Vibrational Excitation Of Molecules Research Articles

Modelling of Energy-Dependent Electron Interactions in the Earth’s Mesosphere

Electrons are produced in the Earth’s quiet nighttime mesosphere by ionization by cosmic rays and ionization of NO by Lyman-α radiation. They are removed by attachment or recombination processes that are usually assumed in modelling to occur at the ambient temperature. However, the electrons have initial energies that are much higher than at thermal equilibrium, and so must have a range of energies as they progress towards equilibrium via interactions with atoms and molecules. As attachment and recombination rates are dependent on the electron energy, it is possible that modelling that considers the actual energy of the electrons will give different results to those based on assuming that the electrons are at the ambient temperature. In this work, starting with electrons at a higher initial energy, the detailed electron interactions (including elastic scattering and vibrational excitation of molecules) are tracked in a time-step simulation. This simulation is implemented by treating electrons in subranges of the electron energy spectrum as chemical species. This allows an investigation of two phenomena in the nighttime mesosphere: the origin of the D-region ledge and the production of radiative emissions from vibrationally excited molecules. It is found that there is negligible difference in the electron densities calculated using the ambient temperature or detailed interaction models, so this study does not provide an explanation for the D-region ledge. However, in the latter model, emissions at various wavelengths are predicted due to reactions involving vibrationally excited molecules. It is also found, using the time-step calculation, that it would take several hours for the predicted electron density to approach equilibrium.

Vibrational CARS measurements in a near-atmospheric pressure plasma jet in nitrogen: II. Analysis

The understanding of the ro-vibrational dynamics in molecular (near)-atmospheric pressure plasmas is essential to investigate the influence of vibrational excited molecules on the discharge properties. In a companion paper Kuhfeld et al (2021 J. Phys. D: Appl. Phys. 54 305204), results of ro-vibrational coherent anti-Stokes Raman scattering (CARS) measurements for a nanosecond pulsed plasma jet consisting of two conducting molybdenum electrodes with a gap of 1 mm in nitrogen at 200 mbar are presented. Here, those results are discussed and compared to theoretical predictions based on rate coefficients for the relevant processes found in the literature. It is found, that during the discharge the measured vibrational excitation agrees well with predictions obtained from the rates for resonant electron collisions calculated by Laporta et al (2014 Plasma Sources Sci. Technol. 23 065002). The predictions are based on the electric field during the discharge, measured by electric field induced second harmonic generation Kuhfeld et al (2021 J. Phys. D: Appl. Phys. 54 305204), Lepikhin et al (2020 J. Phys. D: Appl. Phys. 54 055201) and the electron density, which is deduced from the field and mobility data calculated with Bolsig+ Hagelaar and Pitchford (2005 Plasma Sources Sci. Technol. 14 722–33). In the afterglow a simple kinetic simulation for the vibrational subsystem of nitrogen is performed and it is found, that the populations of vibrational excited states develop according to vibrational-vibrational transfer on timescales of a few microseconds, while the development on timescales of some hundred microseconds is determined by the losses at the walls. No significant influence of electronically excited states on the populations of the vibrational states visible in the CARS measurements () was observed.

Mid-Infrared Scattering in γ-Al2O3 Catalytic Powders.

The energy efficiency of heterogeneous catalytic processes may be improved by using mid-infrared light to excite gas-phase reactants during the reaction, since vibrational excitation of molecules has been shown to increase their reactivity at the gas-catalyst interface. A primary challenge for such light-enabled catalysis is the need to ensure close coupling between light-excited molecules and the catalyst throughout the reactor. Thus, it is imperative to understand how to couple infrared light efficiently to molecules near and inside catalytic material. Heterogenous catalysts are often nanoscale metal particles supported on high surface area, porous oxide materials and exhibit feature sizes across multiple scattering regimes with respect to the mid-infrared wavelength. These complex powders make a direct measurement of the scattering properties challenging. Here, we demonstrate that a combination of directional hemispherical measurements along with the in-line transmission measurement allow for a direct measurement of the scattered light signal. We implement this technique to study the scattering behavior of the catalytic support material γ-Al2O3 (with and without metal loading) between 1040 and 1220 cm-1. We first study how both the mean grain size affects the scattering behavior by comparing three different mean grain sizes spanning three orders of magnitude (2, 40, and 900 µm). Furthermore, we study how the addition of metal catalyst nanoparticles, Ru, or Cu, to the support material impacts the light scattering behavior of the powder. We find that the 40 µm grain size scatters the most (up to 97% at 1220 cm-1) and that the addition of metal nanoparticles narrows the scattering angle but does not decrease the scattering efficiency. The strong scattering of the 40 µm grains makes them the most ideal support material of those studied for the given spectrum because of their ability to distribute light within the reactor. Finally, we estimate that less than 100 mW of laser power is needed to cause significant excitation for testing mid-infrared catalysis in a Harrick Praying Mantis diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reactor, a magnitude easily available using commercial mid-infrared lasers. Our work also provides a mid-infrared foundation for a wide range of studies of light-enabled catalysis and can be extended to other wavelengths of light or to study the scattering behavior of other complex powders in other fields, including ceramics, biomaterials, and geology.

