The electronic states of the azines. VII. 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 (VUV) 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 (¹np^*, E_max˜3.1 eV) is highly structured. The corresponding triplet state is detected in EEL at 2.7 eV. Other valence states of type np^* are assigned to EEL bands at 3.6 eV (^1,3n p^*) and ˜6.0 eV(¹np^*). The ¹pp^* states are positioned about 5.0, 6.7, 7.8 and 7.9 eV, the last two relating to the benzene ¹E_1u state. Electron energy loss processes observed around 4.1 and 4.9 eV are attributed to excitation of ³pp^* states. Low-lying Rydberg states of type ¹n3s and ¹n3p 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. © 1995.