High Vibrational Temperature Research Articles

Non-LTE spectroscopy of the tetradecad region of methane recorded in a hypersonic flow

Laboratory spectroscopic data is essential for the modeling of hot exoplanet atmospheres, since molecules such as methane, a major component of hot-Jupiter-type exoplanet atmospheres, have a complex vibrational energy structure that makes computational predictions difficult at high temperatures for ro-vibrational transitions involving highly excited vibrational sates. To better inform line lists used in radiative transfer modeling, the ro-vibrational spectrum of methane has been recorded in the tetradecad region between 1.7 and 1.65 μm (5880–6060 cm−1) through non-local thermodynamic equilibrium (non-LTE) cavity ringdown spectroscopy (CRDS). Non-LTE conditions, characterized by a low rotational temperature (∼39 K) and a high vibrational temperature (up to 1130 K), have been obtained by hypersonic expansion of a pre-heated mixture of argon and methane in a contoured Laval nozzle. The high vibrational temperature increases the intensity of new hot bands, while the very low rotational temperature greatly simplifies their rotational structure, thus facilitating their identification. A close comparison of the recorded CRDS data to the TheoReTS database reveals both inefficient vibrational relaxation between polyads and efficient vibrational relaxation between vibrational states forming a polyad. These effects result in an overpopulation of the lowest vibrational energy level of each polyad, an effect not widely currently incorporated in non-LTE radiative transfer models. A series of new hot band transitions originating from the pentad and octad polyads were assigned and are provided as a line list for use in future databases.

Fast and facile synthesis of carbonate-modified NiFe layered double hydroxide nanosheets by dielectric barrier discharge microplasma: mechanism and application in enhanced water oxidation

A fast and facile approach was designed to fabricate carbonate (Ci)-modified NiFe layered double hydroxide (LDH) nanosheets array on carbon cloth (CC) by dielectric barrier discharge (DBD) microplasma. The whole synthetic process can be completed within 1 h at ambient temperature and pressure. The prepared NiFe LDH-Ci/CC emerges a superior catalytic activity for oxygen evolving reaction in alkaline media, which only demands an overpotential of 240 mV at 20 mA cm−2 with a high stability for at least 90 h, and shows an excellent turnover frequency value of 0.323 mol O2 s−1 at 350 mV. Time-resolved measurements of direct emission spectra for nitrogen second positive system N2(C-B) were measured in the DBD microplasma discharge. And a high vibrational temperature (Tvib, 3100 K) and rotational temperature (Trot, 340 K) were obtained, indicating a great chemical reactivity. In addition, the intermediate products of hydroxyl radicals (·OH) were identified and the possible synthesis mechanism was tentatively proposed. A fast and facile approach was designed to fabricate NiFe LDH-Ci/CC by DBD microplasma.

Rotational Spectra of Vibrationally Excited AlO and TiO in Oxygen-rich Stars

Abstract Rotational transitions in vibrationally excited AlO and TiO—two possible precursors of dust—were observed in the 300 GHz range (1 mm wavelength) toward the oxygen-rich asymptotic giant branch (AGB) stars R Dor and IK Tau with ALMA, and vibrationally excited AlO was observed toward the red supergiant VY CMa with the SMA. The J = 11 → 10 transition of TiO in the v = 1 and 2 levels and the N = 9 → 8 transition in the v = 2 level of AlO were identified toward R Dor, the J = 11 → 10 line of TiO was identified in the v = 1 level toward IK Tau, and two transitions in the v = 1 and 2 levels of AlO were identified toward VY CMa. The newly derived high vibrational temperature of TiO and AlO in R Dor of 1800 ± 200 K and prior measurements of the angular extent confirm that the majority of the emission is from a region within ≲2R ⋆ of the central star. A full radiative transfer analysis of AlO in R Dor yielded a fractional abundance of ∼3% of the solar abundance of Al. From a similar analysis of TiO a fractional abundance of ∼78% of the solar abundance of Ti was found. The observations provide indirect evidence that TiO is present in a rotating disk close to the star. Further observations in the ground and excited vibrational levels are needed to determine whether AlO, TiO, and TiO2 are seeds of the Al2O3 dust in R Dor, and perhaps in the gravitationally bound dust shells in other AGB stars with low mass-loss rates.

