Effects of strong electric field and solvation on the excitation process of mineral oil molecules

Studying the excitation process of mineral oil molecules under the influence of electric fields and different environments is crucial for understanding the development of insulating oil streamer discharge at the atomic scale. In this paper, based on density functional theory, quantitative characteristic descriptors, hole-electron distribution, and transition density matrix (TDM) analysis, the excitation energies of the first five excited states are used to analyse the excitation process of mineral oil molecules under various conditions. The results obtained under four different electric field strengths (0, 0.0001 a.u., 0.001 a.u. and 0.01 a.u.) and three different environments (vacuum, mineral oil, and natural ester) are compared. The results show that for electric field strengths ranging from 0 to 0.001 a.u., the excitation types of the three mineral oil molecules are localized excitations. However, when the electric field intensity reaches 0.01 a.u., the excitation type of mineral oil molecules changes to charge transfer excitation due to the strong electric field. There are significant differences in the positions of holes and electrons in the excitation processes of the mineral oil molecules under different conditions. While the excitation process of chain hydrocarbon molecules under vacuum and in solution exhibits some differences, the excitation processes of dicycloalkanes and aromatic hydrocarbons are not affected by environmental factors. The electronic transitions in the excitation process of chain hydrocarbons and dicycloalkanes are sigma->sigma* transition processes, whereas in aromatic hydrocarbons, it is primarily pi->pi* transition processes. Compared to pi orbitals, sigma orbitals are lower energy empty orbitals, and sigma* orbitals are higher energy empty orbitals. As a result, the excitation energies of the chain hydrocarbons and dicycloalkanes are higher than that of aromatic hydrocarbons. Furthermore, a strong electric field changes the excitation type of mineral oil molecules, accompanied by a significant decrease in their excitation energy. The selection of insulating oil molecules, whose excitation process is sigma->sigma* or modifying the transition mode of electrons in the excitation process through doping and modification, is an important approach to improve the excitation energy of insulating oil. The findings of this research have the potential to enhance our understanding of the microscopic mechanisms that influence the development of streamer discharge in liquid insulating materials.