Vibrational Kinetics Research Articles

Encoding of Whisker Vibration by Rat Barrel Cortex Neurons: Implications for Texture Discrimination

Rats, using their whiskers, have excellent capabilities in texture discrimination. What is the representation of texture in rat somatosensory cortex? We hypothesize that as rats "whisk" over a surface, the spatial frequency of a grooved or pebbled texture is converted to a temporal frequency of whisker vibration. Surface features such as groove depth or grain size modulate the amplitude of this vibration. Validation of the hypothesis depends on showing that vibration parameters have distinct neuronal representations in cortex. To test this, we delivered sinusoidal vibrations to the whisker shaft and analyzed cortical neuronal activity. Seven amplitudes and seven frequencies were combined to construct 49 stimuli while recording activity through a 10 x 10 microelectrode array inserted into the middle layers of barrel cortex. We find that cortical neurons do not explicitly encode vibration frequency (f) or amplitude (A) by any coding measure (average spike counts over different time windows, spike timing patterns in the peristimulus time histograms or in autocorrelograms). Instead, neurons explicitly encode the product of frequency and amplitude, which is proportional to the mean speed of the vibration. The quantity Af is an invariant because neuronal response encodes this feature independently of the values of the individual terms A and f. This was true across a wide time scale of firing rate measurements, from 5 to 500 msec. We conclude that vibration kinetics are rapidly and reliably encoded in the firing rate of cortical ensembles. Therefore, the cortical representation of vibration speed could underlie texture discrimination.

On the Use of Global Kinetics Models for the Investigation of Energy Deposition and Chemistry in RF Argon–Oxygen Plasmas Working in the Torr Regime

A global plasma model is used to investigate the chemistry and energy deposition in 13.56 MHz radio-frequency capacitively coupled oxygen–argon discharges under conditions usually used for the deposition of tin oxide films. These models are based on the solution of a stationary electron Boltzmann equation coupled to species balance equations including the vibrational kinetics equations of O2. The results obtained showed that vibrational non equilibrium of O2-molecule is not significant. The dissociation degree of O2 was found to be around a few percents and the discharge was often moderately electronegative even for small O2 contents in the feed gas. The ionization and energy dissipation mechanisms are mainly governed by the collisional processes involving O2 for an oxygen feed gas composition greater than 20%. Results also showed that the predicted densities of the charged species and the electronegative character of the discharge are strongly linked to the assumption made on the homogeneity of the power deposition. On the contrary, the predicted density of O-atom is not sensitive to this assumption.

The Mechanisms of Formation and Decay of Atoms in N2-H2 Plasmas

The formation rate and number densities of H(2S) and N(4S) atoms, rate constants of their heterogeneous decay, and some electrophysical characteristics of plasma were measured in the positive column of a dc discharge in an H2–N2 gas mixture at a pressure of 266 Pa and a discharge current of 50 mA. From these data, by the combined solution of the Boltzmann equation, vibrational kinetics equations for Н2(X1Σg) and N2(X1Σg), chemical kinetics equations, and plasma energy and heat balance equations, it was found that vibrationally excited nitrogen molecules must play a substantial role in the formation of both hydrogen and nitrogen atoms.

Numerical Analysis of Carbon Isotope Separation by Plasma Chemical Reactions in Carbon Monoxide Glow Discharge

The separation of carbon isotopes in CO glow discharge has been studied, in which the formation of stable products enriched in 13C is analyzed by the numerical simulation of kinetic model. Vibrational kinetics and vibrationally induced chemical reaction of CO molecules are considered in the kinetic model as well as electron impact reactions and isotope scrambling reactions of isotopically enriched products. The reaction yield and final isotope enrichment of the stable products are derived as a function of mean electron energy. When mean electron energy is 2.1 eV in the case of Maxwellian electron energy distribution and 3.3 eV in the case of Druyvesteyn one, the kinetic model can reproduce experimental isotope enrichment of precipitated carbon atoms in discharge reactor. The calculation suggests optimum mean electron energy of the plasma for isotope separation as 1.0 eV and, in this case, it is expected that 13C enrichment coefficient for precipitated carbon atoms is about 10 and its reaction yield is about 0.5%.

