The electronic states of pyrimidine studied by VUV photoabsorption and electron energy-loss spectroscopy

Abstract

The electronic state spectroscopy of pyrimidine C₄H₄N₂ has been investigated using both high resolution VUV photoabsorption in the energy range 3.7 to 10.8 eV (335 to 115 nm) and lower resolution electron energy loss in the range 2 to 15 eV. The low energy absorption band, assigned to the (π*) ← 7b₂(n_N) (1¹B₁ ← 1¹A₁) transition, at 3.85(4) eV and the vibrational progressions superimposed upon it have been observed for the first time, due to the availability of a high-resolution photon beam (0.075 nm), corresponding to 3 meV at the midpoint of the energy range studied. Vibronic coupling has been shown to play an important role dictating the nature of the observed excited states, especially for the lowest ¹B₁ state. The 2¹B₁ state is proposed to have its origin at 7.026 eV according to the vibrational excitation reported in this energy region (7.8–8.4 eV). New experimental evidence of 4¹A₁ state with a maximum cross section at 8.800 eV is supported by previous ab initio quantum chemical calculations. Rydberg series have been assigned converging to the three lowest ionisation energy limits, 9.32 eV (²B₂), 10.41 eV (²B₁) and 11.1 eV (²A₁ + ²A₂) with new members reported for the first time and classified according to the magnitude of the quantum defects (δ). Additionally, the absolute differential cross section for inelastic electron scattering has been measured for the most intense band from 6.9 to 7.8 eV assigned to ¹ππ* (3¹A₁ + 2¹B₂).