Probe Charges Research Articles

Interaction of a ballistic probe with gaseous media

Free-flying metal probes are used to determine charge densities in gaseous media containing free charge or low density plasma. The trajectory of the probe is ensured either by gravity or by propelling the probe to a certain velocity at the launch site. While travelling, the probe charge changes from its launch-site magnitude to that related to the space charge density existing along the trajectory. The degree to which the probe's arrival-site charge magnitude matches the space charge density in the area of interest depends on the probe shape and on the charge exchange processes between the probe body and the medium. The paper studies a probe acting as a free-flying charge carrier in air, and discusses the problems that may lead to an imbalance between the charge collected by the probe in the area of interest and the charge measured at the arrival site. The analysis and the described experiments are of the ballistic type: a small, triboelectrically pre-charged metal probe was propelled on a horizontal path, and the charge carried by the probe was measured at several points along the trajectory by means of contact-free induction rings; the initial and final charges were determined by static Faraday cups. A charge disparity was found under certain conditions, and its degree explained by the effects of the charge carrier potential. The studied probe charges ranged from 10 to 50 nF, and the fly-times needed to cross a one-meter path ranged from 20 to 40 ms. The probe to gas charge exchange experiments and their analysis yielded conditions under which the probe lost approximately 10 % of its charge. The results of our study may be of interest to those who intend to use the free-flying probe technique for the determination of space charge density.

Remarks on confinement driven by axion-like particles in Yang–Mills theories

Features of screening and confinement are studied for a non-Abelian gauge theory with a mixture of pseudoscalar and scalar couplings, in the case where a constant chromoelectric, or chromomagnetic, strength expectation value is present. Our discussion is carried out using the gauge-invariant but path-dependent variables formalism. We explicitly show that the static potential profile is the sum of a Yukawa and a linear potential, leading to the confinement of static probe charges. Interestingly, similar results have been obtained in the context of gluodynamics in curved spacetime. For only pseudoscalar coupling, the results are radically different.

Confinement from constant field condensates

For ( 2 + 1 ) - and ( 3 + 1 ) -dimensional reformulated SU ( 2 ) Yang–Mills theory, we compute the interaction potential within the framework of the gauge-invariant but path-dependent variables formalism. This reformulation is due to the presence of a constant gauge field condensate. Our results show that the interaction energy contains a linear term leading to the confinement of static probe charges. This result is equivalent to that of the massive Schwinger model.

Duality and confinement in [formula omitted] models driven by condensation of topological defects

We study the interplay of duality and confinement in Maxwell-like three-dimensional models induced by the condensation of topological defects driven by quantum fluctuations. To this end we check for the confinement phenomenon, in both sides of the duality, using the static quantum potential as a testing ground. Our calculations are done within the framework of the gauge-invariant but path-dependent variables formalism which are alternative to the Wilson loop approach. Our results show that the interaction energy contains a linear term leading to the confinement of static probe charges.

Condensation of topological defects and confinement

We study the static quantum potential for a theory of antisymmetric tensor fields that results from the condensation of topological defects, within the framework of the gauge-invariant but path-dependent variables formalism. Our calculations show that the interaction energy is the sum of a Yukawa and a linear potentials, leading to the confinement of static probe charges.

A comparative study of quantitative structure–activity relationship methods based on gallic acid derivatives

By using hologram quantitative structure–activity relationship (HQSAR) and comparative molecular field analysis (CoMFA) methods, the relationships between the structures of 49 gallic acid derivatives and their analgesic activity have been investigated to yield statistically reliable models with considerable predictive power. The best HQSAR model was generated using atoms, bond and connectivity as fragment distinction parameters and fragment size 5–7 from a hologram length of 307 with 3 components. High conventional r 2 and cross-validation r 2 values were obtained. CoMFA analyses varying lattice size and location, grid spacing, probe charges and using, Tripos standard and Indicator force field were performed. The best model was developed with 4 components using sp3-hybridized carbon atom with +1.0 charge as probe, grid spacing (2 Å), lattice offset (1.0, 3.0, -2.5). The CoMFA model showed a conventional correlation coefficient r 2 of 0.889 and a cross-validation equals to 0.633. The robustness and predictive ability of the HQSAR and CoMFA models have been validated by means of an external test set. The results indicate that both models possess high statistical quality in the prediction of analgesic potency of novel gallic acid analogs.

