Browsing by Author "Stebbings, R. F."
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Item Absolute differential cross sections for charge-transfer of keV-energy hydrogen ion and oxygen ion ions with atomic oxygen(1989) Schafer, David Allen; Stebbings, R. F.The major constituent in the atmosphere of the Earth at altitudes between 200 and 500km is atomic oxygen.$\sp1$ Collisions involving atomic oxygen are therefore important in this region of the atmosphere. Due to the difficulty in producing atomic oxygen in the laboratory, there are very few reported measurements of scattering cross sections with an atomic oxygen target.$\sp{2-6}$ We have undertaken an experimental program designed to measure absolute differential cross sections for various collision processes involving atomic oxygen. The oxygen atoms are produced through dissociation of O$\sb2$ in a microwave discharge, producing a target of atomic and molecular oxygen whose densities are inferred from measurements with an electron impact time of flight mass spectrometer.Item Absolute differential cross sections for electron capture and loss by keV hydrogen atoms(1990) Smith, Gerald J.; Stebbings, R. F.Absolute differential cross sections for electron capture and loss by neutral hydrogen atoms incident on various gases are presented. The measurements cover a laboratory angular range of 0.02$\sp\circ$ to 1.77$\sp\circ$ and a laboratory energy range of 2.0 to 5.0 keV. The target gases include H$\sb2$, O$\sb2$, N$\sb2$, Ar and He. Integrated experimental cross sections are compared with total cross sections reported by other investigators.Item Absolute differential cross-sections for keV energy ion and atom collisions with oxygen atoms(1990) Hakes, Charles Lynn; Stebbings, R. F.Oxygen atoms are the most abundant constituent in the Earth's atmosphere between 200km and 600km, and thus are the most probable targets for precipitating ions and atoms at these altitudes. Differential cross sections for collisions involving oxygen atoms supply information necessary for numerically modeling the conditions in the Earth's upper atmosphere. Absolute cross sections, differential in angle, for electron capture by 500, 1500, and 5000eV protons in collisions with oxygen atoms are presented. The target, a mixture of atomic and molecular oxygen produced in a microwave discharge, is confined to a very short ($<$1mm) teflon-coated cell. The abundance of atomic and molecular oxygen in the target is determined using an electron-impact-ionization time-of-flight-mass spectrometer. The mass spectrometer signal, yielding the atomic oxygen density in the target cell, is calibrated by comparing the direct scattering of He(500eV) + O to the direct scattering of O(2000eV) + He. The charge transfer cross sections are integrated and compared to total cross sections reported in the literature.Item Absolute partial cross sections for electron-impact ionization of argon, hydrogen, nitrogen, oxygen, and carbon dioxide from threshold to 1000 eV(1996) Straub, H. Charles; Stebbings, R. F.Absolute partial cross sections from threshold to 1000 eV are reported for electron-impact ionization of Ar, H$\sb2$, N$\sb2$, O$\sb2$, and CO$\sb2$. Data are presented for the production of Ar$\sp{n+}\ (n=1{-}4)$ from Ar; the production of H$\sbsp{2}{+}$ and H$\sp+$ from H$\sb2$; the production of N$\sbsp{2}{+},\ \rm N\sp{+}+N\sbsp{2}{2+}$, and N$\sp{2+}$ from N$\sb2$; the production of O$\sbsp{2}{+}$, $\rm O\sp{+}+O\sbsp{2}{2+}$, and O$\sp{2+}$ from O$\sb2$; and the production of CO$\sbsp{2}{+}$, CO$\sp{+}$, CO$\sbsp{2}{2+},$ O$\sp{+}$, C$\sp{+}$, O$\sp{2+}$, and C$\sp{2+}$ from CO$\sb2$. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output provides a clear demonstration of the complete collection of energetic fragment ions from dissociative ionization. For singly charged parent ions, the overall uncertainty in the absolute value of the cross sections reported here is $\pm$3.5% and is marginally higher for multiple and dissociative ionization. Previous cross section measurements are compared to the present results. Additionally, combining the flight time and detection location of fragment ions yields some qualitative information about their energy distribution.Item Accurate determination of high vacuum pressure(1994) Straub, H. Charles; Stebbings, R. F.Two methods for the accurate determination of high vacuum pressure are reviewed with particular attention paid to the 10$\sp{-6}$ to 10$\sp{-5}$ Torr range. The first method is characterized by calculating the pressure generated from a knowledge of the characteristics and geometry of the vacuum chamber. Static and dynamic expansion techniques are examined. The second method establishes a pressure in the vacuum chamber by balancing an input of gas against a pumping speed and measuring the resulting pressure with a suitable gauge. McLeod, spinning rotor, hot-filament ion, and capacitance diaphragm gauges are discussed. Additionally, new results for ion and capacitance diaphragm gauges are presented including an extension of the range of capacitance diaphragm gauges down to 10$\sp{-6}$ Torr through the use of averaging and a new technique for the direct calibration of capacitance diaphragm gauges between 10$\sp{-6}$ and 10$\sp{-4}$ Torr which uses gravity to deflect the diaphragm and generate apparent pressures.Item