Observation of a significant drop of electron density in cascaded arc argon plasma doped with oxygen gas using laser Thomson scattering

In this work, the electron density (n e) and electron temperature (T e) of cascaded arc argon plasma regulated by adding electronegative oxygen gas have been investigated using laser Thomson scattering diagnostic technique. The results indicate that the addition of O2 gas causes a significant decrease of n e, which drops from 1020 m−3 to 1017 m−3. This is mainly attributed to the dissociative recombination reaction between electrons and O2 + molecular ions. Meanwhile, the formation of negative ions, O2 − and O−, consumes electrons and further makes n e decrease. But, T e remains nearly unchanged with the increase of O2 ratio from 0% to 10%. This is probably due to that the electron energy loss by the electron collisions with O2 molecules in the ground state balances the electron heating induced by the super-elastic collisions with the highly vibrational excited O2 molecules.

Editorial: Biographical Memoirs, Volume 69

Editorial: Biographical Memoirs, Volume 69

Alexander Dalgarno. 5 January 1928—9 April 2015

Alexander (Alex) Dalgarno greatly advanced the quantitative study of fundamental atomic and molecular processes, contributed significantly to atmospheric science and ‘established molecular astrophysics as a unified intellectual field of great scientific endeavour, impact and achievement’ ( Flannery 2010 ). Alex developed and applied techniques that simplify calculations and lead to reliable solutions, enabling him to make landmark contributions to the knowledge of collisionally induced charge transfer, rotational and vibrational excitation of molecules, spin exchange and ultracold chemistry. His wide-ranging curiosity, disciplined steps to broaden his programme and ability to identify dominant physical processes and calculate their rates led to his many important contributions to atmospheric science. For example, Alex greatly expanded the knowledge of terrestrial airglow features, photoabsorption and collisional processes in the terrestrial ozone layer and deposition by energetic electrons in the atmospheres of other planets. In molecular astrophysics, he applied that same systematic approach to studies of a range of environments from the early Universe to present-day UV-irradiated interstellar clouds, shocks and supernova ejecta. Alex made availability to students a priority and encouraged them to pursue problems that they devised, despite his seemingly inexhaustible supply of suitable projects. His community service included his 29-year long editorship of Astrophysical Journal Letters , starting in 1973, and the founding of the Institute of Theoretical Atomic and Molecular Physics at the Harvard-Smithsonian Center for Astrophysics, in 1988, owing to his concerns for the health of the fields of theoretical atomic and molecular physics and fundamental quantum mechanics, which the Institute did much to reinvigorate.

Suppression of clustering of CF3Br molecules with argon atoms by CO2 laser radiation in gas-dynamic expansion of a CF3Br–Ar mixture: bromine isotope selectivity

We present the study of suppressing the clustering of CF3Br molecules with argon atoms under gas-dynamic expansion of the CF3Br–Ar mixture at a nozzle exit by a cw CO2 laser. The main attention is paid to the analysis of possibility of implementing the selective control of bromine isotope clustering. The method relies on the vibrational excitation of molecules with the laser radiation, which leads to the suppression of their clustering with argon atoms. The experimental setup and the method of study are briefly described. The results of evaluation of the efficiency and selectivity of suppression of molecular–atomic clustering for different compositions of the gas above the nozzle and separations between the irradiation zone and the nozzle exit section are presented. It is shown that under the resonance vibrational excitation of gas-dynamically cooled CF3Br molecules at the nozzle exit one can implement bromine isotope selective suppression of clustering of molecules with argon atoms. It is found that by controlling the clustering of CF3Br molecules with argon atoms one can implement considerably higher enrichment and selectivity factors than in the case of controlling the clustering of CF3Br molecules with each other. Thus, using the CF3Br–Ar mixture (the ratio of pressures 1 : 100 and 1 : 200) the enrichment factors for the bromine isotopes Kenr(81Br) = 1.50 ± 0.13 and 1.30 ± 0.09, respectively, were obtained under the conditions of irradiating the jet at the 9R(30) line of the laser (ν = 1084.635 cm−1). The selectivity α of the suppression of clustering of CF3Br molecules with argon atoms, achieved in this case, amounted to 4.02 ± 0.19 and 2.31 ± 0.11, respectively. The results show that the method allows one to selectively control the clustering of molecules, comprising isotopes of heavy elements that possess a small isotopic shift in the IR absorption spectra.