Direct simulation of non-equilibrium kinetics under shock conditions in nitrogen

We study the interplay of vibrational kinetics, dissociation, translational and rotational relaxation in a strong shock wave in nitrogen by Direct Simulation Monte Carlo simulation (DSMC). The input data for vibrational and chemical processes are all in the form of cross-sections, mostly determined by molecular physics methods. In particular, we use for the first time very recent Quasi-Classical Trajectory (QCT) results for cross-sections of multi-quantum VT energy exchange and dissociation in N+N 2 collisions. Non-equilibrium distributions are observed and discussed.

Ultrafast Intramolecular Electron Transfer Studied by Picosecond and Stationary Raman Spectroscopy

Abstract Ultrafast photoinduced intramolecular electron transfer (ET) of betaine-30 (B-30) is studied in different solvents by resonance Raman spectroscopy. We apply stationary and picosecond time-resolved Raman techniques combined with ab initio Hartree–Fock calculations. Picosecond Raman spectroscopy of the first excited singlet state allows to monitor changes of vibrational frequencies due to ET. From the intense Raman lines of the stationary resonance Raman spectra we predict relevant geometric changes of B-30 due to ET which are confirmed by calculations of geometrical changes between the ground and excited electronic states of B-30. The torsional motion between the phenolate and the pyridinium rings as well as nitrogen pyramidalization play an essential role in electron transfer. Vibrational modes accepting the bulk of excess energy after back-electron transfer are identified by time-resolved anti-Stokes resonance Raman spectroscopy. In particular, the highest-frequency mode of the resonance Raman spectrum exhibiting a large Franck–Condon factor is most effective in accepting energy. Mode specific excitation of B-30 after back-electron transfer results in nonequilibrium vibrational populations within the first few picoseconds and subsequent quasi-equilibrium populations of hot Raman active modes. The observed vibrational kinetics can be qualitatively understood by a solvent dependent interplay of direct vibrational excitation and intramolecular vibrational energy redistribution.

Vibrational non-equilibrium and `hot atoms' in the oxidation of carbon monoxide on catalytic surfaces

Non-equilibrium vibrational kinetics, as recently developed for adlayers, is applied to the catalytic oxidation of CO by either molecular or atomic oxygen. This model allows one to fit experimental distribution functions of the product CO 2(g) molecules leaving the surface with hyperthermal energies. An LH * mechanism involving non-equilibrium vibrational distribution functions (vdfs) of the relevant adspecies is presented showing the basic consequences of non-equilibrium at the catalytic surface. In this picture `hot atoms' are seen as mobile, vibrationally excited adspecies with their vibrational ladders populated according to non-equilibrium Treanor distribution functions.

ENHANCEMENT OF H− PRODUCTION IN HOLLOW CATHODE DISCHARGE BY USING HYDROGEN AND NEON MIXTURE

ABSTRACT The purpose of this study is to find a way for more effective H− production in hollow cathode glow discharge. It is achieved by using hollow cathode plasma of H2 and Ne mixture. Based on an appropriate model including the vibrational kinetics a quantitative calculation of H− concentration in pure H2 and (H2+90%Ne) hollow cathode discharges is performed. For (H2+90%Ne) mixture the calculated H−concentration of 3.2×108 cm−3 has one order of magnitude higher value than for the pure H2 hollow cathode discharge. This enhanced H− concentration in the mixture is due to higher electron concentration giving rise to more effective dissociative attachment of electron to H2.

Simulation of the Processes of Formation and Dissociation of Neutral Particles in Air Plasma: Kinetics of Neutral Components

The results are given of calculations of the concentration of basic neutral particles and of the energy balance in an air plasma in the positive column of a d.c. discharge at pressures of 30 to 300 Pa and currents of 20 to 110 mA. The calculations are based on a combined solution of the Boltzmann equation; equations of vibrational kinetics for the ground states of molecules of N2, O2, and NO; and of the equations of chemical kinetics for these molecules, their excited states, and dissociation products. The results are compared with the measured concentrations of O(3 P) atoms and NO molecules and with the effective vibrational temperatures of N2(X 1Σ+ g ). The measured values of gas temperatures and temperatures of the reactor wall are used to find the fractions of energy transforming to heat in the plasma volume, which are compared with the calculation results.

