Mass Spectrometric Study of the Molecular Sublimation of Graphite

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Abstract
The emission of neutral and charged carbon molecules from spectroscopically pure graphite has been studied by rate of vaporization experiments in a 180° mass spectrometer. Small filaments (diameter: 0.015 in.), heated by ac up to 2600°K, yielded neutral particles which were converted into positive ions by bombardment with low‐energy electrons. C3+, C2+, and C1+ ions were found. Although the effects of fragmentation and of differences in ionization cross sections may not be negligible, it is believed that carbon vapor consists largely of C3 molecules. Graphite was found to emit negative ions containing from one to eight carbon atoms. C2—, C4—, and C3— were the most abundant species, in that order. A search was made for carbon particles emitted as positive ions, but none were found. Heats of activation were obtained for the neutrals and the more abundant negative ions from the slopes of logp vs 1/T plots. Identical heats measured for C1 and C3 (178±10 kcal/mole at 2400°K) suggest that these two species may sublime at rates that are interdependent. Thus, this value may not be representative of the true heat of sublimation of C1. For C2, a heat of 199±20 kcal/mole was measured at 2400°K. Electron affinities have been computed for C1, C2, and C3, and estimated bond energies deduced for C2 and C3. Some of the earlier measurements of the heat of sublimation of graphite, obtained from rates of vaporization and equilibrium effusion, but without mass analysis, have been reinterpreted, taking into account the complex character of the sublimation processes.

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