Laser cooling and atom trapping techniques have enabled atomic physicists to attain temperatures on the order of 100 nK. In this regime, a trapped gas must be described within the framework of quantum statistical mechanics. In 1995, Bose-Einstein condensation (BEC) was achieved for the first time in such a system by three independent groups. The success of attaining BEC has encouraged research in quantum degenerate Fermi gases (DFG). One of the most exciting opportunities for a DFG is the prospect for Cooper pairing in a two-component gas. The experimental techniques for obtaining degenerate gases, as well as the mechanism for Cooper pairing, hinges on understanding and characterizing ultracold collisional interactions. This thesis is a twofold study in this exciting field. The first section is an in-depth treatment of collisions in atomic lithium. Lithium has two isotopes, 6Li which is a fermion and 7Li which is a boson. This makes it an ideal candidate for studying quantum degenerate gases. In a series of experiments using a magneto-optical trap, we have spectroscopically measured the ground and excited state molecular potentials for both isotopes. From this work we have been able to obtain the most accurate potentials for lithium to date. These precise measurements have enabled us to extract the most accurate value to date for the 2P atomic radiative lifetime of lithium. Furthermore, we have developed a coupled-channel treatment of two-body collisions which has enabled us to calculate the scattering amplitude, elastic cross section, and rate of spin-exchange decay for any ground state collision in lithium as a function of temperature and magnetic field. Armed with this knowledge, we have made progress on the design and construction of an electromagnetic trap for lithium. This trap will enable the study of a degenerate Fermi gas of 6Li as well as the possibility of observing a Cooper pairing transition. The trap performance has been characterized for each isotope and current work is focussed on cooling lithium down to degeneracy. Measurements of spin-exchange decay have been made and compare well with the collisional theory that has been developed.

A thermal-energy beam of Xe(nf) Rydberg atom has been utilized to study electron tunneling during atom/surface interactions. The Xe Rydberg atoms, which are created by laser-induced photoexcitation of Xe(3P 0) metastable atoms, are incident at near grazing angle on a flat Au(1 1 1) surface. At some critical distance, the Rydberg electron will tunnel into a vacant level in the Au surface The resulting ions are observed by applying a pulsed collection field which is big enough to overcome the image charge of the product ions. Measurement of the ion signal as a function of the applied field gives the threshold field which depends critically on the atom/surface separation at which ionization occurs. Analysis of the data for the current range of (13 ≤ n ≤ 20)n provides results in good agreement with first-principles complex scaling calculations that employ accurate atom-surface interaction potentials. Measurements show that the ionization distance depends critically on the flatness of the surface. Surprisingly, measurements using red and blue Stark states yield quite similar ionization distances and this is being investigated theoretically.

An apparatus has been developed to create spin mixtures of ultracold fermionic 6Li in order to investigate the behavior of strongly interacting Fermi gases, and in particular, search for a superconducting phase transition. 6Li atoms are initially confined in a dual species magneto-optical trap (MOT) along with bosonic 7Li. Due to an overlap between spectral features of the two species, a D2/D1 trap/repump scheme is used for the 6Li. The atoms are subsequently cooled to quantum degeneracy in a magnetic trap through optimized dual forced evaporation and transferred to an all optical trap where spin mixtures are created with RF sweeps and pulses. The atomic interactions are controlled through the use of magnetic Feshbach resonances and an externally applied bias magnetic field of up to 1000 G. This thesis details the construction, operation, and performance of the 6Li portion of the apparatus, as well as current results.

We have designed and constructed an apparatus capable of simultaneously creating both a quantum degenerate Bose and Fermi gas of the two stable isotopes of lithium, 7Li and 6Li, respectively. This apparatus has the capability of optically confining pre-cooled samples of atoms, and applying an arbitrary magnetic bias field between zero and 1000 G. The optical confinement allows us to work with any spin state or mixture, of 6Li or 7Li. The applied external field can tune the atoms through atom-atom scattering resonances, known as Feshbach resonances. These resonance change the effective coupling between atoms, allowing us to study the behavior of the atoms in both the weak and strong coupling limits. With this tool we have been able to produce large lithium Bose-Einstein condensates, stable attractive Bose-Einstein condensates in one dimension, coherent Fermi mixtures, strongly interacting Fermi mixtures, and have induced the formation of bosonic molecules from an interacting Fermi mixture.

Atomic oxygen, an abundant upper atmospheric species, is an important target in collisions involving fast ions and neutrals precipitating into the Earth's thermosphere. To facilitate the laboratory study of these collisions and to measure their differential cross sections, an appropriate atomic oxygen target has been developed. Oxygen atoms for the target are produced by electron-impact dissociation of O$\sb2$ in a discharge created in a microwave cavity resonant at 2.54 GHz. Thirty percent of the added molecular oxygen at 50 mTorr is dissociated in the discharge. The atomic and molecular oxygen mixture flows through a phosphoric acid coated pyrex tube to a fluorocarbon coated target cell. The dissociation fraction in the target cell is determined by titration with NO$\sb2$ and by analysis with a time-of-flight mass spectrometer.

