Negative Electron Affinities Research Articles

First-principles investigation of oxidized Si- and Ge-terminated diamond (100) surfaces.

Diamond is a semiconductor material with remarkable structural, thermal, and electronic properties that has garnered significant interest in the field of electronics. Although hydrogen (H) and oxygen (O) terminations are conventionally favored in transistor designs, alternative options, such as silicon (Si) and germanium (Ge), are being explored because of their resilience to harsh processing conditions during fabrication. Density-functional theory was used to examine the non-oxidized and oxidized group-IV (Si and Ge)-terminated diamond (100) surfaces. The (3 × 1) reconstructed surfaces feature an ether configuration and show relative stability compared with the bare surface. Hybrid-functional calculations of the electronic properties revealed reduced fundamental bandgaps (<1eV) and lower negative electron affinities (NEAs) than those of H-terminated diamond surfaces, which is attributed to the introduction of unoccupied Si (Ge) states and the depletion of negative charges. Furthermore, oxidation of these surfaces enhanced the stability of the diamond surfaces but resulted in two structural configurations: ether and ketone. Oxidized ether configurations displayed insulating properties with energy gaps of ∼4.3 ± 0.3eV, similar to H-terminated diamond (100) surfaces, whereas bridged ether configurations exhibited metallic properties. Oxidization of the metallic ketone configurations leads to the opening of relatively smaller gaps in the range of 1.1-1.7eV. Overall, oxidation induced a shift from NEAs to positive electron affinities, except for the reverse-ordered ketone surface with an NEA of -0.94eV, a value comparable to the H-terminated diamond (100) surfaces. In conclusion, oxidized group-IV-terminated diamond surfaces offer enhanced stability compared to H-terminated surfaces and display unique structural and electronic properties that are influenced by surface bonding.

Energetic, electronic structure and work function of the (0001), (10 [formula omitted] 0), and (11[formula omitted] 0) BaAl2O4 surfaces; A first-principles study

In this study, the GGA-PBE method was applied to explore the electronic structure and properties of the BaAl2O4 surfaces. We found that the most stable surfaces are non-polar surfaces containing Ba and O atoms in their outermost layers. From an energetic point of view, the surface energy of these surfaces is arranged as follows; γ(101‾0)<γ(0001)<γ(112‾0). The electronic structure of each surface is discussed in detail through PDOS curves and particular interest has been devoted to the origin of surface states. On the other hand, our results showed that the work function value of BaAl2O4 strongly depends on the nature of its surface termination. These results revealed that the BaO-terminated (101‾0) and (0001) surfaces have work function values of 0.95 eV and 0.8 eV and negative electron affinities (NEA) of −0.3eV and −0.56eV, respectively. Based on these results, we suggest that BaAl2O4 may be a good candidate for use in thermionic converters and electron-emission devices.

Перенос электронов через границу полупроводник-вакуум с отрицательным и положительным электронным сродством: влияние скачка массы

The effect of jump in electron mass at the crystal-vacuum interface on photoemission from semiconductors is considered. In the effective mass approximation, the angular and energy dependences of the electron transmission coefficient through interfaces with a jump in mass and potential steps of different signs, corresponding to negative and positive electron affinities, are considered. It is shown that due to the jump in mass, there are a critical energy and a critical angle of incidence of electrons, which separate qualitatively different angular and energy dependences of the transmission coefficient, respectively. The jump in mass makes it possible for electrons to transfer (up to complete transmission) through a positive potential step with a normal component of kinetic energy below the height of the step. The calculated dependences of the emission quantum yield of thermalized electrons on the affinity value are compared with the experimental data on photoemission from p-GaAs(Cs,O). Possible reasons for the significant differences between the experiment and the calculation are analyzed: a complex potential profile, including the near-surface band bending in the semiconductor and the image charge potential in vacuum, scattering in the (Cs,O) layer, and the need to go beyond the effective mass approximation and to take into account full Bloch nature of electron wave functions in semiconductors.

