Compound-negative-ion-resonant states and threshold-electron-excitation spectra of monosubstituted benzene derivatives

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Abstract
Threshold‐electron‐excitation (TEE) spectra for fluorobenzene, benzaldehyde, and benzoic acid are presented, discussed, and compared with photoabsorption spectra. The TEE spectra clearly indicate excitation of optically forbidden states. Short‐lived transient‐negative ions have been found to form at low energies (< 2 eV) for nine monosubstituted benzene derivatives. These arise from the quasitrapping of slow electrons in the two lowest unoccupied π orbitals of the benzene‐derivative molecules that, owing to the perturbation introduced by the substituent, are no longer degenerate as in benzene. The positions of these double compound‐negative‐ion resonant states are 1.27, 1.74; 0.61, 1.67; 0.55, 1.88; 0.4, 1.6; 1.30, 2.25; ?, 1.90; 0.66, 1.10; 0.71, 1.12; and 0.63, 1.33 eV for fluorobenzene, phenol, aniline, toluene, N‐methylaniline, anisole, thiophenol, benzaldehyde, and benzoic acid, respectively. These are discussed in terms of the net π‐charge transfer between the substituent and the benzene ring.