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

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 ir 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 n_charge transfer between the substituent and the benzene ring.