The electronic states of the azines. III. Pyrimidine, studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration calculations

Differential cross sections for the electron impact excitation of pyrimidine

“…Here either one or two Gaussian functions were employed as fitting functions for each spectral feature, with the positions and widths of each feature having been established through consideration of the available photo-absorption spectra, 3,[21][22][23] threshold-electron excitation spectra, 23 and theoretical calculations. [22][23][24] A full discussion of the present spectral assignments has been reported in our earlier paper 17 relating to the ICS, so we do not repeat those details here. At each incident energy and scattering angle, the amplitudes of the Gaussian functions were varied, while keeping the positions and widths fixed, in a least-squares fitting procedure to determine the best overall fit to the spectrum.…”

“…3,[21][22][23] In particular, the photo-absorption studies on pyrimidine, in combination with benzene and other azabenzenes, 21 have provided key insights into how the introduction of N atoms into the benzene ring influences the nature of the n-π * and π -π * transitions. These experiments have been interpreted with the assistance of numerous theoretical calculations to provide a detailed understanding of the singlet electronic states.…”

“…These experiments have been interpreted with the assistance of numerous theoretical calculations to provide a detailed understanding of the singlet electronic states. [22][23][24][25][26] The pyrimidine photo-absorption spectrum is dominated by three broad features attributed to strong dipole-allowed transitions at 5.2, 6.7, and 7.6 eV. Specifically, the bulk of the intensity observed for these features has been assigned to π -π * transitions to the 1 B 2 , 1 A 1 , and 1 A 1 + 1 B 2 symmetry states, respectively.…”

“…The available data on the singlet → triplet excitation processes is less comprehensive than that for the dipole-allowed transitions, with the weak transitions and complexity of the spectra making assignment difficult. Here only the near-threshold electron energy loss spectra, 23 hot photo-absorption spectra, 24 and theoretical calculations [22][23][24] have provided a somewhat limited description of the low-lying triplet states. We have recently demonstrated that the electron-impact excitation of pyrimidine to higher lying triplet states may be particularly important at low incident electron energies.…”

Elastic and inelastic cross sections for low-energy electron collisions with pyrimidine

“…15 Measurements have also been performed for condensed pyrimidine, 16 for which vibrational and electronic excitation cross sections were determined. Electron energy loss and VUV spectroscopy experiments 17,18 have provided insight into the electronic excited states of the molecule. Resonance formation in pyrimidine has also been studied experimentally, starting from the pioneering work of Nenner and Schulz.…”