The electronic states of the azines .7. 1,2,4-triazine, studied by photon absorption, near-threshold electron energy loss spectroscopy and ab initio multi-reference configuration interaction calculations

Ultraviolet photoelectron (UPS), ultraviolet (UV) and vacuum ultraviolet (VU) absorption and electron energy loss (EEL) spectra are presented for 1,2,4-triazine in the gas phase and interpreted using the results of ab initio multi-reference configuration interaction calculations. The lowest optical band ((1)n pi*, E(max) approximate to 3.1 eV) is highly structured. The corresponding triplet state is detected in EEL at 2.7 eV. Other valence states of type n pi* are assigned to EEL bands at 3.6 eV ((1,3)n pi*) and similar to 6.0 eV ((1)n pi*). The (1) pi pi* states are positioned about 5.0, 6.7, 7.8 and 7.9 eV, the last two relating to the benzene (1)E(lu) state. Electron energy loss processes observed around 4.1 and 4.9 eV are attributed to excitation of (3) pi pi* states. Low-lying Rydberg states of type (1)n3s and (1)n3p have been assigned, but Rydberg series are not strong in the experimental spectra. The UPS has been interpreted by means of non-diagonal Green's function and Tamm-Dancoff calculations.