Ion arrival time distributions in high pressure mass spectrometry



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The primary concern of this investigation was the measurement of absolute rate constants of ion-molecule reactions in the near thermal energy range. By the variation of the extraction field the energy dependence of these rates were qualitatively determined. The method consisted of using a conventional mass spectrometer modified to allow operation of the ion source at pressures as high as several torr. The electron beam was pulsed for a short duration followed by the measurement of the arrival time distribution. This arrival time distribution yielded upon simple mathematical analysis the rate constant. Since the apparatus was a hybrid drift tube-chemical ionization mass spectrometer, it was also well suited to the examination of certain phenomena which occur under chemical ionization conditions. Consequently, this work branched off into a study of the effects of ion source geometry on ion arrival times. In addition, the ability to measure ion arrival times and hence drift times proved an invaluable experimental tool in the unraveling of two recent controversial problems: 1) The rate of H+ transfer in the H[lowered 3, raised +] - H[lowered 2] system; 2) the achievement of equilibrium in the solvation of the hydrogen ion by water in the gas phase. Finally, several suggestions are made concerning improvements in the experimental apparatus and future experiments to be performed in this area.