Electro-optical, electronic and conformational transitions of interstellar alkyl molecules at different temperatures: An ab-initio study

In this paper, electronic structures of iso-propyl cyanide (i-C3H7CN), gauche-normal-propyl cyanide (g-n-C3H7CN) and anti-normal-propyl cyanide (a-n-C3H7CN) have been optimized using DFT/B3LYP/aug-cc-pVDZ and DFT/CAM-B3LYP/aug-cc-pVDZ methods in neutral and anionic states. Electro-optical properties, HOMO, LUMO and MEP surfaces have been computed in order to check the optical behaviour and chemical reactivity of these interstellar molecules. The a-n-C3H7CN molecule exhibits better electro-optical behaviour than g-n-C3H7CN and iso-C3H7CN molecules. Global reactivity descriptors, adiabatic and vertical electron affinities of these straight and branched alkyl molecules have been investigated. These molecules have negative electron affinities, which means that they emit energy in anionic form. The distribution of chemical bonding and electron lone pairs inside molecules has been discussed using ELF map. Infrared and absorption spectra of these interstellar molecules have been studied in the light of experimental data. Further, formations of g-n-C3H7CN from i-C3H7CN, a-n-C3H7CN from i-C3H7CN and a-n-C3H7CN from g-n-C3H7CN have been examined using transition state at 10 ​K, 298.15 ​K and 500 ​K temperatures. Mechanism of formation of g-n-C3H7CN and a-n-C3H7CN from i-C3H7CN at 10 ​K, 298.15 ​K and 500 ​K temperatures along with their physico-chemical properties have been elucidated.

Photoemission and photon-enhanced thermionic emission: Effect of jump in electron mass

The effect of jump in electron mass on the electron transfer through interfaces with positive and negative potential steps is considered in relation with practically important photoemission and photon-enhanced thermionic emission from semiconductors with positive and negative effective electron affinities. For a zero potential step, a direct similarity is established between the expressions for electron reflection and transmission and Fresnel equations for p-polarized light, with the ratio of masses replacing the ratio of dielectric constants. For non-zero potential steps of both signs, due to the jump in mass, there are critical electron energies that separate the energy regions with qualitatively different angular dependences of the transmission coefficient. The influence of the jump in mass on the spectra of photoemission quantum yield, which are measured experimentally under transition from negative to positive electron affinity at the p-GaAs(Cs,O)-vacuum interface, is considered. It is shown that, for a decisive proof of the jump-in-mass contribution to the electron transfer through semiconductor-vacuum interface, measuring the energy and angular distributions at the state of positive electron affinity is more advantageous, as compared to the state of negative electron affinity.

Density Functional Prediction of Quasiparticle, Excitation, and Resonance Energies of Molecules With a Global Scaling Correction Approach.

Molecular quasiparticle and excitation energies determine essentially the spectral characteristics measured in various spectroscopic experiments. Accurate prediction of these energies has been rather challenging for ground-state density functional methods, because the commonly adopted density function approximations suffer from delocalization error. In this work, by presuming a quantitative correspondence between the quasiparticle energies and the generalized Kohn–Sham orbital energies, and employing a previously developed global scaling correction approach, we achieve substantially improved prediction of molecular quasiparticle and excitation energies. In addition, we also extend our previous study on temporary anions in resonant states, which are associated with negative molecular electron affinities. The proposed approach does not require any explicit self-consistent field calculation on the excited-state species, and is thus highly efficient and convenient for practical purposes.

Approximate equation-of-motion coupled-cluster methods for electron affinities of closed-shell molecules.

We investigate performance of the equation-of-motion coupled-cluster method at the single and doubles level (EOM-CCSD) and a series of approximate methods based on EOM-CCSD on electron affinities (EA) of closed-shell cations and neutral molecules with positive and negative EAs in this work. Our results confirm that P-EOM-MBPT2 can provide reasonable EAs when molecules with significant multireference character are not considered and its mean absolute error on EAs of these molecules is around or less than 0.2 eV. Its accuracy is comparable to that of the more expensive EOM-CCSD(2) method. Results of EOM-CCSD(2), P-EOM-MBPT2, and CIS(D∞) indicate that the [[H, ac +], T2] term in the 1h2p-1h block is more important on EAs than the term neglected in the 1h2p-1h2p block in P-EOM-MBPT2. We proposed an economical method where EAs from CIS(D∞) are corrected by treating this [[H, ac +], T2] term in the 1h2p-1h block perturbatively [corr-CIS(D∞)]. EAs with corr-CIS(D∞) agree very well with those of P-EOM-MBPT2 with a difference of less than 0.02 eV. Computational scaling of this method is N4 for the iterative part and N5 for some non-iterative steps. Its storage requirement is only of OV3. Corr-CIS(D∞) is an economical and reliable method on EAs, and it can be applied to EAs of large molecules.

