Negative ion properties of p-benzoquinone: Electron affinity and compound states

Abstract

Bound and excited negative ion states of para‐benzoquinone (PBQ) are studied from experiments involving collisions of electrons and cesium beams with PBQ. The electron affinity of PBQ was measured to be 1.89^+0.2_−0.3 eV using the cesium collisional ionization technique. PBQ⁻(C₆H₄O⁻₂) was the dominant negative ion observed following collisions of cesium beams with PBQ. Direct electron attachment to PBQ produced the fragment ions C₅H₄O⁻, C₅H₃O⁻, C₄O₂H⁻₂, C₂HO⁻, and C₂H⁻ as well as the parent C₆H₄O⁻₂ ion. Our observations showed that the cross section for producing the metastable parent negative ion, C₆H₄O⁻₂, peaks at 1.40±0.1 eV which is in disagreement with a previous value of 2.1 eV. Attachment of thermal electrons to PBQ at low pressure (−4 mm Hg) was not observed. Compound negative ion states were observed to peak at 0.70, 1.35, and 1.90 eV using the SF₆ scavenger technique. The energies of these compound states agree qualitatively with the energies if the manifold of unfilled π* orbitals. The long‐lived state at 1.4 eV is attributed to either (a) direct attachment of the incident electron into the ²b_1u orbital followed by internal conversion to the ground ²B_3g of PBQ⁻ or (b) simultaneous excitation of a core electron and attachment of the incident electron into an excited orbital (i.e., core excited Feshbach resonance) as previously suggested. Evidence is presented for the former explanation.