Interpreting Quasi-Thermal Effects in Ultrafast Spectroscopy of Hydrogen-Bonded Systems.

Vibrational excitation of molecules in the condensed phase relaxes through vibrational modes of decreasing energy to ultimately generate an equilibrium state in which the energy is distributed among low-frequency modes. In ultrafast vibrational spectroscopy, changes in the vibrational features of hydrogen-bonded NH and OH stretch modes are typically observed to persist long after these high-frequency vibrations have relaxed. Due to the resemblance to the spectral changes caused by heating the sample, these features are typically described as arising from a hot ground state. However, these spectral features appear on ultrafast time scales that are much too fast to result from a true thermal state, and significant differences between the thermal difference spectrum and the induced quasi-thermal changes in ultrafast spectroscopy are often observed. Here, we examine and directly compare the thermal and quasi-thermal responses of the hydrogen-bonded homodimer of 7-azaindole with temperature-dependent FTIR spectroscopy and ultrafast mid-IR continuum spectroscopy. We find that the thermal difference spectra contain contributions from both dissociation of the hydrogen bonds and from frequency shifts due to changes in the thermal population of low-frequency modes. The transient spectra in ultrafast vibrational spectroscopy are also found to contain two contributions: initial frequency shifts over 2.3 ± 0.11 ps associated with equilibration of the initial excitation, and frequency shifts associated with the excitation of several fingerprint modes, which decay over 21.8 ± 0.11 ps, giving rise to a quasi-thermal response caused by a distribution of fingerprint modes being excited within the sample ensemble. This resembles the thermal frequency shifts due to population changes of low-frequency modes, but not the overall thermal spectrum, which is dominated by features caused by dimer dissociation. These findings provide insight into the changes in the vibrational spectrum from different origins and are important for assigning, analyzing, and comparing features in thermal and ultrafast vibrational spectroscopy of hydrogen-bonded complexes.

Kinetic modelling of NH3 production in N2–H2 non-equilibrium atmospheric-pressure plasma catalysis

Detailed plasma kinetics modelling is presented of a low electron energy N2–H2 atmospheric-pressure discharge for ammonia synthesis. The model considers both electron and vibrational kinetics, including excited N2(X, ν) and H2(X, ν) species, and surface reactions such as those occurring by the Eley–Rideal and Langmuir–Hinshelwood mechanisms and dissociative adsorption of molecules. The predictions of the model are compared to the measured NH3 concentration produced in a packed-bed dielectric barrier discharge reactor as a function of process parameters such as input gas composition and applied voltage. Unlike typical low-pressure plasma processes, under the plasma conditions considered here (reduced electric field E/N in the range 30–50 Td, electron density of the order 108 cm−3), the influence of ions is not significant. Instead, the reactions between radicals and vibrationally-excited molecules are more important. The active species in surface reactions, such as surface-adsorbed atomic nitrogen N(s) or hydrogen H(s), are found to be predominantly generated through the dissociative adsorption of molecules, in contrast to previously proposed mechanisms for plasma catalysis under low-pressure, high-E/N conditions. It is found that NH radicals play an important role at the early stages of the NH3-generation process, NH in turn is produced from N and H2(ν). Electron kinetics is shown to play a critical role in the molecular dissociation and vibrational excitation reactions that produce these precursors. It is further found that surface-adsorbed atomic hydrogen H(s) takes a leading role in the formation of NH3, which is another significant difference from the mechanisms in conventional thermo-chemical processes and low-pressure plasmas. The applied voltage, the gas temperature, the N2:H2 ratio in the input gas mixture and the reactivity of the surface material are all found to influence the ammonia production. The calculated results reproduce the observed trends in the dependence of NH3 production on the N2/H2 ratio in the input gas mixture and the applied voltage.

