattering angles between 0.04$\sp\circ$ and 1.0$\sp\circ$. Comparisons are made between measured differential cross sections and those calculated from various model potentials, revealing the range of validity of potentials appearing in the literature. Experimental cross sections are also integrated over the angular range to obtain partial total cross sections, which are compared with those appearing in the literature.

A single-mode, frequency-stabilized LNA ring laser has been developed which routinely provides output powers $\sbsp{\sim}{>}$300 mW at 1.083 $\mu$m when pumped with 5 W of the 514 nm output from an Argon ion laser. A modified CR899-21 Ti:Sapphire ring laser from Coherent Laser Group of Palo Alto produces 300 mW at 1.083 $\mu$m when pumped with 21 W of all-lines Argon ion laser light. The importance of intense, stable 1.083 $\mu$m laser sources is demonstrated through several experiments. Optical pumping with either laser produces He(2$\sp3$S) atom beams of high polarization which can be maintained for periods of many hours. This allows investigation of Penning Ionization using spin labeling techniques, spin-polarized metastable atom deexcitation spectroscopy, and a novel method for absolute calibration of Mott polarimeters. Saturation of the He(2$\sp3$S) $\rightarrow$ He(2$\sp3$P) transition allows study of the interaction of mixed He(2$\sp3$S)/He(2$\sp3$P) atoms beams with surfaces. Construction of a magneto-optical trap for He(2$\sp3$S) atoms is possible utilizing the high power provided by the laser systems. Both lasers provide ample power for all applications currently envisioned involving optical pumping and optical manipulation of He(2$\sp3$S) metastable atoms.