An apparatus for electron impact ionization cross-section measurements(1990) Renault, Pascal Dominique; Stebbings, R. F.Cross sections for electron impact ionization of molecules and rare gases are necessary for the modeling of phenomena related to the earth's upper atmosphere and to plasmas. The construction of an apparatus to improve the accuracy of measurements of such cross sections is reported. The ions are produced in a static gas target crossed by an electron beam and are counted with a new ion detector technology, which insures a increased accuracy of the ion flux measurement and of the target length. The target gas density is measured using an ionization gauge calibrated against a high accuracy capacitive transducer. This apparatus also features a pulsed mode of operation, which allows a product analysis through time of flight measurement.Item An atomic oxygen target for absolute differential cross section measurements(1992) Sieglaff, Dean Ronald; Stebbings, R. F.An atomic oxygen target suitable for absolute differential cross sections has been developed. Molecular oxygen gas is partially dissociated in a microwave discharge and flowed through a collision cell to serve as a fast beam target. Determination of the atomic oxygen density is carried out using an electron impact time-of-flight mass spectrometer, which uses the exit aperture of the cell as a sample leak.Item Construction of a crossed beams apparatus to study chemiionization(1971) Riola, John Peter; Stebbings, R. F.A high vacuum, crossed beams apparatus has been constructed which will be used to study Penning ionization and associative ionization. The first experiments to be performed will involve collisions between He(ls2s 21,3S) and various atoms and molecules. The absolute total ionization cross sections, the separate ionization cross sections for all final state positive ions, and the energy distribution of the Penning electrons will be measured. Chapters 1 and 2 of this thesis will describe previous theoretical and experimental work in the field, respectively. Chapter 3 will discuss the design and initial use of the Crossed Beams Apparatus.Item Experimental determination of absolute partial cross-sections for the electron impact ionization of argon and nitrogen(1994) Renault, Pascal Dominique; Stebbings, R. F.This thesis presents a description of a novel technique for the accurate experimental determination of absolute partial electron impact ionization cross sections of atoms and molecules by a calibrated time of flight mass spectrometer. The simple design of the present apparatus insures the total collection of the energetic fragment ions. Measurements for argon and nitrogen targets are presented over a range from threshold to 1 keV. Previously published electron impact data on argon, nitrogen, and oxygen are reviewed. Computer programs for the modeling of the data and fragment ions energy analysis are also presented.Item Low-energy elastic scattering of oxygen atoms by atmospheric species(1992) Smith, Gerald J.; Stebbings, R. F.This thesis reports measurements of absolute differential cross sections for elastic scattering of atomic oxygen at 0.5- and 1.5-keV laboratory energies. Measurements for targets of He, Ne, Ar, Kr, Xe, H$\sb2$, N$\sb2$, O$\sb2$, CO, CO$\sb2$, H$\sb2$O, CH$\sb4$, CF$\sb4$, SF$\sb6$, SO$\sb2$, and NH$\sb3$ are performed over a laboratory angular range of 0.2 to 5.0 degrees. Using a partial wave analysis, the cross sections for several targets are inverted and estimates of model interaction potentials are made. These potentials are then used to predict the elastic scattering cross sections at a laboratory energy of 100 eV.Item Penning ionization studies using a crossed-beam technique(1971) Howard, John Stephen; Stebbings, R. F.A modulated crossed-beam apparatus for the investigation of Penning ionization at thermal energies has been designed and constructed. Details are given for the production of a beam of helium metastable atoms by electron impact excitation. The production of well-collimated neutral beams of stable species using multi-channel capillary arrays is analyzed, and preliminary measurements on the performance of one such array are presented. Production of atomic hydrogen by thermal dissociation of is H2 discussed. A detection system is described which collects product ions and separates those due to ionization of beam particles from those due to ionization of residual gas. Preliminary analysis of the appropriate operating conditions for the pulse counting system is presented. To determine the types and abundances of ions produced in He*-neutral collisions, a magnetic sector mass spectrometer has been designed. Extraction of the product ions from the interaction region and subsequent acceleration and steering of these ions through the spectrometer are analyzed.Item The absolute differential cross-section for charge transfer scattering of 1.5 keV protons with atomic oxygen(1994) Sieglaff, Dean Ronald; Stebbings, R. F.The absolute cross section, differential in laboratory scattering angle, for the process $\rm H\sp{+} + O\to H + O\sp{+}$ using 1.5 keV-energy protons is reported. The total cross section value of (8.7 $\pm$ 1.9) $\times$ 10$\sp{-16}$ cm$\sp2$ derived from the integration of the differential cross section is compared with existing cross section data.