Negative Electron Affinities and Derivative Discontinuity Contribution from a Generalized Gradient Approximation Exchange Functional.

Two methods to calculate negative electron affinities systematically from ground-state density functional methods are presented. One makes use of the lowest unoccupied molecular orbital energy shift provided by approximate inclusion of derivative discontinuity in the nearly correct asymptotic potential (NCAP) nonempirical, constraint-based generalized gradient approximation exchange functional. The other uses a second-order perturbation calculation of the derivative discontinuity based on the NCAP exchange-correlation potential. On a set of thirty-eight molecules, NCAP leads to a rather accurate description that is improved further through the perturbation correction. The results presented show the importance of the asymptotic behavior of the exchange-correlation potential in the calculation of negative electron affinities as well as demonstrating the versatility of the NCAP functional.

Electron-affinity and surface-stability of aluminium-oxide terminated diamond surfaces

Control over the chemical termination of diamond surfaces has shown great promise in the realization of field-emission applications, the selection of charge states of near-surface color-centers such as NV, and the realization of surface-conductive channels for electronic device applications. Efficient electron-emission exploiting the negative electron-affinity requires a stable surface treatment that can operate over a useful range of temperatures, for which H-termination fails. For compounds involving oxygen, surface geometry and layer stoichiometry are thought to be critical parameters in engineering the desired affinity. In this study, we show that large negative electron-affinities (−2 to −3 eV) are found for surfaces with an AlO3 stoichiometry, and the formation is exothermic. Higher proportions of Al increases the likelihood of metallic bonding between Al atoms, decreasing the bond-polarity and increasing the electron affinity (making it less negative). AlO2 and Al2O3 present unfavorable electron affinities, and have a lower thermal stability.

Simple DFT Scheme for Estimating Negative Electron Affinities.

A simple density functional theory (DFT) scheme is proposed for estimating negative vertical electron affinities of neutral systems, based on a consideration of the integer discontinuity and density scaling homogeneity. The key feature is the derivation of two system-dependent exchange-correlation functionals, one appropriate for the electron deficient side of the integer and one appropriate for the electron abundant side. The electron affinity is evaluated as a linear combination of frontier orbital energies from self-consistent Kohn-Sham calculations on the neutral system using these functionals. For two assessments comprising a total of 43 molecules, the scheme provides electron affinities that are in good agreement with experimental values and which are an improvement over those from the DFT method of Tozer and De Proft [ J. Phys. Chem. A 2005 , 109 , 8923 ]. The scheme is trivial to implement in any Kohn-Sham program, and the computational cost is that of a series of generalized gradient approximation Kohn-Sham calculations. More generally, the study provides a prescription for performing low-cost, self-consistent Kohn-Sham calculations that yield frontier orbital energies that approximately satisfy the appropriate Koopmans conditions, without the need for exact exchange.

Unique Solvating Effect in Azabenzene Clathrate Hydrates

The water solvent is a well-known solvating medium which can stabilize unstable species in bulky water or molecular clusters, but little is known about the solvating performance of the clathrate hydrate cages and changes of molecular properties of the solutes. In this work, on the basis of the experimentally observed clathrate hydrate structures, we theoretically explore their structures and properties, taking azabenzenes (pyridazine, pyrimidine, pyrazine, and pyridine) and benzene as the guests which have negative electron affinities, focusing on stability, electron affinity, vibrational shifts, proton transfer, and especially unique solvating effect of the clathrate hydrate cavity, using the density functional theory calculations. Calculations indicate that inclusion of the guest azabenzene/benzene in the clathrate hydrate cages could considerably change the structures and properties including stability increase, the C–H stretching blue shifts of the guests, and electron affinity conversion from the neg...

