Experimental Observation of the Resonant Doorways to Anion Chemistry: Dynamic Role of Dipole-Bound Feshbach Resonances in Dissociative Electron Attachment.

Anion chemical dynamics of autodetachment and fragmentation mediated by the dipole-bound state (DBS) have been thoroughly investigated in a state-specific way by employing the picosecond time-resolved or the nanosecond frequency-resolved spectroscopy combined with the cryogenically cooled ion trap and velocity-map imaging techniques. For the -, -, or -iodophenoxide anion (-, -, or -IPhO), the C-I bond rupture occurs via the nonadiabatic transition from the DBS to the nearby valence-bound states (VBS) of the anion where the vibronic coupling into the S (πσ*) state (repulsive along the C-I bond extension coordinate) should be largely responsible. Dynamic details are governed by the isomer-specific nature of the potential energy surfaces in the vicinity of the DBS-VBS curve crossings, as manifested in the huge different chemical reactivity of -, -, or -IPhO. It is confirmed here that the C-I bond dissociation is mediated by DBS resonances, providing the foremost evidence that the metastable DBS plays the critical role as the doorway into the anion chemistry especially of the dissociative electron attachment (DEA). The fragmentation channel is dominant when it is mediated by the DBS resonances located below the electron-affinity (EA) threshold, whereas it is kinetically adjusted by the competitive autodetachment when the DBS resonances above EA convey the electron to the valence orbitals. The product yield of the C-I bond cleavage is strongly mode-dependent as the rate of the concomitant autodetachment is much influenced by the characteristics of the individual vibrational modes, paving a new way of the reaction control of the anion chemistry.