Temporary Anions of Benzoxazole in Charge-Transfer Cluster Photodetachment.

The photoelectron spectra of cluster anions of superoxide (O) 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 O·BzOx → O·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 O 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 (π) at 0.891 eV and resonance B (π) 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 O·BzOx charge-transfer states are partially stabilized relative to the free-electron limit by interactions with the O molecule. These interactions depend on the electronic states of both species. The theory predicts that at the O·BzOx cluster geometry, the O(Σ)·BzOx(A) and O(Δ)·BzOx(A) states lie at 0.56 and 0.47 eV (vertically) above the respective neutral states. The O(Σ)·BzOx(B) resonance is found 1.43 eV (vertically) above O(Σ)·BzOx. Intense signatures of both BzOx resonances and the three above-mentioned charge-transfer cluster states, O(Σ)·BzOx(A), O(Δ)·BzOx(A), and O(Σ)·BzOx(B) are observed in the 355 nm (3.495 eV) and 532 nm (2.330 eV) photoelectron spectra of the O·BzOx cluster anion.