Ionization potentials for Cl-C4 alkyl free radicals have been re-measured by electron impact, using an energy-resolved electron beam from a double-hemispherical electrostatic monochromator. From these values and kinetic data for heats of formation of the radicals, ionic heats of formation for C1-C4 alkyl ions have been calculated. New values for the four isomeric butyI ions are given. The ionization efficiency curves for the simpler radicals show linear sections corresponding to excitation of vibrational levels; for larger radicals these segments are not resolved and only a smooth upward curvature is seen.In disagreement with earlier results, no evidence for formation of low-lying excited states of the ions could be found, either for parent or fragment ions.Canadian Journal of Chemistry, 48, 955 (1970) Introduction Earlier data on the ionization potentials (IP) of alkyl free radicals obtained with conventional mass spectrometric ion sources has been shown, by retarding potential difference (RPD) electron impact (1, 2) and photon impact (PI) (3,4, 5), to be generally too high by 0.1 to 0.6 V. The reason for this over-estimate is that the probability of ionization to the zeroth vibrational level of the ion may for some species be quite small, becoming rapidly larger for vibrational levels a few tenths of a volt higher, corresponding to allowed vertical transitions in the Franck-Condon sense. Using electrons with a relatively wide (0.6-1.0 V) spread, the energy resolution at the onset of ionization is low, and the customary data treatment yields apparent ionization onsets corresponding approximately to the part of the ionization efficiency curve where the steep rise begins.

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The photochemistry of monofluorobenzene in the vapor phase has been examined over a range of wavelengths extending from 2350 to 2710 Á. The sensitized emission of biacetyl was used to determine triplet-state yields for exciting wavelength of 2470, 2550, 2590, and 2670 Á. The effect of cyclohexane on the fluorescence yield and on the sensitized emission yield has been examined.