Increased arc length and stability in a magnetic gliding arc discharge using a cylindrical notched cathode

Intermediate arc discharge plasmas allow for working with ions at lower temperatures than thermal plasmas and can generate greater selectivity in certain chemical reactions due to the high vibrational and electron temperature. In this paper, a magnetic gliding arc discharge (MGA), which is an intermediate plasma, was developed and is described. The reactor is operated with CO2 as the working gas, but open to atmosphere. It presents increased arc length and stability by using a cylindrical notched cathode (CNC). The constructed rectangular section of the notch channel on the cylindrical cathode surface allowed the arc to be confined within the channel with enhanced stability. The changes in the cathode geometry allowed the system to reach radial arc length of 40 mm, plasma disc diameter of 102 mm, and from 300 to 400 mA of current. A low cost electro-optical device was set up to determine the arc rotation frequency and is described in detail. Measurements of frequency (f) as a function of current (I) indicates that the developed MGA-CNC follows the relation f∝ I0.5, which agrees with the theoretical prediction. In order to compare the MGA-CNC data acquired with other reported MGAs, an analysis considering the distinct magnetic field intensities (Br) and different cathode diameters (D) was performed. Thus, a linear relationship was observed for f D/(Br=D/2)1/2 as a function of I1/2, which may represent the general experimental behavior of different MGA operated in air at atmospheric pressure. The arc diameter (d) in the region near the cathode (at r = D/2) can be determined from the slope α of the plot (α = 6.03 m kg−1/2); the obtained value (d = 3.2 ± 0.7 mm) is coherent with measured values elsewhere.

The High Temperature Vibrational Partition Function of Stretching of Triatomic Systems

The statistical thermodynamic model for the vibrational partition function with separated stretching and bending is developed. The model is studied on the example of hbox {CO}_{2} molecule for temperature up to 20,000 K with the aim to describe efficient dissociation by deposition of energy mainly to the stretching modes of vibration. The observed separation of bending mode at lower temperatures suggest that it is possible to construct such kinetic model of plasma in which the high vibrational temperature of stretching and the low vibrational temperature of bending are obtained resulting in an efficient dissociation. In particular, the proposed model is extended to ideal-gas version where all the interactions between atoms are taken into account. The idea behind such approach is to eliminate contributions to partition functions stemming from non-interacting dissociated fragments of the molecule. The application areas of such partition functions are discussed and the full vibrational partition functions based on that model are compared with the known data.

Highly efficient conversion of methane using microsecond and nanosecond pulsed spark discharges

Plasma-assisted methane (CH4) activation is a promising way for a hydrogen (H2) production. In this paper, we describe our studies of microsecond and nanosecond pulsed spark discharge plasmas use in a CH4 pyrolysis for a H2 production. The dependence of CH4 conversion and gas discharge product composition on discharge power, discharge gap length and gas flow rate are studied. The electrical and optical characteristics of the discharges are also studied to reveal discharge plasma parameters and chemical reactions leading to CH4 pyrolysis. Experimental results show that H2 and acetylene (C2H2) are the major gas discharge products accompanied by trace gas discharge products, such as ethane (C2H6), ethylene (C2H4) and carbon. The highest CH4 conversion and H2 yield, 91.2% and 38.4%, respectively, are achieved with an energy conversion efficiency of 44.3% using the microsecond pulsed spark discharge at a gap length 6 mm and a gas flow rate 50 mL/min. The carbon balance under the studied operating parameters varies from 66.7% to 92.8%. The morphology of carbon deposition is presented by two crystal forms identified by SEM and Raman spectral analyses. Finally, comparatively low electron temperature and high vibrational molecular temperature are observed in our experiments, which suggests that V-V transition for CH4 excitation process and V-T transition for CH4 heating process play important roles in CH4 pyrolysis sustained by the pulsed spark discharge.

Coupling strength assumption in statistical energy analysis.

This paper is a discussion of the hypothesis of weak coupling in statistical energy analysis (SEA). The examples of coupled oscillators and statistical ensembles of coupled plates excited by broadband random forces are discussed. In each case, a reference calculation is compared with the SEA calculation. First, it is shown that the main SEA relation, the coupling power proportionality, is always valid for two oscillators irrespective of the coupling strength. But the case of three subsystems, consisting of oscillators or ensembles of plates, indicates that the coupling power proportionality fails when the coupling is strong. Strong coupling leads to non-zero indirect coupling loss factors and, sometimes, even to a reversal of the energy flow direction from low to high vibrational temperature.

