Absolute OH Research Articles

Effects of N2, CO2 and H2O dilutions on temperature and concentration fields of OH in methane Bunsen flames by using PLIF thermometry and bi-directional PLIF

The effects of N2, CO2 and H2O dilutions on the two-dimensional (2D) temperature and concentration fields of OH in methane Bunsen flames were investigated by employing two-line planar laser-induced fluorescence (PLIF) thermometry and bi-directional PLIF technique, respectively. The spatial resolution of the measured temperature and concentration fields of OH radical was 41.7μm. It was found that the measured temperature fields in methane Bunsen flames with different diluents exhibited a non-uniform and a symmetrical structure. In terms of decreasing the flame temperature, the CO2 diluent achieved a good result compared with N2 and H2O diluents. The total uncertainties in PLIF thermometry measurements were estimated to be ±8.44%, ±8.17% and ±8.27% for each diluent, respectively. The OH peak absorption cross sections at different heights were obtained in methane Bunsen flames with different diluents. Finally, the 2D absolute OH concentration filed in methane Bunsen flames diluted with three different diluents were obtained by using bi-directional PLIF technique. The peak absolute OH number densities in the flames with H2O, N2 and CO2 diluents were 9.84×1017cm−3, 8.57×1017cm−3 and 7.84×1017cm−3, with the overall uncertainties of ±20.56%, ±19.04% and ±23.43%, respectively. The numerical simulations of chemical kinetics were performed by using a physical model of one-dimensional laminar premixed flame and a detail methane reaction mechanism of GRI-Mech 3.0 so as to investigate the difference between the simulation results and experimental values. The comparison results for the temperature and OH concentration indicated that the reaction mechanism of GRI-Mech 3.0 might underestimate the chemical effects of CO2 and H2O diluents on the production of OH radical in premixed methane flames.

OH radical production in an atmospheric pressure surface micro-discharge array

The generation of OH radicals from an array of surface micro-discharges working in atmospheric pressure He/Ar/H2O mixtures is investigated. The absolute OH density and its temporal-and-spatial dynamics are detected by UV broadband absorption spectroscopy (UV-BAS) and laser-induced fluorescence (LIF) spectroscopy. The measured absolute density of OH(X) state is about 1021 m−3 in Ar/H2O mixture reaching a peak at 0.05% of H2O. In the case of He/H2O mixtures however, the peaking at ~1019 m−3 is approximately two orders of magnitude lower and decreases monotonously with increasing H2O content. From a control standpoint, the ratio of the Ar/He mixture may be adjusted to tune the OH density over two orders of magnitude and to modulate the H2O content dependence of the OH density. The capability of modulating the OH radical production over a large density range is of practical interest for many applications such as atmospheric chemistry and biochemistry. With the array of atmospheric micro-discharges sustained over a large electrode area, a uniform distribution of its OH density can be achieved in a plane parallel to the electrodes thus enabling spatially controlled surface treatment of large samples.

Model studies of short‐term variations induced in trace gases by particle precipitation in the mesosphere and lower thermosphere

AbstractThe 3‐D global chemistry and transport model (3dCTM) was used to investigate NO, OH, and O3 from January 2002 to May 2010 between 60 km and 133 km. Their daytime and nighttime mean zonal means (55°–75° geomagnetic latitude) were analyzed with respect to short‐term variations associated with particle precipitation. The corresponding ionization rates were derived from the 3‐D atmospheric ionization module Osnabrück (AIMOS), which is based on particle flux measurements. The trace gas variations with respect to their background were investigated by using a superposed epoch analysis. The 27 day signature associated with particle precipitation is found in NO, while it is only indicated in OH and O3 during winter. A varying solar spectrum associated with the 11 year solar cycle causes modifications of this signal up to 10%, while the main patterns are conserved. Published observations show a clear 27 day signal, qualitatively agreeing with the model results at altitudes >70 km except for O3 in Northern Hemisphere winter. Further differences occur with respect to the magnitude of the trace gas variations, primarily attributed to the different trace gas background and dynamical variations of the background atmosphere. Absolute OH variations are overestimated by the 3dCTM during winter, while the opposite is true for O3. These differences might originate from an unknown offset in AIMOS, incorrect chemical reaction rates, a different background of H2O and O3, and the model dynamics. However, their nonlinear relationship and their altitude of largest response are qualitatively captured in Southern Hemisphere winter.

