N2-CH4 Mixtures Research Articles

Characterization of non-Boltzmann CN X2Σ+ behind shock waves in CH4-N2 via broadband ultraviolet femtosecond absorption spectroscopy.

This article describes the temporal evolution of rotationally and vibrationally non-Boltzmann CN X2Σ+ formed behind reflected shock waves in N2-CH4 mixtures at conditions relevant to atmospheric entry into Titan. A novel ultrafast (i.e., femtosecond) laser absorption spectroscopy diagnostic was developed to provide broadband (≈400 cm-1) spectrally resolved (0.02nm resolution) measurements of CN absorbance spectra belonging to its B2Σ+ ← X2Σ+ electronic system and its first four Δv = 0 vibrational bands (v″ = 0, 1, 2, 3). Measurements were acquired behind reflected shock waves in a mixture with 5.65% CH4 and 94.35% N2 at initial chemically and vibrationally frozen temperatures and pressures of 4400-5900K and 0.55-0.75bar, respectively. A six-temperature line-by-line absorption spectroscopy model for CN was developed to determine the rotational temperature of CN in v″ = 0, 1, 2, and 3, as well as two vibrational temperatures via least-squares fitting. The measured CN spectra revealed rotationally and vibrationally non-Boltzmann population distributions that strengthened with increasing shock speed and persisted for over 100 µs. The measured vibrational temperatures of CN initially increase in time with the increasing CN mole fraction and eventually exceed the expected post-shock rotational temperature of N2. The results suggest that strong chemical pumping is ultimately responsible for these trends and that, at the conditions studied, CN is primarily formed in high vibrational states within the A2Π or B2Σ+ state at characteristic rates, which are comparable to or exceed those of key vibrational equilibration processes.

Erratum to: Development of natural gas liquefaction processes using mixed refrigerants: a review of featured process configurations and performance

The original version of this article unfortunately contained a mistake. In p.750, the sentence “Therefore, it is hard to recommend operation stated, and two-phase states of MRs are necessary for component separation, which might limit the use of gas expanders and N2-CH4 mixtures” should be “Therefore, it is hard to recommend operation parameters and comparable SPC ranges. As already stated, two-phase states of MRs are necessary for component separation, which might limit the use of gas expanders and N2-CH4 mixtures.”

Layered structure around an extended gliding discharge column in a methane-nitrogen mixture at high pressure

The current work aims at investigating the detailed spatial structure of the thin plasma column of a gliding arc (GA) discharge extended in N2-CH4 gas mixtures, using visualization techniques. The GA discharge was operated at up to 5 atm in a high-pressure vessel with extensive optical access. The results show that the emission intensity from the plasma column increased tenfold with the addition of 0.1% CH4 in nitrogen, compared to that in pure N2. Furthermore, an additional layer located around the GA discharge column is detected. Imaging through spectral filters and spectral analysis of the emitted signal indicate that the emissions of this outer layer are mostly from the CN A-X and CH A-X transitions. This outer layer can propagate and extinguish dynamically, similar to the flame front in combustion. Besides, the separation of this outer layer to the plasma core decreases with pressure. The layered structure and its dynamical behaviors can be explained by a plasma-sustained radical propagation mechanism. The high-power plasma column can produce a high-temperature zone with rich atomic species, surrounded by the relatively cold N2-CH4 mixture. At the mixing layer between the high-temperature zone and the N2-CH4 mixture, some highly exothermic reactions occur to produce excited CN and CH species, which emit their specific spectra. As the high-temperature zone expands with time, the outer layer propagates outward. However, with the propagation continuing, the radical species involved in the outer layer formation are rapidly consumed, and thus, this layer disappears when it propagates too far away from the plasma column.