VIBRATIONALLY EXCITED HCN IN THE LUMINOUS INFRARED GALAXY NGC 4418

Infrared pumping and its effect on the excitation of HCN molecules can be important when using rotational lines of HCN to probe dense molecular gas in galaxy nuclei. We report the first extragalactic detection of (sub)millimeter rotational lines of vibrationally excited HCN, in the dust-enshrouded nucleus of the luminous infrared galaxy NGC 4418. We estimate the excitation temperature of T_vib ~ 230 K between the vibrational ground and excited (v_2=1) states. This excitation is most likely due to infrared radiation. At this high vibrational temperature the path through the v_2=1 state must have a strong impact on the rotational excitation in the vibrational ground level, although it may not be dominant for all rotational levels. Our observations also revealed nearly confusion limited lines of CO, HCN, HCO+, H13CN, HC15N, CS, N2H+, and HC3N at lambda ~ 1 mm. Their relative intensities may also be affected by the infrared pumping.

低圧空気極小プラズマジェットのノズル出口直後における N2 2+バンドの放射特性

Low-pressure air micro-plasmajets with a length of 10mm were generated through a nozzle with a diameter of 0.7mm at the pressures of 31.3 and 5.3kPa. Spectroscopic measurements were conducted at the point of 0.5mm from the nozzle exit on the central axis. The N2+ 1- and N2 2+ bands were predominant in the wavelength region of 280 to 420nm and the band shape was almost independent of the pressure. From temperature determination by a spectral matching method, it was found that the plasmajets were in a thermal nonequilibrium state and that vibrational temperature was much higher than rotational one. The experimental intensity distribution was reconstructed by the equilibrium radiation theory, in good agreement except for the N2 2+ bands with v′ ≥ 2 as in case of atmospheric air micro-plasmajet. For these bands, much better agreement was obtained by taking the effects of predissociation and non-Boltzmann rotational population distribution for N2C3Πu state into consideration in the theory. Discussions were made about potential cause of high vibrational temperature and slight difference between experimental and theoretical band shapes of the N2 2+ bands at the low-pressures.

Investigation of NO production and flame structure in plasma enhanced premixed combustion

A pulsed plasma source is used to stabilize lean premixed methane flames. Nitric oxide (NO) production is measured using probe sampling and chemiluminescence analysis. While the discharge is a potential source of NO, it is observed that the flame also consumes some NO. The NO production in the flame is modeled by use of a modified opposed diffusion flame simulation (OPPDIF), augmented by the initial radical pool created by the discharge. The simulations are in qualitative and quantitative agreement with the measurements, despite the simplified modeled geometry. The structure of this plasma assisted premixed flame is discussed within the framework of two interacting premixed streams, as simulated by the OPPDIF configuration. Under certain conditions, we observe a cold pre-flame associated with the plasma-excited premixed stream, the products of which (OH radicals) have an unusually high vibrational temperature and low rotational temperature in electronically excited states, when compared to the OH characteristics found in a conventional lean premixed flames. While the role of the OH radicals in this cold pre-flame stream is not fully understood, we believe it plays an important role in igniting the surrounding combustible mixture, as confirmed by the simulations.

Nitrogen vibrational excitation in a N2/He pulsed planar-ICP RF discharge

A pure inductive regime of pulsed RF discharge in N2–He mixtures at 0.2 Torr has been investigated by time resolved Langmuir probe and optical emission spectroscopy. A planar coil ICP with a Faraday shield has been used. Mixture compositions of 5–100% N2 and duty cycles, TON = 1–15 ms and TOFF = 15 ms, have been explored. The Langmuir probe analysis evidences a Maxwell electron energy distribution in the discharge and post-discharge, characterized by electron density and temperature that depend on both duty cycle and composition. The vibrational excitation of N2(C) state is inferred by the N2 second positive emissions. Its kinetic analysis reveals the formation of a high vibrational excitation of ground state nitrogen. A vibrational temperature higher than 10 000 K can be reached at the end of the 5 ms discharge pulse on the levels v = 0–3 of the X state. Such a high vibrational temperature slows the post-discharge relaxation of the electron temperature.

