Ionization and Dissociation by Electron Impact; Methylene, Methyl, and Methane

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Abstract
The appearance potentials of C+, CH+, and CH2+ in the mass spectra of methylene, methyl, and methane are found to be mutually consistent and when combined with the spectroscopic value of the ionization potential of the carbon atom (11.26 ev) lead to 15.5±0.5 ev/molecule for the heat of atomization of methane. This value (15.5) when further combined with the heat of formation of methane (—0.78 ev/molecule) and the dissociation energy of H2 (4.48 ev/molecule) yields 5.76 ev/atmos or 133 kcal/mole for the heat of sublimation of graphite. The set of appearance potentials confirmed the spectroscopic value for the heat of dissociation of CH+ (3.6 ev/molecule) and yield the following additional energetic quantities: D(CH2–H) = 3.75±0.3 ev/molecule, D(CH–H) = 3.99±0.3 ev/molecule, D(CH3+–H) = 1.34±0.1 ev/molecule, D(CH2+–H) = 5.54±0.2 ev/molecule, and D(CH+–H) = 3.37±0.2 ev/molecule. Earlier determinations of Iz(CH4), Iz(CH3) and D(CH3–H) are confirmed (within the experimental error) and the ionization potential of the methylene radical, Iz(CH2), is found to be 11.75±0.1 ev/molecule. Combination of the appearance potentials of CH2+ in the methane and diazomethane mass spectra gives 46±6 kcal/mole for the heat of formation of diazomethane.

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