Ionization and Dissociation of Polyatomic Molecules by Electron Impact. I. Methane

Abstract

The first of a series of mass-spectrograph studies of the products of ionization of polyatomic molecules by low speed electrons is described. The following are the positive ions detected in methane together with their appearance potentials in volts: C$\mathrm{H}_{4}^{}{}_{}{}^{+}$ (13.1\ifmmode\pm\else\textpm\fi{}0.4); C$\mathrm{H}_{3}^{}{}_{}{}^{+}$ (14.4\ifmmode\pm\else\textpm\fi{}0.4); C$\mathrm{H}_{2}^{}{}_{}{}^{+}$ (15.7\ifmmode\pm\else\textpm\fi{}0.5, 22.9\ifmmode\pm\else\textpm\fi{}0.8); C${\mathrm{H}}^{+}$ (23.3\ifmmode\pm\else\textpm\fi{}0.6); ${\mathrm{C}}^{+}$ (26.7\ifmmode\pm\else\textpm\fi{}0.7); ${\mathrm{H}}^{+}$ (22.7\ifmmode\pm\else\textpm\fi{}0.5, 29.4\ifmmode\pm\else\textpm\fi{}0.6); $\mathrm{H}_{2}^{}{}_{}{}^{+}$ (27.9\ifmmode\pm\else\textpm\fi{}0.5); and $\mathrm{H}_{3}^{}{}_{}{}^{+}$ (25.3\ifmmode\pm\else\textpm\fi{}1.0). The following negative ions were also observed: C$\mathrm{H}_{2}^{}{}_{}{}^{\ensuremath{-}}$; C${\mathrm{H}}^{\ensuremath{-}}$; ${\mathrm{C}}^{\ensuremath{-}}$; and ${\mathrm{H}}^{\ensuremath{-}}$. From the higher appearance potential of ${\mathrm{H}}^{+}$ and the appearance potentials of ${\mathrm{C}}^{+}$ and ${\mathrm{C}}^{\ensuremath{-}}$ three independent upper limits are calculated for the heat of dissociation of C${\mathrm{H}}_{4}$ to normal atoms which lead, respectively, to the following three upper limits for the heat of sublimation of carbon: $L(\mathrm{C})\ensuremath{\le}6.1\ifmmode\pm\else\textpm\fi{}0.6$ volts or 141\ifmmode\pm\else\textpm\fi{}14 kcal.; $L(\mathrm{C})\ensuremath{\le}5.7\ifmmode\pm\else\textpm\fi{}0.7$ volts or 131\ifmmode\pm\else\textpm\fi{}16 kcal.; $L(\mathrm{C})\ensuremath{\le}6.0\ifmmode\pm\else\textpm\fi{}0.9$ volts or 139\ifmmode\pm\else\textpm\fi{}21 kcal. Thus of the three values of $L(\mathrm{C})$ recently proposed from data on predissociation in CO, these results are inconsistent with the value $L(\mathrm{C})=168.8$ kcal. but are quite consistent with the value $L(\mathrm{C})=123.6$ kcal. Correlations of known ionization energies with the best obtainable data on the relative energies of the possible stable states of a carbon and four hydrogen atoms give reasonable interpretations of all observed appearance potentials. Upper limits for the ionization potentials of C${\mathrm{H}}_{3}$, C${\mathrm{H}}_{2}$, and CH are deduced. These are: $I({\mathrm{CH}}_{3})\ensuremath{\le}9.9$ volts; $I({\mathrm{CH}}_{2})\ensuremath{\le}12.0$ volts; $I(\mathrm{CH})\ensuremath{\le}11.7 or 16.2$ volts. Two general conclusions are drawn. The first is that most ions are only formed when the parent molecules are broken into as many parts as possible. The second is that interpretations of all but four appearance potentials are possible on the assumption that the total excitational and kinetic energy of products formed in an electron impact from a C${\mathrm{H}}_{4}$ molecule be less than two volts.