Field-dependent molecular ionization and excitation energies: Implications for electrically insulating liquids

• Published in: AIP advances 2014-03, Vol.4 (3), p.037117-037117-13
• Main Authors: Davari, N., Åstrand, P.-O., Unge, M., Lundgaard, L. E., Linhjell, D.
• Format: Article
• Language: English
• Subjects: Alkanes; Cyclohexane; Density functional theory; Diketones; Electric fields; Electrical insulation; Esters; Excitation; Ionization potentials; Liquids; Mathematical analysis; Propagation modes; Time dependence
• ISSN: 2158-3226; 2158-3226
• DOI: 10.1063/1.4869311

The molecular ionization potential has a relatively strong electric-field dependence as compared to the excitation energies which has implications for electrical insulation since the excited states work as an energy sink emitting light in the UV/VIS region. At some threshold field, all the excited states of the molecule have vanished and the molecule is a two-state system with the ground state and the ionized state, which has been hypothesized as a possible origin of different streamer propagation modes. Constrained density-functional theory is used to calculate the field-dependent ionization potential of different types of molecules relevant for electrically insulating liquids. The low singlet-singlet excitation energies of each molecule have also been calculated using time-dependent density functional theory. It is shown that low-energy singlet-singlet excitation of the type n → π* (lone pair to unoccupied π* orbital) has the ability to survive at higher fields. This type of excitation can for example be found in esters, diketones and many color dyes. For alkanes (as for example n-tridecane and cyclohexane) on the other hand, all the excited states, in particular the σ → σ* excitations vanish in electric fields higher than 10 MV/cm. Further implications for the design of electrically insulating dielectric liquids based on the molecular ionization potential and excitation energies are discussed.