Compound-negative-ion resonance states and threshold-electron excitation spectra of N-heterocyclic molecules: Pyridine, pyridazine, pyrimidine, pyrazine, and sym-triazine

The threshold‐electron excitation spectra of the N‐heterocyclic molecules pyridine, pyridazine, pyrimidine, pyrazine, and sym‐triazine have been studied with a newly constructed electron spectrometer. These are presented, discussed, and compared with photoabsorption and, whenever possible, with electron impact spectral data. Optically forbidden and $\mathrm{n\to \; \pi *}$ transitions have been observed. Two compound negative ion resonance (CNIR) states (``shape'' resonances) have been detected below the first excited electronic states of all N‐heterocyclic molecules studied, except sym‐triazine for which only one CNIR state has been observed. These CNIR states are discussed in terms of the effect of a perturbation introduced by the more electronegative nitrogen atom(s) in the benzene ring on the two lowest (degenerate) antibonding orbitals of benzene.