The Titan Haze Simulation experiment on COSmIC: Probing Titan’s atmospheric chemistry at low temperature

The aim of the Titan Haze Simulation (THS) experiment is to contribute to a better understanding of aerosol formation in Titan’s atmosphere through the study of the chemical formation pathways that link the simpler gas phase molecules resulting from the first steps of the N2–CH4 chemistry, to the more complex gas phase precursors of aerosols; and more specifically, to investigate the role of polycyclic aromatic hydrocarbons (PAHs) and nitrogenated polycyclic aromatic hydrocarbons (PANHs), among other hydrocarbons, in this process. In the THS experiment developed at the NASA Ames Cosmic simulation facility (COSmIC), Titan’s atmospheric chemistry is simulated by a pulsed plasma jet expansion at temperature conditions (∼150K) close to those found in Titan’s atmosphere in regions where aerosols are formed. In addition, because of the very short residence time of the gas in the plasma discharge, only the initial steps of the chemistry occur, making the COSmIC/THS a unique tool to study the first and intermediate (when adding heavier precursors to the initial N2–CH4 mixture) steps of Titan’s atmospheric chemistry at low temperature as shown in the study presented here. We further illustrate the potential of COSmIC/THS for the simulation of Titan’s atmospheric chemistry by presenting very promising results from a preliminary comparison of the laboratory data to data from the Cassini Plasma Spectrometer–Ion Beam Spectrometer (CAPS–IBS) instrument.

New insights into the structure and chemistry of Titan’s tholins via13C and 15N solid state nuclear magnetic resonance spectroscopy

Tholins are complex C,N-containing organic compounds produced in the laboratory. They are considered to provide materials that are analogous to those responsible for the haze observed in Titan’s atmosphere. These compounds present an astrobiological interest due to their ability to release amino acids upon hydrolysis. Their chemical structure has been investigated using a large number of techniques. However, to date no detailed nuclear magnetic resonance (NMR) study has been performed on these materials despite the high potential of this technique for investigating the environment of given nuclei. Here 13C and 15N solid state NMR spectroscopy was applied to obtain new insights into the chemical structure of tholins produced through plasma discharge in gaseous N2CH4 mixtures designed to simulate the atmosphere of Titan. Due to the low natural abundance of these isotopes, a 13C and 15N-enriched tholin sample was synthesized using isotopically enriched gas precursors. Various pulse sequences including 13C and 15N single pulse, 1H13C and 1H15N cross-polarisation and 1H15N13C double cross-polarisation were used. These techniques allowed complete characterisation of the chemical and structural environments of the carbon and nitrogen atoms. The NMR assignments were supplemented and confirmed by ab initio electronic structure calculations for model structures and molecular fragments.

Emission of cyan upon excitation of nitrogen, air, and N2-CH4 mixture by discharge pulses in an inhomogeneous electric field

We have studied spectral and amplitude-time characteristics of the emission by nitrogen, air, and the N2-CH4 mixture upon excitation by nanosecond and microsecond high-voltage discharge pulses in an inhomogeneous electric field in the pressure range of 0.01–3 atm. In the pulsed and pulse-periodic discharge regimes, we have observed emission bands of the violet system of cyan, the transition B2Σ+ → X2Σ+, which were comparable in intensity with bands of the (2+) system of nitrogen. We show that, in the pulsed regime, the highest intensity of the violet system of cyan is observed in the N2-CH4 mixture. We show that, in the pulse-periodic discharge in nitrogen with a small amount of a carbon-containing admixture, upon contracting the discharge, the efficiencies of formation and emission of the violet system of cyan considerably increase. We confirm that admixtures of oxygen to nitrogen lead to suppression of the emission of the violet system of cyan molecules.

Capacitively coupled plasma used to simulate Titan's atmospheric chemistry

A complex chemistry in Titan's atmosphere leads to the formation of organic solid aerosols. We use a radio-frequency (RF) capacitively coupled plasma discharge produced in different N2–CH4 mixtures (from 0% to 10% of CH4) to simulate this chemistry. The work presented here was devoted to the study of the plasma discharge. In our experiment, the electron density is measured by the resonant cavity method and is about 1015 m−3 in pure N2 plasma at 30 W excitation RF power. It decreases by a factor of 2 as soon as CH4 is present in the discharge, even for a proportion as small as 2% of CH4. An optical emission spectroscopy diagnostic is installed on the experiment to study the evolution of the N2 bands and to perform actinometry measurements using Ar lines. This diagnostic allowed us to measure variations in the electron temperature and to show that a decrease in the density of the electrons can be compensated by an increase in their energy. We have also used an experimental setup where the plasma is tuned in a pulsed mode, in order to study the formation of dust particles. We observed variations in the self-bias voltage, the RF injected power and the intensities of the nitrogen bands, which indicated that dust particles were formed. The characteristic dust formation time varied, depending on the experimental conditions, from 4 to 110 s. It was faster for higher pressures and for smaller proportions of CH4 in the gas mixture.