Threshold dependence of the vibrational excitation of molecules on laser radiation intensity

The collisionless vibrational excitation of a polyatomic molecule in an IR laser radiation field has been theoretically studied. It has been shown that (i) the degree of vibrational excitation (namely, number 0000 of vibrational quanta of a molecular mode near-resonant with the IR laser field that are absorbed by the molecule) is low if laser pulse intensity P (energy flux density in the laser beam) is lower than a certain critical value Pcr; (ii) the degree of excitation abruptly increases after crossing the boundary where P = Pcr; (iii) this effect is attributed to two properties inherent in polyatomic molecules, namely, the anharmonicity of the vibrational mode interacting with the laser field and the energy exchange with other modes; and (iv) at P > Pcr, number 00000 is determined only by energy density Φ = PτP, where τP is the laser pulse duration, 00000 monotonically increases with increasing Φ. The model is in good agreement with the experimental data.

Light-Induced Mid-Infrared Emission of Liquid Carbon Tetrachloride and Benzene

Light-induced infrared emission spectroscopy (LIRES) is a novel technique that permits to receive high-quality spectra in the mid-infrared region. Low-intensity visible light connected to a highly sensitive FTIR spectrometer is more advantageous for studying any samples, including biological samples without any damage. This technique permits obtaining unique information on the molecule structure via vibrational excitation fundamental frequencies, overtones, and combination modes. It also enables a direct observation of vibrational radiation transitions in vibrationally excited molecules as well as the channels of vibration energy redistribution, which is not allowed with any other method. In this work, the LIRES is being tested as a technique for studying of vibrationally-excited molecules of carbon tetrachloride and benzene in the liquid phase. On the other hand, using transparent liquids, we had tried to understand some of the physical phenomena that can drive emission in mid-IR. The characteristics of the infrared emission of both liquid species produced by different wavelength radiation from various types of light systems (100 - watt Xe-lamp and Nd:YAG laser; lambda = 1064 nm (8 mW) and lambda = 532 nm (4 mW)) are presented. We demonstrated that the IR-signal, as well as spectral properties of carbon tetrachloride and benzene, was dependent on the wavelength and power of excitation beam. Results obtained with different light sources show that the visible light produces a nonlinear IR-emission signal in transparent liquids. We believe that the visible light is the source of the nonlinear response and is producing the vibration excitation as well as photostimulated transformations of the molecules possessing the high activity for the nonlinear response.

Absorption Dynamics of Nitric Oxide in Gas Mixtures Excited by Pulsed Discharge

Vibrational relaxation of NO molecules excited by a pulsed e-beam sustained discharge (EBSD) with the use of CO laser probing was studied. We developed numerical model of vibrational kinetics in ensemble of NO molecules due to comparing the experimental and calculated data on absorption dynamics of vibrational excited NO molecules.

Thermal Characteristics of Air in the Problem of Hypersonic Motion of Bodies in the Earth’s Atmosphere

Introduction. The air fl ow around bodies moving in the atmosphere with hypersonic velocities is accompanied by a sharp increase in the air temperature due to the high energy of the fl ow and its dissipation in strong shock waves (SW) characteristic of such fl ows [1]. As the velocity of motion increases, there is an increase in the air temperature behind the shock wave, which leads to the initiation of a number of high-temperature processes such as vibrational excitation of molecules and their dissociation and the formation of molecular and atomic ions. These physicochemical processes have their own relaxation times. When these times are much longer than the characteristic time of the change in the fl ow parameter (in the stationary fl ow the above situation is equivalent to a relaxation length larger than the linear size), the thermodynamical properties of air can be described in the ideal gas approximation with a constant adiabatic index. In the opposite case of a short relaxation time, the thermal parameters of the medium can be assumed to be in equilibrium. To describe the intermediate region where the times are comparable, it is necessary to use physical and chemical kinetics equations [2]. It is known that the above high-temperature processes can not only infl uence locally the fl ow parameters, but also determine its character in general. Nevertheless, considering the hypersonic fl ow without account for the nonequilibrium processes is justifi ed [3], since it permits forming an approximate hydrodynamic fl ow pattern. The present paper gives the results of calculations of the equilibrium properties of a high-temperature air and describes a model of the processes of physicochemical kinetics, which, along with the gas dynamics equations, forms the basis for simulating the hypersonic motion of bodies in the Earth’s atmosphere. Equilibrium Characteristics of Air. In the ideal gas approximation, the system of gas-dynamic equations is closed by the equation of state

Resonant electron–molecule vibrational excitation cross sections and rate coefficients for atmospheric plasmas

A complete set of resonant electron–molecule vibrational excitation cross sections and rate coefficients for N2, NO and O2 atmospheric components are given.

