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

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Abstract
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).

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