Berichtigung: Barium as Honorary Transition Metal in Action: Experimental and Theoretical Study of Ba(CO)+ and Ba(CO)−

In this Communication, for the calculations of the anion [Ba(CO)]− neutral Ba and the carbonyl anion CO− were considered as interacting fragments. The choice was made on the basis of the experimental literature values for the electron affinities (EAs) of Ba (0.108 eV, average value of 2P1/2 and 2P3/2)1 and CO (1.37 eV).2 The authors now learned that the experimental value for the EA of CO is incorrect. High-level quantum chemical calculations suggest that CO has a negative EA,3 i.e. the attached electron in CO− is unbound and the anion is a resonance species that may only be observed as metastable resonating species via sophisticated spectroscopy.6 This means that the bonding analysis of the anion [Ba(CO)]− and the calculation of the bond dissociation energy (BDE) must be carried out using the fragments Ba− and neutral CO. The results of the relevant calculations are reported here. The calculated BDE for the reaction [Ba(CO)]− → Ba− + CO at DLPNO-CCSD(T)/def2-QZVPPD using the CCSD/def2-TZVPP optimized geometries is De = 15.6 kcal mol−1, significantly lower than the previous value of 56.1 kcal mol−1 that was obtained with respect to Ba and CO− at the same level of theory. The numerical results of the EDA-NOCV analysis of [Ba(CO)]− using Ba− in the 2D state and CO as fragments are shown in Table 1, together with the previous data using Ba (1S) and CO−(2Π) as fragments. The intrinsic interaction energy with the former fragments ΔEint = −25.6 kcal mol−1 is also significantly lower than the previous value ΔEint = −71.4 kcal mol−1. BaCO− Fragments Ba− (2D) + CO (1Σ+) Ba (1S) + CO− (2Π) ΔEint −25.6 −71.4 ΔEhybrid[a] 4.2 5.8 ΔEPauli 51.0 52.5 ΔEelstat[a] −36.5 (45.2 %) −66.1 (51.0 %) ΔEorb[a] −44.3 (54.8 %) −63.5 (49.0 %) ΔEorb(1)[b] −27.9 (63.0 %) −41.1 (64.7 %) ΔEorb(2)[b] −11.2 (25.3 %) −16.4 (25.8 %) ΔEorb(rest)[b] −5.2 (11.7 %) −6.0 (9.5 %) ΔEprep 1.9 23.2 BDE 23.7 48.2 The partitioning of the orbital term into the most important pairwise interactions gives in both cases two dominant interactions. The largest contribution ΔEorb(1) comes from the donation of the unpaired π electron at Ba− to the LUMO of CO (Figure 1 a). This was also found in the original work as the strongest orbital interaction in the cation Ba(CO)+, where the electronic reference state of Ba+ cation is the excited 2D state that makes it to become a donor Ba+→CO. The deformation density, which is associated with the second strongest orbital interaction shows that ΔEorb(2) comprises two alterations of the electronic charges (Figure 1 b). There is a polarization at barium, where the occupied 6 s AO of Barium mixes with vacant p(σ) and d(σ) orbitals and a concomitant donation from the HOMO of CO into the σ hybrid AO at Ba. Shape of the most important interacting MOs of fragments and plot of the associated deformation densities Δρ of the most important pairwise orbital interactions ΔEorb(1) and ΔEorb(2) BaCO−. The direction of the charge flow is red→blue. Thus, the most important finding in the original work remains the same: The chemical bonds of barium in the carbonyl complexes [Ba(CO)]+ and [Ba(CO)]− use mainly the 5 d AOs as valence orbitals. The use of the (n−1) d functions as genuine valence orbitals not only by barium but also by the lighter alkaline earth atoms calcium and strontium has in the meantime been supported by the experimental observation of the octacarbonyls E(CO)8 (E = Ca, Sr, Ba), which mimic transition metal complexes.7 The authors are grateful to Prof. Peter Schwerdtfeger, who pointed out the error of choosing the wrong interacting fragments for [Ba(CO)]−.

Thermodynamics of radical scavenging of symmetric carotenoids and their charged species

For nine symmetric natural carotenoids, a comprehensive thermodynamics study of processes associated with their radical scavenging activity is proposed. We have investigated the hydrogen atom transfer (HAT) from the parent carotenoid, mono-radical species, radical cations and radical anions. Electron transfer and proton transfer reactions were also studied. Terminal units and carbon atoms in their vicinity were identified as thermodynamically favoured reaction sites of the HAT mechanism. Rhodoxanthin, canthaxanthin and astaxanthin, as strong antioxidants, without any pro-oxidative effect, were found to have bond dissociation enthalpies (BDE) higher than 300 kJ mol−1 and the most negative electron affinities. The electron transfer to a carotenoid is exothermic, while other studied reactions are endothermic. In solvent, the electron transfer reactions may be preferred instead of hydrogen atom transfer.

