Field-dependent ionization potentials and excitation energies of molecules of relevance for electrically insulating liquids

The ionization potential (IP) and excitation energies of selected molecules relevant for electrically insulating liquids are investigated at high electric fields using density-functional theory calculations. The field-dependent IP is calculated by constrained density-functional theory and it is shown that this method is capable of determining the field-dependent IP of molecules with different electronic properties. The number of available excited states decreases with increasing electric field. At specific electric fields, different for each molecule, excited states vanish and it is hypothesized that this phenomenon influences streamer propagation and may be the origin at the molecular level for different streamer modes.