No description available.

No description available.

No description available.

No description available.

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.

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.