In search of the best DFT functional for dealing with organic anionic species.

Quantum chemical computational methods are thought to have problems in dealing with unstable organic anions. This work assesses the ability of different Density Functional Theory (DFT) functionals to reproduce the electron affinity and reduction potential of organic compounds. The performance of 23 DFT functionals was evaluated by computing the negative electron affinities (from 0 eV to -3.0 eV) and reduction potentials in acetonitrile (from 0 to -2.7 V). In general, most of the hybrid GGA functionals work fine in the prediction of electron affinities, BPW91, B3PW91 and M06 being the best in each class of functionals (pure, hybrid and meta-GGA functionals, respectively). On the other hand, the ab initio post-Hartree-Fock methods, MP2 and coupled-cluster (CCSD(T)), as well as the double hybrid functionals, B2PLYP and mPW2PLYP, usually fail. For compounds with EAs lower than -1.75 eV, a method for stabilizing the anion, based on solvation with the IEFPCM model, was employed. In this case, BPW91, PBE0 and M06-HF could be the recommended option for the pure, hybrid and meta-GGA functionals, respectively. The situation improves for the evaluation and prediction of redox potentials. In this case the performance of the DFT functionals is better, in part because the solvent assists in the stabilization of the anions. Nevertheless, there is a systematic bias in the calculation of absolute redox potentials, which could be corrected by using a redox partner that helps by the cancellation of errors. In this case, the hybrid and meta-GGA functionals B3PW91, PBE0, TPSSh and M06 are also among the best for computing redox potentials with a mean absolute deviation (MAD) lower than 0.13 V.

Mass spectrometric determination of Morse parameters for the fifty-four superoxide states dissociating to the lowest limit.

Superoxide is the most significant homonuclear diatomic anion in biochemistry. Theory predicts 12 doublet (X, A-K) and 12 quartet (a-l) electronic states split by spin orbital coupling into 54 states dissociating to the (3) P(O) + (2) P(O(-) ) limit. Dissociation energies for the 27 bonding states with positive electron affinities have been determined from mass spectrometric data. However, the 27 antibonding states with negative electron affinities have not been experimentally characterized. The electron affinity of the hydrogen atom per electron, the Hylleraas, is the fundamental measure of electron correlation. It has been used to assign and evaluate experimental electron affinities of atoms and diatomic molecules. The 27 negative electron affinities of oxygen are estimated from the 27 positive values and the Hylleraas. These values are used to determine frequencies and internuclear separations by fitting theoretical electron impact distributions to the gas-phase mass spectrometric atomic oxygen anion distribution peaking at about 6.5 eV. The dissociation energies, internuclear distances and frequencies giving the first complete set of Morse potential energy curves for the 54 superoxide states dissociating to the lowest limit are reported from mass spectrometric data. The potentials are compared to theoretical and empirical literature curves. The existence of the 27 bonding and 27 antibonding spin orbital coupling superoxide states dissociating to (3) P(O) + (2) P(O(-) ) is established from mass analyzed thermal, photon, and electron ionization data. There are electron affinities from 0 to 0.15 eV, and onsets and peaks for dissociative electron attachment that cannot be explained by the 54 states. These support the existence of the 36 superoxide spin states dissociating to [(1) D(O) + (2) P(O(-) )] and [(1) S(O) + (2) P(O(-) )] predicted by quantum mechanics. Copyright © 2016 John Wiley & Sons, Ltd.

Unconventional molecule-resolved current rectification in diamondoid-fullerene hybrids.

The unimolecular rectifier is a fundamental building block of molecular electronics. Rectification in single molecules can arise from electron transfer between molecular orbitals displaying asymmetric spatial charge distributions, akin to p–n junction diodes in semiconductors. Here we report a novel all-hydrocarbon molecular rectifier consisting of a diamantane–C60 conjugate. By linking both sp3 (diamondoid) and sp2 (fullerene) carbon allotropes, this hybrid molecule opposingly pairs negative and positive electron affinities. The single-molecule conductances of self-assembled domains on Au(111), probed by low-temperature scanning tunnelling microscopy and spectroscopy, reveal a large rectifying response of the molecular constructs. This specific electronic behaviour is postulated to originate from the electrostatic repulsion of diamantane–C60 molecules due to positively charged terminal hydrogen atoms on the diamondoid interacting with the top electrode (scanning tip) at various bias voltages. Density functional theory computations scrutinize the electronic and vibrational spectroscopic fingerprints of this unique molecular structure and corroborate the unconventional rectification mechanism.

