Browsing by Author "Straub, H. Charles"

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    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$3.5% and is marginally higher for multiple and dissociative ionization. Previous cross section measurements are compared to the present results. Additionally, combining the flight time and detection location of fragment ions yields some qualitative information about their energy distribution.
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    Accurate determination of high vacuum pressure
    (1994) Straub, H. Charles; Stebbings, R. F.
    Two methods for the accurate determination of high vacuum pressure are reviewed with particular attention paid to the 10$\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.