Ion—Molecule Reactions in the Xenon-Sensitized Ionization of Ethylene

Xenon was irradiated with alpha particles in the absence of electric fields at pressures up to 40 torr containing 10−3−5 torr ethylene. Part of the gas bleeds into a mass spectrometer. Experiments with a movable collimating slit and a theoretical treatment of the conditions in the ion source establish a qualitative picture for the sampling conditions and ionic reaction times. Most of the detected ions originate from a 1-mm-radius volume directly over the leak. The average ionic reaction times are a few milliseconds. Electrons are sampled with greatly reduced efficiency or not at all. The xenon-sensitized ionization of ethylene produces mostly C2H4+ as primary ion. The reactions of C2H4+ with ethylene at xenon pressures above 10 torr lead mainly to the simple chain CnH2n+ (n=even). Approximate rate constants for the addition reactions were determined. C4H8+ adds at least 100 times more slowly to ethylene than C2H4+. Further decreases of rate constants occur as the chain develops such that the rate constant for the C10H20+ addition to ethylene is some four orders of magnitude lower than that of C2H4+. At lower xenon pressures stabilization of the excited addition product (CnH2n*)+ is not sufficiently rapid and decomposition CkH2k−1 (k=odd) occurs. The stabilization to decomposition ratio is studied as a function of xenon pressure.