Negative electron affinities from conventional electronic structure methods

If the potential V describing the interaction between an excess electron and a ground-state neutral or anionic parent is sufficiently attractive at short range, electron-attached states having positive electron affinities (EAs) can arise. Even if the potential is not attractive enough to produce a bound state, metastable electron-attached states may still occur and have lifetimes long enough to give rise to experimentally detectable signatures. Low-energy metastable states arise when the attractive components of V combine with a longer-range repulsive contribution to produce a barrier behind which the excess electron can be temporarily trapped. These repulsive contributions arise from either the centrifugal potential in the excess electron’s angular kinetic energy or long-range Coulomb repulsion in the case of an anionic parent. When there is no barrier, this kind of low-energy metastable state does not arise, but improper theoretical calculations can lead to erroneous predictions of their existence. Conventional electronic structure methods with, at most, minor modifications are described for properly characterizing metastable states and for avoiding incorrectly predicting the existence of metastable states with negative EAs where no barrier is present.

The effect of oxidative and paper degradation impurities on partial discharge characteristics of hexadecane

Partial discharge (PD) characteristics of hexadecane were studied in a needle-plane electrode geometry under AC field with a tungsten needle of 20 μm tip radius. PD experiments were conducted on samples containing known concentrations of oil/paper degradation compounds. Partial discharge inception voltage (PDIV), streamer repetition rates, and phase resolved PD (PRPD) patterns were acquired. Ionization potentials (IP) and electron affinities (EA) of hexadecane and a selection of additives were calculated with density functional theory (DFT) and correlated to PD characteristics. IP did not have a significant effect on the number of streamers initiated as most additives had higher IP relative to hexadecane. The presence of electronegative oxygen changes substantially the PD characteristics and, for most additives, increases the number of positive and negative streamers initiated. The greatest changes in PD characteristics, a reduced number of negative streamers, was observed for compounds with large electron capture cross section, even those with negative EA.

A comparison of the chemical reactivity of naringenin calculated with the M06 family of density functionals

BackgroundChemicals generically referred to as flavonoids belong to the group of phenolic compounds and constitute an important group of secondary metabolites due to their applications as well as their biochemical properties. Flavonoids, which share a common benzo- γ-pyrone structure, constitute a kind of compound which are highly ubiquitous in the plant kingdom.FindingsThe M06 family of density functionals has been assessed for the calculation of the molecular structure and properties of the Naringenin flavonoid. The chemical reactivity descriptors have been calculated through Conceptual DFT. The active sites for nucleophilic and electrophilic attacks have been chosen by relating them to the Fukui function indices and the dual descriptor f(2)(r). A comparison between the descriptors calculated through vertical energy values and those arising from the Koopmans’ theorem approximation have been performed in order to check for the validity of the last procedure.ConclusionsThe M06 family of density functionals (M06, M06L, M06-2X and M06-HF) used in the present work leads to the same qualitatively and quantitatively similar description of the chemistry and reactivity of the Naringenin molecule, yielding reasonable results. However, for the case of the M06-2X and M06-HF density functionals, which include a large portion of HF exchange, the calculations considering the validity of the Koopmans’ theorem lead to negative electron affinities.

The effect of aromatic and sulfur compounds on partial discharge characteristics of hexadecane

Partial discharge (PD) characteristics of hexadecane were studied in a needle-plane electrode geometry under AC field with a 20 μm tip radius tungsten needle. PD experiments were conducted on samples containing known concentrations of aromatic and corrosive sulfur compounds. Partial discharge inception voltage (PDIV), streamer repetition rates, and phase resolved PD (PRPD) patterns were acquired. Ionization potentials (IP) and electron affinities (EA) of hexadecane and the additives were calculated with density functional theory (DFT) and correlated with the PD characteristics. Low IP and negative EA of the additives relative to hexadecane increased the number of positive and negative streamers initiated, except for additives that contained an electron donating group on the aromatic ring. Low IP and positive EA for polyaromatic hydrocarbons (PAH) and corrosive sulfur compounds decreased the number of negative streamers initiated due to large electron capture cross sections. Copper sulfide particulates caused the greatest changes in PD characteristics as a result of its semi-conductive nature.