CN(B 2Σ+) formation and emission in a N2–CH4 atmospheric pressure dielectric barrier discharge

We have investigated CN(B 2Σ+ → X 2Σ+) violet system emission and laser induced fluorescence in an atmospheric pressure pulsed dielectric barrier discharge and found a high degree of vibrational non-equilibrium in both CN(B, v) and CN(X, v′). The vibrational distributions depend strongly on the gas feed composition and on the discharge/post-discharge regime. Analysis of the time resolved laser and emission spectroscopy measurements leads to the conclusion that two main chemi-luminescent mechanisms are active in the CN(B) excitation. One is the C + N + M → CN(A, B) + M recombination, that is dominant in the post-discharge; the other one, active in the discharge, is probably the reaction N + CH → CN(A, B) + H. Both reactions give intense emission when the discharge is operated in a pure N2 gas feed in the presence of the surface deposit produced by a N2–CH4 mixture discharge. C and CH must then be produced starting from some intermediate species coming from the surface. When CH4 is added to the gas feed, the recombination reaction is strongly inhibited, while CH can be produced more easily from the methane contained in the gas phase. It is also likely that CN(X) is produced by the same reactions after radiative cascade from CN(A, B) states.

Spectroscopy study and modelling of an afterglow created bya low-pressure pulsed discharge in N2-CH4

Time-resolved emission spectroscopy is used to investigate therelaxation of N2(B 3Πg), N2(C 3Πu) and CN(B 2Σ)states in the time afterglow of a low-pressure N2-CH4 pulsed discharge,with time duration of 1 ms and in the range [CH4]/[N2] = 0-2%. Thedecays in the relative measured concentrations in the afterglow areinterpreted by modelling the relaxation of a set of time-varying kineticmaster equations for the various species produced in the discharge, withconditions at the beginning of the afterglow calculated from a time-dependentkinetic model for the pulsed discharge. It is observed that theN2(B 3Πg) state is populated in the afterglow mainly via thereaction N2(A 3Σu+) + N2(X 1Σg+, 5⩽v⩽14)→N2(B 3Πg) + N2(X 1Σg+, v = 0), sincethe pulse duration is large enough to populate theN2(X 1Σg+, v) levels at its end and, to a smaller extent, alsoby pooling of N2(A 3Σu+). The N2(C 3Πu) state ispopulated by pooling of N2(A 3Σu+) only, whereas theCN(B 2Σ) state is created through reactions involving eitherN2(A 3Σu+) states or N2(X 1Σg+, v) levels incollisions with CN(X 2Σ+) molecules. The agreement between measuredand calculated concentrations of N2(B 3Πg) and N2(C 3Πu)states is very good in pure N2 and it may be considered satisfactory in thecase of N2-CH4 mixtures, and for the CN(B 2Σ) state theagreement between theory and experiment is also reasonably good.

Shock-tube analysis of argon influence in Titan radiative environment

The effects of argon addition, in the range of 0-20% in a N2-CH4 mixture on the nonequilibrium radiation emitted behind a normal shock wave, have been investigated in a free-piston-driven shock tube. The intensity of spontaneous emission, for the B2S+ —» X 2S+ electronic transition of CN molecules, is measured at a shock velocity of 5700 m/s propagating in a 200-Pa test gas mixture. Rotational and vibrational temperature profiles in the shock layer are obtained by matching three spectral lines simultaneously recorded in the A*> = 0 band with theoretical spectra calculations. The results show that the nonequilibrium radiation overshoot weakly increases with argon addition, whereas the equilibrium intensity value is not affected. The characteristic relaxation time of radiation is also affected so as to be reduced by argon addition. The vibrational relaxation time for CN molecules is also determined from the temperature profiles, but the accuracy is difficult to assess since the temperatures are weakly dependent functions of ratios of intensities.

