Knowledge Bank

Potential energy surfaces have been calculated for the lowest resonant states of $CH4-$. These calculations are used to correlate the dissociative electron attachment (DA) $spectra1,2$ and the threshold electron excitation spectrum (TEES) of $methane.3$ The peak at 2.5 eV in the TEES spectrum is assigned to the $X~2A1CH4-$ compound state which is bound with respect to the lowest dissociation limits: $CH2-(A~2A1)+H2(X1\Sigma g+)$ and $CH3-(X~1A1)+H(2S)$. Thus, this resonance state can decay only by autoionization. This assignment is consistent with no $CH3-$ being formed by dissociative electron attachment to $CH4$ (the lowest excited states of $CH3-$ are not energetically accessible). The $H-/CH4$ peak at 9.0 eV and the $CH2-/CH4$ peak at 10.4 eV of the DA $spectrum1$ are assigned to the $A~2T2CH4-$ compound state. It is proposed that the inverse isotope $effect1$ for H production $[\sigma (H-/CH4)\approx 0.8\sigma (D-/CD4)]$ and the large direct isotope effect for $CH2-$ production $[\sigma (CH2-/CH4)>260\sigma (CD2/CD4)]$ may be the result of the Jahn-Teller splitting of the triple degeneracy of the $A~2T2$ resonance state.