Abstract
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.
Highlights
The molecular ionization potential (IP) in high electric fields is an important parameter in pre-breakdown and breakdown phenomena in electrically insulating liquids.[1,2] In addition to the applied electric field, a molecule is influenced by the electric moments of its neighboring molecules
The IP and excitation energies in different directions of the electric field are studied for benzene, cyclohexane and n-tridecane, propylene carbonate and methyl butyrate, benzil, 4,4 -dihydroxybenzil, 2,3-heptanedione and p-benzoquinone
The constrained densityfunctional theory (DFT) method is demonstrated as an efficient method for calculating the fielddependent IP of different types of molecules
Summary
The streamer speed decreases at specific voltage In another recent work, azobenzene and N,N-dimethylaniline were added to an ester liquid and it was shown that azobenzene with lower excitation energies than N,N-dimethylaniline causes a significant increase in the acceleration voltage.[15]. Among the different quantum-chemical methods, densityfunctional theory (DFT) calculations are accurate and computationally efficient based on the choice of exchange-correlation functionals.[16] Methods for calculating the field-dependent IP has been developed recently.[1,17] In the first studies, a point-charge model is used[1,2] that is a quantum-classical method based on the electrostatic interactions between a negative point-charge and a cation to find the transition state for the dissociation of an electron in an electric field. A more recent method is based on constrained DFT (CDFT),[17] which is a full quantum-chemical approach that includes both the electrostatic interactions and the exchange effects between the electron and the cation. The IP and excitation energies of different types of molecules in the electric field are studied and compared to each other, as well as with our previous work.[1,2,17]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