Quantitative measurement of hydroxyl radical (OH) concentration in premixed flat flame by combining laser-induced fluorescence and direct absorption spectroscopy**Project supported by the National Natural Science Foundation of China (Grant No. 11272338), the Science and Technology on Scramjet Key Laboratory Funding, China (Grant No. STSKFKT 2013004), and the China Scholarship Council.

An accurate and reasonable technique combining direct absorption spectroscopy and laser-induced fluorescence (LIF) methods is developed to quantitatively measure the concentrations of hydroxyl in CH4/air flat laminar flame. In our approach, particular attention is paid to the linear laser-induced fluorescence and absorption processes, and experimental details as well. Through measuring the temperature, LIF signal distribution and integrated absorption, spatially absolute OH concentrations profiles are successfully resolved. These experimental results are then compared with the numerical simulation. It is proved that the good quality of the results implies that this method is suitable for calibrating the OH-PLIF measurement in a practical combustor.

Is it possible to deduce the ground state OH density from relative optical emission intensities of the OH(A2Σ+-X2Πi) transition in atmospheric pressure non-equilibrium plasmas?—An analysis of self-absorption

The measurement of absolute densities of reactive species and radicals such as OH is of growing interest for many plasma applications. In this paper, we extend the use of a self-absorption model for atomic emission spectroscopy to molecular emission spectroscopy. The proposed analysis of self-absorbed molecular emission spectra is a simple and inexpensive method to determine OH(X) densities and rotational temperatures compared to laser induced fluorescence. We compare the recorded absolute OH density in a non-equilibrium diffuse atmospheric-pressure RF glow discharge by this method with broadband UV absorption considering a number of rotational lines with J′ ⩽ 6.5, the detection limit of the line integrated OH(X) density with this method is of the order of 2 × 1019 m−2. The accuracy of the density is sensitive to the rotational temperature of the OH(A) state and the non-equilibrium rotational population distribution.

Absolute OH and O radical densities in effluent of a He/H2O micro-scaled atmospheric pressure plasma jet

The effluent of a micro-scaled atmospheric pressure plasma jet (μ-APPJ) operated in helium with admixtures of water vapor ( ppm) has been analyzed by means of cavity ring-down laser absorption spectroscopy and molecular beam mass spectrometry to measure hydroxyl (OH) radical densities, and by two-photon absorption laser-induced fluorescence spectroscopy to measure atomic oxygen (O) densities. Additionally, the performance of the bubbler as a source of water vapor in the helium feed gas has been carefully characterized and calibrated. The largest OH and O densities in the effluent of and , respectively, have been measured at around 6000 ppm. The highest selectivity is reached around 1500 ppm, where the OH density is at ∼63% of its maximum value and is 14 times larger than the O density. The measured density profiles and distance variations are compared to the results of a 2D axially symmetric fluid model of species transport and reaction kinetics in the plasma effluent. It is shown that the main loss of OH radicals in the effluent is their mutual reaction. In the case of O, reactions with other species than OH also have to be considered to explain the density decay in the effluent. The results presented here provide additional information for understanding the plasma-chemical processes in non-equilibrium atmospheric pressure plasmas. They also open the way to applying μ-APPJ with He/H2O as a selective source of OH radicals.

Absolute OH density measurements in an atmospheric pressure dc glow discharge in air with water electrode by broadband UV absorption spectroscopy

Spatially resolved absolute OH radical density measurements are performed in an atmospheric pressure glow discharge generated in ambient air with water electrode by broadband UV absorption spectroscopy. The radial distributions of OH density and gas temperature are obtained for the positive column, anode and cathode regions both for water-cathode and water-anode discharges. It is found that for both polarities of the water electrode the radial profiles of the ground state OH density and gas temperature are significantly broader than the total discharge emission intensity and the emission intensity originating from OH() only. Exceptional large OH densities exceeding 1023 m−3 are found. The OH kinetics are discussed in detail.