Simulation of the Processes of Formation and Dissociation of Neutral Particles in Air Plasma: Vibrational Kinetics of Ground States of Molecules

The results are given of the calculations of populations of vibrational levels of N2(X 1·+ 8 ), О2(X 3·- 8 ), and NO(X 2Π) molecules in an air plasma whose parameters correspond to the positive column of a DC discharge at pressures of 30–300 Pa and currents of 20–110 mA. Different processes are analyzed that form the distribution of molecules over levels, including the electron impact, V–Vand V–T exchange, chemical reactions, and heterogeneous deactivation. The calculation results for N2(X 1·+ 8 ) are compared with experiment. It is found that the values of the effective vibrational temperature characterizing the populations of N2(X 1·+ 8 ) are close to the values realized in a nitrogen plasma, and the difference of the vibrational temperatures of О2(X 3·- 8 ) and NO(X 2Π) from the gas temperatures is not significant, which is due to high frequencies of V–T relaxation of vibrational states of these molecules in collisions with О(3 P) atoms.

A one-dimensional, self-consistent model of charged particle transport and vibrational kinetics in weakly ionized hydrogen

A one-dimensional model of weakly ionised plasmas of pure hydrogen has been developed, which allows to describe the plasma dynamics, the ionisation and dissociation kinetics. The model consists of a state to state, reaction diffusion model which includes eV, EV, VV, VT, dissociation and attachment processes over the vibrational ladder of H 2 ( X 1Σ g + , v=0, 14). The plasma kinetics is described by the Particle in Cell method with Monte Carlo Collisions applied to electrons and four ion species (H 3 +, H 2 +, H +, and H −). The electron/molecule process rates are calculated as a function of position taking into account the translational non-equilibrium of electrons and the vibrational non-equilibrium of molecules. As a case study we consider a parallel plate, radio frequency discharge plasma. Results are shown for two different values of the gas pressure. The role of collisional and electrostatic phenomena is discussed in the light of experimental results.

Non-equilibrium vibrational kinetics and `hot atom' models in the recombination of hydrogen atoms on surfaces

Non-equilibrium vibrational kinetics as recently developed for adlayers is applied to experimental data on H-atom recombination and abstraction at solid surfaces. This model allows one to fit vibrational and translational energy distribution functions of the excited diatoms leaving the surface. Eley–Rideal and Langmuir–Hinshelwood mechanisms involving vibrationally excited adatoms are presented together with a criterion for establishing their relative contribution to the total reaction rate. Analogies of this multilevel `hot atom' (HA) model with recently employed two and three levels HA kinetic schemes are presented and discussed.

Nonequilibrium Vibrational Kinetics of Air Hitting a Catalytic SiO Surface

The effects of a SiO2 partially catalytic surface on the nonequilibrium chemical and vibrational kinetics of a Ž ve-species (N, N2, O, O2 , NO) air mixture in the boundary layer of hypersonic reentry vehicles have been investigated. To this aim, boundary-layerequationscoupled to vibrationalmaster equations have been solved. The surface atomic recombination reactions have been considered as pumping the last bound vibrational level of N2 and O2 molecules. The results show a combined in uence of gas-phase and surface reactions on NO formation.

Energy coupling efficiency of a hydrogen microwave plasma reactor

Zero-dimensional and two-dimensional plasma models and optical emission spectroscopy are used in tandem to investigate the power coupling efficiency for a pure hydrogen microwave plasma. The zero-dimensional model accounts for the vibrational kinetics of H2, the chemistry of H2 and H excited states, and the kinetics of ground-state species. The set of species conservation equations are then coupled to the electron Boltzmann equation (to account for the non-Maxwellian electron energy distribution function) and the total energy equation for solution. The two-dimensional model makes use of a simpler thermochemical description of the plasma. The chemistry is described with nine species and thirty chemical reactions. Three energy modes are considered to describe the plasma’s thermal nonequilibrium, and Maxwellian distribution functions for kinetic and vibrational modes are assumed. The non-Maxwellian nature of the electron energy distribution function is separately accounted for. Experimentally, the absolute line emission intensity is utilized to obtain number densities of up to five hydrogen excited states using the following transitions: Hα (6563 Å), Hβ (4861 Å), Hγ (4340 Å), Hδ (4102 Å), and Hε (3970 Å). The first three transitions were used for a 38 Torr, 1000 W hydrogen discharge, and all five transitions were used for a 121 Torr, 4000 W hydrogen discharge. The absolute continuum emission from the plasma was compared to numerical predictions. The comparison of the numerical and experimental data indicates that 90%–100% of the input power is deposited in the plasma and that both the line and continuum emission match within a factor of 3, with the exception of the high energy excited states for the 4000 W plasma. A control volume heat transfer analysis validates the energy coupling.