H2CN+ · nN2 clustering formation and the atmosphere of Titan

Measurements performed by Voyager 1 have revealed a dense atmosphere around Titan mainly composed of N2 with some CH4 and other minor components, including hydrocarbons and organic–nitrogen molecules1–4. Based on Voyager and some laboratory data, it is reasonable to assume that other complex organic–nitrogen molecules are formed in Titan's atmosphere5–7. Three-body association reactions are probably involved in the synthesis of such compounds5,7,8. It has been suggested5 that the clustering of H2CN+ with N2 or CH4 could have an important role in these reaction mechanisms. Recently,8 we observed the H2CN+ · N2 cluster ion in a N2–CH4 mixture irradiated by α particles. Here we report the results of laboratory measurements of equilibrium constants for the clustering of N2 with H2CN+ leading to H2CN+ ⋅ nN2 (n = 1–6) formation, which should help in understanding the ion–molecule chemistry in the atmosphere of Titan.

Collision Induced Microwave Absorption in N2, CH4, and N2–Ar, N2–CH4 Mixtures at 1.1 and 2.3 cm−1

Collision induced microwave absorption is reported in pure N2, N2–Ar, N2–CH4, mixtures, and in pure CH4 in the 35 and 70 GHz regions (1.1 and 2.3 cm−1) at a temperature of 22 °C. The measurements are accomplished using overmoded high Q cavities capable of pressurization of up to 5000 p.s.i.g. The apparatus and method are described. With the high sensitivity attained, the results in pure N2 from 30 → 250 amagat reveal terms in the square and cube of the density from which the relaxation times are calculated. The linear dependence on frequency of the collision induced absorption up to 2.3 cm−1 is established. Higher order dependence on the density is observed in the N2–Ar and N2–CH4 mixtures. Various estimates of the quadrupole moment of N2 are given, making use of earlier results in other frequency regions.

Measurement of the space charges produced in streamers

An investigation was made of the slow phase of cathode-directed streamers in gas discharges, which developed according to the streamer mechanism. A 3 cm gap was used with a uniform electric field and low overvoltages (about 20% above the DC breakdown voltage). All experiments were carried out in an N2-CH4 mixture (total about 100 Torr). By means of a probe electron avalanche, the ionic space charge which accumulates in the gap during the development of the streamer was measured. Streak pictures of the discharge were recorded simultaneously, so it was possible to show the correlation of the ionic space charge with the growth of the streamer.

The influence of secondary avalanches on the development of the cathode-directed streamer

Investigations have been made, with an electro-optical streak camera, of the low-current phases of gas discharges (3 cm gap, uniform electric field, and medium over-voltages of some 10% above the DC breakdown voltage). In N2-CH4 mixtures (pressure about 100 Torr) the gas discharge develops under these conditions according to the streamer mechanism. Interest was concentrated on the effect of secondary avalanches following the primary avalanche to see if they are necessary for the development of the cathode-directed streamer. It is shown that the cathode-directed streamer can start unsupported by successors. Further, it is demonstrated that an artificial successor very high carrier number accelerates the development.

Calculations of Midgap Breakdown in Gases

Calculations were made with an electronic computer for comparison with the measurements by Tholl of midgap breakdown in a N2–CH4 mixture. Tholl used a 3-cm gap, 400-Torr pressure, and an overvoltage of 20.7%. For the calculations, a one-dimensional Townsend avalanche model was used, which includes space charge but omits photoionization in the gas. The input data were adjusted to give agreement with Tholl's measured avalanche velocity and current-growth time constant in the absence of space charge. The initial effect of space charge was calculated to be a reduced current growth rate occurring concurrently with an increased avalanche velocity toward the anode, as noted by Tholl. The calculated values of these quantities for this current regime differ by a factor of ∼2 from the measured values, but this discrepancy could be due to the discharge channel expansion and to energy considerations. The later stage of current buildup is marked by a very rapid current buildup occurring at the time that the ``streamer'' propagates toward the cathode. Good agreement is obtained in this time regime. The cathode-directed streamer is shown to result from a secondary avalanche from the cathode. The moving junction between the primary and secondary avalanches is marked by a reduced channel diameter in Tholl's photographs. It is shown that this constriction of the discharge channel could not be caused by radial fields.