In thermal collisions between Xe(nf) Rydberg atoms and NH(,3) collisional depopulation of the nf state occurs through at least three distinct mechanisms: n-changing collisions, l-changing collisions, and ionizing collisions. Using selective field ionization, the total collisional depopulation rate constants k(,d) are measured for laser-excited Xe(nf) atoms in the n range of 22 to 39. The magnitudes of k(,d) are large, corresponding to reaction cross sections comparable to the geometric size of the Xe(nf) atom at n = 22. In the n range of 25 to 40, Xe('+) ion production provides an absolute measure of the rate constants for collisional ionization. Approximate rate constants for n-and l-changing collisions are also presented for the 31f state. In these experiments, one finds that the state-changing collision rate constants are fairly independent of n and are determined largely by the l-changing collisions. The collisional ionization rate constants measured are smaller than the state-changing rate constants and increase with increasing n, ranging between 0.2 and 4.6 x 10('-7) cm('3)/sec for n between 25 and 40. The present results are compared to those of recent theoretical calculations.

Total charge transfer cross sections for the reaction H('+)+H(ls) (--->) H(ls) + H('+) and the isotopic reaction H('+)+D (ls) (--->) H (ls) + D('+) are presented for collisions at center of mass energies ranging from 0.05 eV to 150 eV. These data were experimentally determined by the technique of merged beams. This reaction is the most fundamental charge transfer reaction and is also one of the primary processes resulting in loss of hydrogen from the upper atmosphere. The apparatus is described and a number of experimental difficulties associated with laboratory study of the reaction are discussed. A summary of theoretical treatments is also presented. The results include the first cross sections measured at collision energies below 1 eV. Agreement with theory at energies above 10 eV is excellent. The results below 5 eV agree with the best quantum mechanical calculations only to within about 30%. Possible causes of the disagreement are discussed.

Effects due to the interaction between the 300K background blackbody radiation (BBR) and xenon nf Rydberg atoms are reported. Transitions to nearby d and g levels are observed and the transition rates are found to be consistent with the predictions of a simple theoretical calculation which makes use of electric dipole matrix elements obtained by use of the quantum defect method. When the radiation background is reduced to that appropriate to a black body at liquid nitrogen temperature by cooling the apparatus, the transition rates are also observed to decrease. The reduction in the transition rates to the d and g states is consistent with the calculated decrease in the ambient blackbody flux. The total net depopulation rate of the f states is reduced as well when the temperature of the environment is lowered. The change in the observed lifetimes of the f states is in good agreement with the calculated decrease in the blackbody-induced depopulation rate.

Ion neutralization and metastable de-excitation spectroscopy, INS and MDS, allow detailed analysis of the surface electronic configuration of metals. The orthodox application of these spectroscopies may be enhanced by electronic spin polarization of the probe beams. For this reason, a spin polarized helium ion beam has been constructed. The electronic spin of helium metastables created within an rf discharge may be spacially aligned by optically pumping the atoms. Subsequent collisions between metastables produce helium ions which retain the orientation of the electronic spin. Extracted ion polarization, although not directly measurable, may be estimated from extracted electron polarization, metastable polarization, pumping radiation absorption and current modulation measurements. Ions extracted from the optically pumped discharge exhibit an estimated polarization of about ten per cent at a beam current of a few tenths of a microampere. Extraction of helium ions from the discharge requires that the ions have a high kinetic energy. However, to avoid undesirable kinetic electron ejection from the target surface, the ions must be decelerated. Examination of various deceleration configurations, in particular exponential and linear deceleration fields, and experimental observation indicate that a linear decelerating field produces the best low energy beam to the target surface. Auger analysis of the target surface allows determination of the degree of surface contamination. Although various analog systems for Auger analysis are available, a digital, computer compatible system has been constructed. The technique of ion neutralization spectroscopy and metastable de-excitation spectroscopy allows probing of the surface electronic structure of a metal target. With spin polarization of the probe helium ion beam, the apparatus provides a means to ascertain the existence and properties of surface magnetization. The apparatus not only may perform the highly surface selective ion neutralization and metastable de-excitation spectroscopies upon a common target surface but also provides the additional diagnostic ability achieved with spin polarization of the probe beam.

The outer electron of a Rydberg atom is so weakly bound to the atomic core that a relatively small electric field may cause the electron to be stripped from the atom. The field strength at which this ionization happens is strongly dependent on the state of the Rydberg atom. Pulsed electric field ionization serves, therefore, not only as a means by which a Rydberg atom may be efficiently detected, but also a means by which the state of the Rydberg atom may be analyzed. The first observation of predominantly diabatic passage to field ionization of a non-hydrogenic Rydberg atom has resulted in an improved understanding of the adiabatic and diabatic ionization processes. The magnitudes of the various field ionization features have been measured and found to agree with theory, and by detailed analysis, it is now possible to label the various field ionization signal features with the appropriate (VBAR)m(,l)(VBAR) quantum numbers. A much better understanding of the laser excitation process, the Rydberg state production, and the response of the excited state to an increasing electric field has enabled measurement of some Rydberg fine-structure intervals, through observation of field ionization quantum beats.

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.