Growth and structure of fullerene-like CNx thin films produced by pulsed laser ablation of graphite in nitrogen

The growth and structure of fullerene-like CNx films produced by laser ablation of graphite in low pressure nitrogen were investigated. Deposition conditions were selected based on investigations of CN and C2 concentration at the condensation surface, vibrational temperature of CN radicals, and kinetic energies of atomic and molecular species. Films were characterized with x-ray photoelectron spectroscopy, Raman spectroscopy, high-resolution transmission electron microscopy, nanoindentation, and stress analyses. The nitrogen content in CNx films directly depended on the concentration of CN radicals at the condensation surface. Formation of fullerene-like structures required a high vibrational temperature of these radicals, which was maximized at about 4 eV for depositions at 10 mTorr N2 and laser fluences of ∼7 J/cm2. The presence of C2 had only a minor effect on film composition and structure. Optimization of plasma characteristics and a substrate temperature of 300 °C helped to produce about 1-μm-thick solid films of CNx (N/C ratio≈0.2–0.3) and pure carbon consisting of fullerene-like fragments and packages. In contrast to carbon films, fullerene-like CNx films exhibited a high elastic recovery of about 80% in using a Berkovich tip at 5 mN load and indentation depths up to 150 nm. Their elastic modulus was about 160 GPa measured from the unloading portion of an indentation curve, and about 250 GPa measured with a 40 Hz tip oscillation during nanoindentation tests. The difference was related to time dependent processes of shape restoration of fullerene-like fragments, and an analogy was made to the behavior of elastomer polymers. However, unlike elastomers, CNx film hardness was as high as 30 GPa, which was twice that of fullerene-like carbon films. The unusual combination of high elasticity and hardness of CNx films was explained by crosslinking of fullerene fragments induced by the incorporated nitrogen and stored compressive stress. The study demonstrated laser ablation as a viable technique for the growth of fullerene-like CNx films, which may be used as hard protective coatings resisting brittle fracture at high loads and extensive substrate deformations.

The triplet lifetime dependence on vibrational energy for gas-phase carbonyl aromatic compounds: benzophenone and fluorenone

The dependences of the intensities and decay rates of CO2 laser induced delayed fluorescence on vibrational energy and bath gas pressure were used to study the dominant relaxation pathways in the lowest triplet state, T1, over a wide range of vibrational energy Evib. With this in view, the excess energy dependences of nonradiative relaxation processes rates in the T1 state, such as intersystem crossing from the T1 to ground S0 states, reversible intersystem crossing between excited S1 and T1 states and intermolecular vibrational relaxation were compared. It was found that under vibrational equilibrium at high vibrational temperature (Tvib ≫ T) the experimentally observed nonexponential dependence of the gas-phase triplet decay time on Evib reflected the excess vibrational energy (in the T1) dependence of the intersystem crossing rate from T1 to S0 that was well approximated by an Arrhenius' activated type rate equation in contrast to exponential decay for isolated molecules.

Plasma chemistry in laser ablation processes

Based on the results of quantitative spectroscopic diagnostics (LIF in combination with time resolved emission spectroscopy) chemical dynamics in laser-produced plasmas of metallic (Ti, Al,), and graphite samples have been examined. The Nd-YAG (1064 nm, 10 ns, 100 mJ) and excimer XeCl (308 nm, 10 ns, 10 mJ) lasers were employed for ablation. The main attention was focused on the elucidation of a role of oxide and dimer formation in controlling spatio-temporal distributions of different species in the ablation plume. The results of the spatial and temporal analysis of a laser-produced plasma in air indicates the existence of diatomic oxides in the ablation plume both in the ground and excited states, which are formed from reactions between ablated metal atoms and oxygen. The efficiency of the oxidation reaction depends on the intensity and spot diameter of the ablation laser beam. The maximal concentration of TiO molecules are estimated to be of 1×10 14 cm −3 at the time of 10 μs after the start of the ablation pulse. A comparison of spatial–temporal distributions of Ti atoms and excited TiO molecules allow us to find a correlation in their change, which proves that electronically excited Ti oxides are most probably formed from oxidation of atoms in the ground and low lying metastable states. The spectroscopic characterization of pulsed laser ablation carbon plasma has also been performed. The time–space distributions as well as the high vibrational temperature of C 2 molecules indicate that the dominant mechanism for production of C 2 is the atomic carbon recombination.

