Field dependence on the molecular ionization potential and excitation energies compared to conductivity models for insulation materials at high electric fields

Abstract

High-resolution pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopic study of the two lowest electronic states of the ozone cation O 3 + Rotationally resolved pulsed field ionization photoelectron study of CO + (X 2 Σ + ,v + =0-42) in the energy range of 13.98-21.92 eVThe aim is to improve the understanding of high-field phenomena (such as preinception currents/conduction, streamer initiation and propagation) in insulating materials in terms of the molecular properties of the substances involved. In high electric fields, ionization is a likely process, and in all such processes, the ionization potential is an important parameter. A fundamental question is how these processes depend on the electric field, and therefore, based on the interaction between a negative point charge and a molecular cation as modeled by density functional theory, a field-dependent model for the ionization potential is developed. In addition, the first excitation energies as a function of the electric field are calculated using time-dependent density functional theory. It is demonstrated that empirical high-field conduction models for cyclohexane and n-tridecane can be explained in terms of the difference between the ionization potential and the first excitation energy. It is also suggested that the reduction of the ionization potential with electric fields, can help explain how fast-mode streamers propagate.

Modeling the transition to fast mode streamers in dielectric liquids

“…The IP is the energy required to ionize a molecule from its ground state, and is an important characteristic of an insulating liquid [5], [10]. Additives with a low IP have been found to facilitate the growth of slow streamers [11] and increase the threshold for fast propagation, the acceleration voltage [1].…”

“…The dynamics of the streamer channel is of importance, however, processes occurring at the tip of the branches, like electron avalanches [4] and photoionization, are viewed as more important. An lowered ionization potential (IP) in a strong electric field, may facilitate photoionization [5].…”

Positive Breakdown Streamers and Acceleration in a Small Point-Plane Liquid Gap and Their Variation with Liquid Properties.

“…Azobenzene was included as an additive because of a hypothesis that the lowest electronic excitation level might have an influence [11,12,19], particularly at the higher local fields which may exist in front of a streamer tip, since the ionisation potential may be so reduced with increased field [10,11] that it approaches the lowest excitation level. However, in this experiment it has an effect similar to but weaker than with DMA, and this experiment alone cannot tell if the effect is caused by the low excitation level or the ionization potential which is lower than in the base liquid but still higher than in DMA.…”

“…One long gap study indicates that this has no consequences for positive streamers [10], while another long-gap study shows significant changes in both acceleration voltage and breakdown voltage [6], so there is a possibility that the results may be influenced by the particles. Table 1 shows a list of liquid combinations used and Table 2 lists the purity and some relevant electronic properties of the liquids, calculated with quantum chemical methods [11,12]. The table shows zero-field gas phase values, which may not be the relevant values, since the ionization potential is typically lowered about 1 eV in the liquid state [13] and is highly dependent upon the local electric field [11,12,14].…”

“…Table 1 shows a list of liquid combinations used and Table 2 lists the purity and some relevant electronic properties of the liquids, calculated with quantum chemical methods [11,12]. The table shows zero-field gas phase values, which may not be the relevant values, since the ionization potential is typically lowered about 1 eV in the liquid state [13] and is highly dependent upon the local electric field [11,12,14]. The excitation levels are in most cases much less dependent upon the field.…”