Evidence of rotational autoionization in the threshold region of the photoionization spectrum of $CH3$

The photoionization spectrum of the threshold region of $CH3$ equilibrated at room temperature has been recorded and compared to the zero electron kinetic energy (ZEKE) spectrum of Blush et al. [J. Chem. Phys. 98, 3557 (1993)]. The ionization onset region is at $∼70 cm−1$ higher energy than previous high-temperature photoionization work [Chupka and Lifshitz, J. Chem. Phys. 48, 1109 (1967)], but still $∼34 cm−1$ lower than that implied by invoking only direct ionization. The residual discrepancy can be accounted for by including fully allowed quadrupole-induced and partially allowed dipole-induced rotational autoionization, thus making the observed onset completely congruous with the ZEKE ionization potential. In addition, the fragment appearance potential of $CH3+$ from $CH4$ was redetermined by accurate fitting as $AP0(CH3+/CH4)=14.322±0.003 eV.$ With the very precise ZEKE ionization potential, this yields the best current value for the bond dissociation energy in methane, $D0(H–CH3)=4.484±0.003 eV$$=103.40±0.07 kcal/mol$ (⁠$104.96±0.07 kcal/mol$ at 298 K).