The He I photoelectron spectra (PES) of 2- and 4-ethynylpyridine were measured, and the origin of each ionization band was clarified. For instance, 4-ethynylpyridine shows five PES bands at 9.58, 9.9, 10.0, 10.75 and 11.84 eV in the lower energy region, the 9.9 eV and 10.75 eV bands in particular being assigned to the ionizations from the aln orbital of the pyridyl group and the in-plane b(2) pi orbital of the ethynyl group, respectively. Furthermore, with the aid of first order perturbation theory, the 10.59 eV band of 2-ethynylpyridine has been ascribed to the ionization from the a(2)-like pi orbital, and the 10.8 eV band to that from the in-plane b(2)-like pi one. Through the process of the band assignments for the ethynylpyridines, it has been revealed that there is a weak interaction between the n orbital on the ortho-substituted nitrogen atom and the in-plane n orbital localized on the ethynyl group of 2-ethynylpyridine. The photoelectron spectra of 1,4-bis(2-pyridyl)-1,3-butadiyne and 1,4-bis(4-pyridyl)-1,3-butadiyne, whose pi-electron systems are regarded electronically as double ones of the corresponding ethynylpyridines, were also measured, and we have succeeded in making band assignments of these compounds on the ground of the correlation energy diagram with the ethynylpyridines.