Studies on Gas Purification by a Pulsed Microwave Discharge at 2.46 GHz in Mixtures of N2/NO/O2 at Atmospheric Pressure

Pulsed microwave discharges operated at atmospheric pressure in gas mixtures containing N2, O2, and NO are investigated experimentally and theoretically for various gas mixture constituents and operating conditions with respect to the ability of exhaust gas purification. The rotational gas temperature and the vibrational temperature of N2 are derived from CARS measurements. The composition of the exhaust gas after treatment is monitored using FTIR spectroscopy. The processes of the chemical, electronic, and vibrational kinetics are described by a model that has been developed to calculate the species densities. The results obtained show that in N2/NO gas mixtures an overall reduction of NOx takes place. In the case of N2/O2/NO gas mixtures, no net reduction of NOx is achieved for a pulsed microwave power below 3600 W, a pulse length of 50 μs, and a typical repetition frequency of 2 kHz.

Modeling of dissociation–recombination in nozzles using strongly non-equilibrium vibrational distributions

The paper presents a quasi-stationary model for dissociation, recombination and vibrational kinetics in a nozzle expansion of (N 2, N) mixture. The model is based on strongly non-equilibrium non-Boltzmann and non-Treanor vibrational distributions. The closed set of equations for gas dynamic parameters is derived, the flow field parameters and vibrational distributions are computed in different sections of the nozzle for various reservoir conditions. The influence of vibrational distributions on gas parameters in the nozzle is discussed as well as the dissociation–recombination effects.

Kinetic theory of low-temperature plasmas in molecular gases

This work reviews a number of important aspects of the kinetic theory of dc ormicrowave discharge plasmas in N2, O2, H2 and their mixtures. Thefundamental problems which are addressed include (1) the determination of thenon-equilibrium electron energy distribution function from the Boltzmannequation and its coupling to the vibrational kinetics; and (2) theself-consistent modelling of such discharges taking into account bulk andsurface kinetic processes, the gas thermal balance and the charged particlebalance. In particular, the mechanisms of dissociation, ionization andproduction of H- ions are discussed. Comparisons between modelpredictions and experiment are presented.

Electron kinetics in atomic and molecular plasmas

In the first part of this paper we briefly review some basicconcepts of kinetic theory. The concept of the velocity distribution functionis first introduced and its meaning is discussed. Then, the Boltzmann equationis presented on physical grounds and it is shown that the fluid equations areits moments. In the second part, the Boltzmann equation for free electrons ina low-temperature plasma is analysed. It is shown how this equation canapproximately be solved for electrons under a HF field of frequency ω(including the particular case of a dc field, which corresponds to the limitω = 0) by using a first-order double expansion in spherical harmonics invelocity space and a Fourier series in time. The electron transportparameters, particle balance and energy balance are analysed from a generalpoint of view. Finally, an application to argon and nitrogen is given. Theeffects of changes in the field frequency on the electron energy distributionfunction, transport parameters and power balance are discussed. The importanceof the coupling between the electron and the vibrational kinetics in N2 isemphasized.

Monte Carlo models of electron and ion transport in non-equilibriumplasmas

The fundamentals of the Monte Carlo method for electron and iontransport in plasmas are explained with emphasis on the coupling of theparticle transport with the space charge fields and chemical kinetics. In thefirst three sections the basics of the null-collision Monte Carlo methods andthe particle in cell/Monte Carlo collision method (PIC/MCC) are explained withpractical suggestions for numerical implementation. The fourth section showsthat the Monte Carlo method can be derived from the linear Boltzmann equation.In the fifth section, we show how the charged particle transport can beself-consistently coupled to the chemical kinetics of the gas phase, inparticular to the vibrational kinetics of molecules. In the penultimate section aone-dimensional, self-consistent model for a parallel-plate radio-frequencydischarge in pure hydrogen is presented.

Dissociation of I2 and the vibrational kinetics in the oxygen-iodine medium

The comparative analysis of the processes responsible for dissociation of molecular iodine in the oxygen-iodine medium is made. The rate of relaxation of vibrationally excited oxygen in the O2—I2—H2O medium is shown to be limited by the VV energy exchange between oxygen and water molecules, and the fraction of vibrationally excited O2 (ν=1) in the active medium of the oxygen-iodine laser can reach several tens of percent. It is noted that the VV exchange in O2(1Δ,ν=1)+I(2P3/2)↔O2(3Σ,ν=1)+I(2P1/2) reaction may be of considerable importance in the kinetics of formation of the active medium in the oxygen-iodine laser.