An analytic algebraic approach to study the influence of molecular alignment and orientation on multiphoton excitation in intense laser fields

The influence of molecular alignment and orientation on multiphoton vibrational excitation of diatomic molecules H2, HD, and D2 is studied by an analytical algebraic approach. The explicit expressions of the time-evolution operator and the excitation probability are given. The long-time average absorbed energy spectra and the time-dependent absorbed energy are obtained. Results show that the impact of molecular orientation on the multiphoton resonant excitation is decided by the molecular type and molecular anharmonicity. This is valuable for controlling the vibrational excitation of molecules, which is relevant to the whole field of molecular physics and physical chemistry. Furthermore, good agreement with numerical simulation is achieved.

Differential Cross Sections for High-Lying Vibrational Excitations (ν = 0 → ν′ = 1, 2, …, 9, 10) of e-H2 Scattering

A theoretical investigation on the differential cross section (DCS) from low-energy electron scattering of high-lying vibrational excited H2 molecules is reported. The body-frame vibrational close-coupling (BFVCC) approach is used to solve the scattering equations. Quantum scattering potentials include static, exchange, and polarization contributions based on ab initio calculations. By including the contributions of 9 partial waves (Nν = 9), 18 Morse vibrational states (Nl = 18), and 16 molecular symmetries (Δ = 0,1, …, 7), the calculated DCSs have good agreement with available experimental measurements and theoretical studies, and show that high angular momenta and good vibrational wavefunctions are necessary to better describe the scattering physics of electron-molecule vibrational excitation collisions.

Negative-Ion-Source Modeling: From Expansion to Extraction Region

In a negative-ion source, the vibrational kinetics of molecules H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and the negative-ion H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> transport are both important in order to understand and optimize H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> extraction. In this paper, two different particle models are developed, simulating the expansion and extraction regions of a radio-frequency inductively coupled plasma negative-ion source. These models show the importance of gas- and plasma-surface interaction in the vibrational excitation of molecules and in the direct production of negative ions from the wall.

Effect of vibrational excitation of molecules on the velocity of sound in a high-temperature diatomic gas

A formula for the velocity of sound, which is valid not only for barotropic gases, is derived on the basis of methods of the kinetic theory of gases. This formula is specified for various stages of relaxation of a high-temperature diatomic gas in the approximation of the model of anharmonic oscillators. A dependence between the populations of vibrational levels of molecules and the velocity of sound is found.

Inelastic inverse chaotic scattering problem

We consider the inverse scattering problem for a chaotic system as the task to reconstruct information about the chaotic invariant set from the knowledge of scattering data. In this Letter we show how a recent method developed for periodically driven systems can be modified to work for inelastic scattering of the type which occurs in vibrational excitation of molecules during collisions. The additional idea is to measure the total time and final momentum of the trajectories. In a corresponding two-dimensional plot, the occurring values fall naturally into disjoint groups easily related to the hierarchical levels of the scattering functions. We pay special attention to the effect of anharmonicity of the internal degree of freedom on the procedure of reconstruction.

The Effect of Vibrational Excitation of Molecules on Plasmachemical Reactions Involving Methane and Nitrogen

An experimental study of plasmachemical reaction involving CH4 and N2 molecules in rf discharge was studied in order to know the effect of vibrational excitation of N2 molecules. When the relative nitrogen concentration was greater than 0.8, the main product of CH4 decomposition was HCN, and the rate of methane decomposition at this condition was faster than that one in pure methane. These results could be confirmed through the mass spectroscopic method. The reason for these results is the vibrational energy of N2 excited by rf discharge. The chain reaction mechanisms of producing HCN by vibrational excitation of N2 were examined closely through numerical simulation. The rate-controlling step was the dissociation reaction of excited nitrogen molecule to the atomic nitrogen, so the process of HCN synthesis was limited by the value of reaction constant, kN.