Differential cross sections for the scattering of atoms and ions are of fundamental interest and they supply information necessary for numerical modeling of problems ranging from fusion reactors to planetary atmospheres. In an inelastic collision some energy is lost to ionization or excitation. Data that distinguishes elastic and inelastic collision processes at low keV energies is scant. The distinction is vital for the matching of theory with data and for modeling some problems such as the Jovian aurora. Several apparatus designs are proposed for modifying the present differential collision cross section instrument to measure doubly-differential cross sections (differential in the solid angle of the scattered particle and the energy lost) and energy loss spectra at low keV energies. A new type of ion source that has significant advantages for time-of-flight experiments is presented.

The two-photon excitation of cesium to high-n Rydberg levels through resonant dissociative states of the dimer is investigated over the wavelength range 6000-5900A. Highly vibrationally excited $\sp1\Sigma\sb{\rm g}\sp+$ ground state molecules are excited to the continuum of $\sp1\Pi\sb{\rm u}$ bound states which dissociate to give 5D atoms. The 5D atoms are subsequently photoexcited to high-n Rydberg states which are detected by selective field ionization techniques. Series corresponding to both (5D $\to$ nF) and (5D $\to$ nP) transitions with components up to n = 150 were observed. The photoexcitation process appears to be very efficient and produces copious quantities of Cs Rydberg atoms.

New insights into many physical phenomena can be obtained through spin dependent analysis and such investigations frequently require an electron spin polarimeter. In the past available polarimeters have been bulky and required very high voltages (100 kV or more) in order to operate. In recent years more portable models which operate in the 20 kV range have been constructed with efficiencies (figures of merit) of $3\times 10\sp{-5}$. A very compact UHV compatible retarding potential Mott polarimeter is described here that provides the highest efficiencies yet achieved with an instrument of this type. With a thorium target and electron accelerating voltage of 25 kV, it provides effective Sherman functions $\rm S\sb{eff}$ between $-$0.15 and $-$0.25. The resulting figures of merit are as high as $\approx1.6\times 10\sp{-4}$ which make the device attractive for use in any application requiring electron spin polarimetry.

A commercial Spectra Diode Laboratories, 1.083 $\mu$m diode laser has been used to efficiently spin polarize an atomic beam of He(2$\sp3$S) metastable atoms by optical pumping on the $\rm 2\sp3S\sb1\rightarrow2\sp3P\sb1$ (D1) transition. The diode laser is an InGaAs/GaAs strained multiquantum well with a distributed Bragg reflector to insure single longitudinal mode operation. The laser has been frequency stabilized to less than 1 MHz/hr drift. The helium metastable atom beam is produced by electron impact and the polarization is measured using a Stern-Gerlach analyzer. Atomic polarizations exceeding 97% have been obtained with $\sim$25 mW of power from the diode laser. A number of experiments currently under construction may also make use of the diode laser. These include the production of a spin polarized helium ion beam by optically pumping the metastable atoms in an rf discharge, and the trapping of helium metastable atoms in a magneto-optical trap (MOT).

Highly excited (Rydberg) atoms are produced and can exist for significant periods of time in various plasma environments. It is necessary to know cross sections for the various collision processes involving these atoms if a complete understanding of the physics of these environments is to be attained. The experimental investigation of collisional phenomena between high Rydberg sodium atoms and electrons is described in this thesis. A fraction of the atoms in a ground state sodium beam is optically excited (using lasers) to well-defined, high Rydberg states (principal quantum numbers chosen around 40). Subsequent to their production the Rydberg atoms are allowed to interact with 25 eV electrons for a specified time interval following which the collision products are detected. The electrons are produced in a beam which is crossed at right angles with the sodium beam. Important collision processes are (1) electron impact ionization of the Rydberg atom, and (2) electron-Rydberg atom interactions in which the quantum state of the atom is changed (state changing collisions). Results are presented for an observed state changing process which is identified as angular momentum state changing only (no change in the value of the principal quantum number). Cross sections for this process are measured to be 10('-10) to 10('-9) cm('2). Problems encountered during the work which prevented the measurement of electron impact ionization cross sections are discussed. In the course of the work two other topics required study: (1) the process of electric field ionization of Rydberg atoms (used to detect the atoms), and (2) the effects of background (300 K) radiation (from the chamber walls, for example) on Rydberg atoms. Results of these studies are also presented.