Mechanism behind exclusive CH3− production from ~11.1 eV ion‑pair states

Determine the physical mechanism that causes ion-pair dissociation of ethyl acetate (CH3COOC2H5) near 11.1 eV to yield exclusively CH3− anions, and ascertain why other anionic fragments are not observed from ion-pair channels e− + CH3COOC2H5 → CH3− + COOC2H5+ + e− and e− + CH3COOC2H5 → CH3− + CH3COOCH2+ + e− in this energy region.

Background

The CH3− ion yield exhibits a rising contribution above ~11 eV that the authors attribute to ion-pair (IP) dissociation, with computed IP thresholds around 11.1 eV for the channels producing CH3− with COOC2H5+ or CH3COOCH2+. Notably, only CH3− is observed from IP dissociation in this region, which is unusual as other anionic fragments are not detected from IP channels.

The authors explicitly note that they lack an explanation for this exclusive CH3− production from low-energy IP states and remark that the presence of low-lying IP states itself is uncommon. Clarifying this mechanism would explain fragment selectivity and the dynamics of IP dissociation in ethyl acetate.