Crack propagation induced heating in crystalline energetic materials

A model is presented for time and spatial dependences of the heating of molecular vibrations and the possible initiation of chemical reaction from heat dissipated in the vicinity of a propagating crack in a molecular crystal. In the model, energy from a moving crack tip is released as phonons in proximity to the crack. Initially the phonons and the molecular vibrations are not in thermal equilibrium. Subsequently, there is a competition between excitation of molecular vibrations by multiphonon up-pumping and diffusion of phonons from the crack region. If the coupling between the locally hot phonon bath and the molecular vibrations is sufficiently large, a transitory high vibrational temperature will be achieved prior to eventual thermal equilibration with the bulk of the crystal. It is found that the peak vibrational temperature can be sufficiently high for a significant time period for chemical reactions to occur. The model calculates the local time-dependent vibrational temperature using reasonable values of the physical input parameters. For a crack tip moving near the speed of sound, the calculations show that vibrational temperatures can reach ∼800 K in 55 ps and exceed 550 K for ∼1 ns after the initial heating. This temperature change is sufficient to produce chemical reaction in a secondary explosive such as HMX, but given the duration and size of the heated region, a single crack should not result in self-sustaining chemical reaction. The role that cracks may play in shock sensitivity is discussed.

Vibrational relaxation of anharmonic oscillators in expanding flows

Although the Landau-Teller vibrational model accurately predicts vibrational excitation process in post-shock and compressing flows, it under-predicts the rate of de-excitation in cooling and expanding flows. In the present paper, detailed calculations of the vibrational relaxation process of N2 and CO in cooling flows are conducted. A coupled set of vibrational transition rate equations and quasi-one-dimensional fluid dynamic equations is solved. Multiple quantum level transition rates are computed using SSH theory. The SSH transition rate results are compared with available experimental data and other theoretical models. Vibration-vibration exchange collisions are responsible for some vibrational relaxation acceleration in situations of high vibrational temperature and low translational temperature. The present results support the relaxation mechanisms proposed by Bray and by Treanor Rich and Rehm.

Laser-induced fluorescence of OH and SiO molecules during thermal chemical vapour deposition of SiO 2 from silane-oxygen mixtures

During low-pressure chemical vapour deposition of SiO 2 from thermally activated mixtures of silane and oxygen, laser-induced fluorescence is observed from OH and SiO molecules. From these measurements the rotational and vibrational temperatures are derived. The continuous nature of the observed oxygen chemiluminescence might be explained by a high vibrational temperature, which is not in equilibrium with the rotational temperature. Oxygen molecules are probably excited in the gas-phase reaction step SiO+O+O 2→SiO 2+O 2 *.

Rotational and vibrational temperatures of naphthalene in a naphthalene-argon supersonic jet

In the A 1B 2u- X 1A g system of naphthalene in a supersonic jet, rotational contour calculations show rotational temperatures of 2–60 K for argon carrier gas pressures of 1520-120 Torr. The b 1u vibration v 24 shows a high vibrational temperature which corresponds to the seeding temperature for pressures <400 Torr.

Measurement of vibrational temperature of CO and N2 using the He(23S) Penning ionization technique

The vibrational temperatures of N2 and CO in binary gas mixture and in the VV quasiequilibrium regime of high vibrational temperature and low kinetic temperature were measured in a flow tube apparatus. The Penning ionization with He (23S) technique was used to determine the vibrational temperatures of both gases. The Treanor-Teare expression relating vibrational temperatures for such conditions was verified. The formulation of the Penning ionization technique for CO allowed the CO rotational energy and the kinetic energy of relative motion between CO and He to play a role in the normally nonenergetically allowed CO(X, 0)→CO+(B, 1) excitation reaction. The unexpected result of a collisional quenching rate that is smaller for CO+(B, 1) than for CO+(B, 0) was also observed.

Evaluation of Rotational Temperature at High Vibrational Temperature in Electron Beam Fluorescence Technique

The influence of vibrational temperature on the rotational distribution function in nitrogen has been investigated. The rotational temperature TR is calculated for various vibrational temperatures Tv and compared with the calculations, where the influence of Tv is neglected.