Temporary Anions of Benzoxazole in Charge-Transfer Cluster Photodetachment

The photoelectron spectra of cluster anions of superoxide (O₂^-) solvated by one molecule of benzoxazole (BzOx) reveal two competing photodetachment mechanisms: a direct photoemission from the solvated cluster core and an indirect pathway involving temporary anion states of benzoxazole accessed via the O₂^-·BzOx → O₂·BzOx^- charge-transfer transitions. Benzoxazole is a bicyclic unsaturated organic molecule that does not form permanent anions. However, its low-lying vacant π* orbitals permit a resonant capture of the electron emitted from the O₂^- cluster core. The non-Hermitian theory using a complex absorbing potential predicts the existence of two BzOx^- π* resonances within the experimental energy range: resonance A (π₁^*) at 0.891 eV and resonance B (π₂^*) at 1.76 eV, relative to the onset of the BzOx + e^- continuum at the ground-state geometry of neutral BzOx. Within the clusters, the O₂·BzOx^- charge-transfer states are partially stabilized relative to the free-electron limit by interactions with the O₂ molecule. These interactions depend on the electronic states of both species. The theory predicts that at the O₂^-·BzOx cluster geometry, the O₂(X³Σ_g^-)·BzOx^-(A) and O₂(a¹Δ_g)·BzOx^-(A) states lie at 0.56 and 0.47 eV (vertically) above the respective neutral states. The O₂(³Σ_g^-)·BzOx^-(B) resonance is found 1.43 eV (vertically) above O₂(X³Σ_g^-)·BzOx. Intense signatures of both BzOx^- resonances and the three above-mentioned charge-transfer cluster states, O₂(X³Σ_g^-)·BzOx^-(A), O₂(a¹Δ_g)·BzOx^-(A), and O₂(³Σ_g^-)·BzOx^-(B) are observed in the 355 nm (3.495 eV) and 532 nm (2.330 eV) photoelectron spectra of the O₂^-·BzOx cluster anion.