Electros: Development of an educational software for simulations in electrostatic

AbstractThis is an introduction to the main design features of a computer‐aided educational package addressed to students of the first years of sciences and engineering. The package includes interrelated tutorials, computer simulation, and self‐evaluators, in which graphical output, hypertext, and animations are widely used. The package is devoted to the study of electrostatic fields and potentials produced by charged dielectrics arbitrarily located by the user in three‐dimensional space. It provides instant numerical evaluation and a graphical display of the electric field and potentials of the charge distribution created by the user, and permits its investigation with probe charges. The software presented in this paper has all the features we consider important for any educational software of physical phenomena simulation: an integrative character, self‐evaluation tests, personalized and active learning process, adaptability to teacher's aims, versatility as a teaching tool, multimedia resources, and simplicity. All these features will be explained in the following sections. © 2004 Wiley Periodicals, Inc. Comput Appl Eng Educ 12: 65–73, 2004; Published online in Wiley InterScience (www.interscience.wiley.com); DOI 10.1002/cae.10050

Analysis of the NMR Spin−Spin Coupling Mechanism Across a H−Bond: Nature of the H-Bond in Proteins

The NMR spin−spin coupling constants (SSCCs) across the H-bond in proteins are sensitive to the electronic structure of the H-bonded system, i.e., the N−H···OC group in proteins. The spin−spin coupling mechanism across the H-bond involves a strong electric field effect, steric exchange interactions, and some weak covalent effects (transfer of electronic charge). The electric field effect is reflected by one-orbital contributions to the SSCC and can be tested with the help of probe charges. A negative charge opposite to the N−H bond leads to increased polarization of the N−H bond, a larger contact density at the N nucleus, and a stronger FC coupling mechanism for those SSCCs involving the N nucleus. Similarly, a positive charge opposite to the OC bond, distorts the O density into the direction of the external charge and in this way decreases the spin density at the O nucleus. All SSCCs across the H-bond depend primarily on the electric field effect and two-orbital steric exchange interactions. The lone pai...

Validating a polarizable model for the glass-forming liquid Ca0.4K0.6(NO3)1.4 by ab initio calculations

Ab initio calculations have been performed in order to investigate a recently proposed polarizable model [M. C. C. Ribeiro, Phys. Rev. B 61, 3297 (2000)] for molecular dynamics (MD) simulation of the molten salt Ca0.4K0.6(NO3)1.4. On the basis of the electronegativity equalization method, polarization effects in the MD simulations have been introduced by a fluctuating charge (FC) model for the nitrate ion. Partial charges in the nitrate ion are obtained by ab initio calculations at several levels of theory, and compared with previously proposed models for MD simulations of nitrate melts. Charge fluctuation is achieved in the ab initio calculations by using positive probe charges placed around a nitrate ion. The parameters of the FC model are corroborated by comparison of the ab initio partial charges with the ones obtained directly by the electronegativity equalization method. Simulated annealing of a cluster including two double-charged cations and two nitrate ions shows that very different structures are obtained depending on whether the FC model or its nonpolarizable counterpart is considered. Ab initio calculations show that the structure of this cluster is strongly dependent on polarization effects in the nitrate ions.

Screening of fractional charges in (2+1)-dimensional QED

We show that the logarithmically rising static potential between opposite-charged sources in two dimensions is screened by dynamical fields even if the probe charges are fractional, in units of the charge of the dynamical fields. The effect is due to quantum mechanics: the wave functions of the screening charges are superpositions of two bumps localized both near the opposite- and the same-charge sources, so that each of them gets exactly screened.

Antitumor agents. 163. Three-dimensional quantitative structure-activity relationship study of 4'-O-demethylepipodophyllotoxin analogs using the modified CoMFA/q2-GRS approach.

Analogs of 4'O-demethylepipodophyllotoxin are considered as potential anticancer agents. We have applied comparative molecular field analysis (CoMFA) and a novel CoMFA/q2-GRS technique recently developed in our group to identify the essential structural requirements for increasing the ability of these compounds to form cellular protein-DNA complex. In addition, a new method to incorporate different types of probe atoms as part of q2-GRS routine has been developed. The best final model with 101 compounds using a combination of four different sets of probe atoms and charges [C (sp3, +1), C (sp3, 0), H (+1), and O (sp3, -1)] yielded a q 2 of 0.584 and the standard error of prediction of 0.660 at 5 principal components. The steric and electrostatic contour plots of the final model were compared with the DNA phosphate backbone environment of the DNA-4'O-demethylepipodophylltoxin analog complex, which was generated using the X-ray structure of the DNA-nogalamycin complex. The comparison reveals that the CoMFA steric and electrostatic fields are compatible with stereochemical properties of the DNA backbone. The results obtained from this study shall guide our future synthetic efforts.

Direct prediction of dissociation constants (pKa's) of clonidine-like imidazolines, 2-substituted imidazoles and 1-methyl-2-substituted-imidazoles from 3D structures using a comparative molecular field analysis (CoMFA) approach

The applicability of a comparative molecular field analysis (CoMFA) method to reproduce and predict the pKa values of 28 clonidine-like imidazoline analogues and 16 2-substituted imidazoles has been investigated with the GRID force field. Molecular fields calculated with an H+ probe and AM1 partial atomic charges produced a correlation with a small standard deviation and a high correlation coefficient with cross validation. It was concluded that the CoMFA treatment of electrostatic effects is suitable for predicting pKa values and thus for the examination of the electronic effects in 3D quantitative structure-activity relationships.