Ionization Potentials of Molecules and Free Radicals and Appearance Potentials by Electron Impact in the Mass Spectrometer

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Abstract
An improved method of measuring ionization efficiency (I.E.) curves by the retarding potential difference technique in the mass spectrometer (to ∼one ion/sec at onset) gives ionization potentials which agree within ± 0.02 eV of spectroscopic values and ± 0.03 eV of photoionization. Ionization potentials I have been measured for the following free radicals (eV): CH3, 9.87; C2H5, 8.34; n-C3H7, 8.13; i-C3H7, 7.57; n-C4H9, 8.01; C2H5O, 9.11; n-C3H7O, 9.20; i-C3H7O, 9.20; n-C4H9O, 9.22. Appearance potentials V have been measured for R+/RX, where R=CH3, C2H5, n-C3H7, n-C4H9 and X=H, Cl, Br, I, and OR; also for RO+/ROX where R=C2H5, i-C3H7, n-C4H9 and X=H, NO, and R; also for I+/n-C4H9I, NO+/C2H5ONO, NO+/n-C4H9ONO, and C3H6D+/CH3CD2CH3. Values of ΔHf(R+), ΔHf(RO+), D(R−X), and D(RO−X) are reported as well as electron affinities of NO, C2H5O, n-C3H7O, i-C3H7O, and n-C4H9O. Whenever two or more independent measurements of V provide the same ΔHf or D, agreement is satisfactory. The linearity of onsets does not correlate with photoionization efficiency curves according to present threshold theory. The “breaks” in I.E. curves do yield thermochemically consistent results.

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