Nazanin Davari

Excitation energies and ionization potentials at high electric fields for molecules relevant for electrically insulating liquids

The electric-field dependence of the molecular ionization potential and excitation energies is investigated by density-functional theory calculations. It is demonstrated that the ionization potential has a strong field dependence and decreases with increasing field. The excitation energies depend weakly on the field and the number of available excited states decreases with increasing field since the ionization potential has a stronger field dependence. Above a specific field, different for each molecule, a two-state model is obtained consisting of the electronic ground state and the ionized state. Implications for streamer propagation and electrically insulating materials are discussed.

Field-dependent ionisation potential by constrained density functional theory

Constrained density functional theory is used in a novel approach to calculate the vertical ionisation potential in an electric field up to 30 MV/cm for water, ethane and ethene. Two density functionals, the Becke, three-parameter, Lee–Yang–Parr functional (B3LYP) and the B3LYP functional modified by the Coulomb-attenuating method (CAM-B3LYP), and two basis sets, cc-pVTZ and aug-cc-pVTZ are employed and compared with an earlier study using a point-charge approach. It is found that the field-dependent ionisation potential obtained from constrained density functional theory is in good agreement with the previous work. Furthermore, the results for the two density functionals are close to each other and the effect of augmentation of the basis set is small for the field-dependent ionisation potential. Limitations of the methodology at high electric fields are discussed. The method presented here demonstrates how constrained density functional theory can be used in general to study molecule–electron interactions.

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

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 b...

Local electric field factors by a combined charge-transfer and point–dipole interaction model

A force-field model for the local electric field as a linear response to a frequency-dependent external electric field is presented based on a combined charge-transfer and point–dipole interaction (CT-PDI) force-field model for frequencies through the first absorption maximum. The local electric field provides a measure of the mutual interactions of the molecules with each other, as is important in problems ranging from dielectric breakdown to solvent polarization and energy transfer. It also indicates how resonant excitation of these molecules can perturb Raman scattering by a third molecule located nearby through an intensity borrowing mechanism. The CT-PDI model is a combination of a modified electronegativity equalization model including non-metallic behaviour and a point–dipole interaction model described by atomic polarizabilities which also includes the time-dependence of the atomic charges and atomic dipole moments. A parametrization of frequency-dependent polarizabilities through the first absorption maximum calculated by time-dependent density-functional theory has been extended for a set of hydrocarbon and azobenzene molecules to provide atom-type parameters for the CT-PDI model. As initial model systems, results are presented for the benzene and azobenzene dimers for the local electric field response at points between the molecules and at the atoms in the molecules. As expected, the response depends critically on the intermolecular distance between the monomers. The azobenzene dimer shows a larger local field response at the atoms in the phenyl rings compared to the benzene dimer and the response at the nitrogen atoms is larger than at the hydrogen and carbon atoms in the azobenzene dimer, which can be rationalized qualitatively by the charge and dipole contributions to the local field factor either adding up or to a large extent cancelling each other. At the absorption frequency, the largest local field factor of the benzene dimer is around 6 and for the azobenzene dimer it is around 12, respectively, at typical distances, indicating that the response may be significant.

Local Field Factors and Dielectric Properties of Liquid Benzene.

Local electric field factors are calculated for liquid benzene by combining molecular dynamic simulations with a subsequent force-field model based on a combined charge-transfer and point-dipole interaction model for the local field factor. The local field factor is obtained as a linear response of the local field to an external electric field, and the response is calculated at frequencies through the first absorption maximum. It is found that the largest static local field factor is around 2.4, while it is around 6.4 at the absorption frequency. The linear susceptibility, the dielectric constant, and the first absorption maximum of liquid benzene are also studied. The electronic contribution to the dielectric constant is around 2.3 at zero frequency, in good agreement with the experimental value around 2.2, while it increases to 6.3 at the absorption frequency. The π → π* excitation energy is around 6.0 eV, as compared to the gas-phase value of around 6.3 eV, while the experimental values are 6.5 and 6.9 eV for the liquid and gas phase, respectively, demonstrating that the gas-to-liquid shift is well-described.

Complex frequency-dependent polarizability through the π → π* excitation energy of azobenzene molecules by a combined charge-transfer and point-dipole interaction model.

The complex frequency-dependent polarizability and π → π* excitation energy of azobenzene compounds are investigated by a combined charge-transfer and point-dipole interaction (CT/PDI) model. To parametrize the model, we adopted time-dependent density functional theory (TDDFT) calculations of the frequency-dependent polarizability extended with excited-state lifetimes to include also its imaginary part. The results of the CT/PDI model are compared with the TDDFT calculations and experimental data demonstrating that the CT/PDI model is fully capable to reproduce the static polarizability as well as the π → π* excitation energy for these compounds. In particular, azobenzene molecules with different functional groups in the para-position have been included serving as a severe test of the model. The π → π* excitation is to a large extent localized to the azo bond, and substituting with electron-donating or electron-attracting groups on the phenyl rings results in charge-transfer effects and a shift in the excitation energy giving rise to azobenzene compounds with a range of different colors. In the CT/PDI model, the π → π* excitation in azobenzenes is manifested as drastically increasing atomic induced dipole moments in the azo group as well as in the adjacent carbon atoms, whereas the shifts in the excitation energies are due to charge-transfer effects.

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.

Frequency-dependent local field factors in dielectric liquids by a polarizable force field and molecular dynamics simulations

A force field model for calculating local field factors, i.e. the linear response of the local electric field for example at a nucleus in a molecule with respect to an applied electric field, is discussed. It is based on a combined charge-transfer and point-dipole interaction model for the polarizability, and thereby it includes two physically distinct terms for describing electronic polarization: changes in atomic charges arising from transfer of charge between the atoms and atomic induced dipole moments. A time dependence is included both for the atomic charges and the atomic dipole moments and if they are assumed to oscillate with the same frequency as the applied electric field, a model for frequency-dependent properties are obtained. Furthermore, if a life-time of excited states are included, a model for the complex frequency-dependent polariability is obtained including also information about excited states and the absorption spectrum. We thus present a model for the frequency-dependent local field ...

Atomistic simulations of the local electric field in dielectric liquids

The linear response of the local electric field to an external electric field is calculated by a force-field model for frequencies through the first molecular excitation energy. Both static and frequency-dependent external fields are applied and results for the local field are presented for liquid benzene as a model system by combining molecular dynamics simulations and the local field model. It is found that the largest local field response is around 8 at the absorption frequency but it depends significantly on the molecular configuration of the liquid.

Field-dependent ionization potential for polyaromatic molecules from constrained density-functional theory

The field-dependent ionization potential is determined by calculating the dissociation energy barrier for the interaction between a cation and an electron in an electric field. A quantum-chemical method based on constrained density-functional theory (CDFT) has been established for this purpose. Here we present the field-dependent ionization potential and excitation energies for polyaromatic molecules relevant for electrically insulating liquids. In the CDFT model, we rely on that the dissociation barrier is located somewhere outside the cation. This causes problems for all molecules at sufficiently high electric fields, but for polyaromatic molecules the problems appear at lower fields as compared to previously studied molecules. This limitation has been investigated in detail and some initial results are presented for a set of polyaromatic molecules including benzene and pyrene. The importance of ionization potential and excitation energies in streamer initiation and propagation are discussed.