Resonances in Electron Scattering on Benzisoxazole

Electron energy loss spectra (EELS) of benzisoxazole reveal signatures of anionic resonances in the vicinity of 1.2 and 2.2 eV incident electron energies. Another low-energy resonance is likely present just below 0.5 eV, but its contributions are difficult to discern definitively because of the overlap with intense near-threshold scattering and thermionic emission features. The observed resonances decay via two competing mechanisms: the excitation of specific vibrational modes and emission of low-energy electrons following statistical thermalization of resonance energy among unspecific vibrations. The thermionic pathway is likely mediated and amplified by vibronic couplings between the resonance states and the weakly bound dipole-bound anion supported by the 3.2 D dipole moment of benzisoxazole. The existence of the stable dipole-bound and metastable π* resonance states is confirmed by equation-of-motion coupled-cluster theory calculations. The non-Hermitian theory using a complex absorbing potential to stabilize the temporary-anion states predicts three such states in benzisoxazole below 3 eV. These states are assigned to the observed experimental features and described as π₁^*, π₂^*, and π₃^* scattering resonances. Additional structureless features in the 3-6 eV range observed in the EELS excitation curves for some vibrations are tentatively ascribed to several additional resonances predicted in that range. The existence of the dipole-bound anion state and its role in resonance decay dynamics set benzisoxazole sharply apart from its isomer benzoxazole.