Surface charge dynamics and OH and H number density distributions in near-surface nanosecond pulse discharges at a liquid / vapor interface

The present work provides insight into surface charge dynamics and kinetics of radical species reactions in nanosecond pulse discharges sustained at a liquid-vapor interface, above a distilled water surface. The near-surface plasma is sustained using two different discharge configurations, a surface ionization wave discharge between two exposed metal electrodes and a double dielectric barrier discharge. At low discharge pulse repetition rates (~100 Hz), residual surface charge deposition after the discharge pulse is a minor effect. At high pulse repetition rates (~10 kHz), significant negative surface charge accumulation over multiple discharge pulses is detected, both during alternating polarity and negative polarity pulse trains. Laser induced fluorescence (LIF) and two-photon absorption LIF (TALIF) line imaging are used for in situ measurements of spatial distributions of absolute OH and H atom number densities in near-surface, repetitive nanosecond pulse discharge plasmas.Both in a surface ionization wave discharge and in a double dielectric barrier discharge, peak measured H atom number density, [H] is much higher compared to peak OH number density, due to more rapid OH decay in the afterglow between the discharge pulses. Higher OH number density was measured near the regions with higher plasma emission intensity. Both OH and especially H atoms diffuse out of the surface ionization wave plasma volume, up to several mm from the liquid surface. Kinetic modeling calculations using a quasi-zero-dimensional H2O vapor / Ar plasma model are in qualitative agreement with the experimental data. The results demonstrate the experimental capability of in situ radical species number density distribution measurements in liquid-vapor interface plasmas, in a simple canonical geometry that lends itself to the validation of kinetic models.

The 27 day solar rotational effect on mesospheric nighttime OH and O3 observations induced by geomagnetic activity.

Observations performed by the Earth Observing System Microwave Limb Sounder instrument on board the Aura satellite from 2004 to 2009 (2004 to 2014) were used to investigate the 27 day solar rotational cycle in mesospheric OH (O3) and the physical connection to geomagnetic activity. Data analysis was focused on nighttime measurements at geomagnetic latitudes connected to the outer radiation belts (55°N/S–75°N/S). The applied superposed epoch analysis reveals a distinct 27 day solar rotational signal in OH and O3 during winter in both hemispheres at altitudes >70 km. The OH response is positive and in‐phase with the respective geomagnetic activity signal, lasting for 1–2 days. In contrast, the O3 feedback is negative, delayed by 1 day, and is present up to 4 days afterward. Largest OH (O3) peaks are found at ~75 km, exceeding the 95% significance level and the measurement noise of <2% (<0.5%), while reaching variations of +14% (−7%) with respect to their corresponding background. OH at 75 km is observed to respond to particle precipitation only after a certain threshold of geomagnetic activity is exceeded, depending on the respective OH background. The relation between OH and O3 at 75 km in both hemispheres is found to be nonlinear. In particular, OH has a strong impact on O3 for relatively weak geomagnetic disturbances and accompanying small absolute OH variations (<0.04 ppb). In contrast, catalytic O3 depletion is seen to slow down for stronger geomagnetic variations and OH anomalies (0.04–0.13 ppb), revealing small variations around −0.11 ppm.

Effects of signal corrections on measurements of temperature and OH concentrations using laser-induced fluorescence

Temperature and OH concentrations derived from OH laser-induced fluorescence (LIF) are known to be susceptible to effects such as collisional quenching, laser absorption, and fluorescence trapping. In this paper, a set of analytical and easy-to-implement methods is presented for treating these effects. The significance of these signal corrections on inferred temperature and absolute OH concentration is demonstrated in an atmospheric-pressure, near-stoichiometric CH4-air flame stabilized on a Hencken burner, for laser excitation of both the A2Σ+←X2Π (0,0) and (1,0) bands. It is found that the combined effect of laser attenuation and fluorescence trapping can cause considerable error in the OH number density and temperature if not accounted for, even with A–X(1,0) excitation. The validity of the assumptions used in signal correction (that the excited-state distribution is either thermalized or frozen) is examined using time-dependent modeling of the ro-vibronic states during and after laser excitation. These assumptions are shown to provide good bounding approximations for treating transition-dependent issues in OH LIF, especially for an unknown collisional environment, and it is noted that the proposed methods are generally applicable to LIF-based measurements.

Time-resolved absolute OH density of a nanosecond pulsed discharge in atmospheric pressure He–H2O: absolute calibration, collisional quenching and the importance of charged species in OH production

The time-resolved OH density in a nanosecond pulsed filamentary discharge in an atmospheric pressure He–H2O(0.05%) mixture is measured using laser induced fluorescence. The lifetime of the excited OH(A) state is found to be strongly time dependent during the plasma pulse, with the shortest decay time occurring at the moment the plasma is switched off. The measured LIF intensity is corrected for this time-resolved quenching and calibrated using Rayleigh scattering. Time-resolved electron density , He (3S1) metastable density , gas temperature and optical emission are presented and used in the interpretation of the observed time dependence of the OH density. Based on these experimental data, it is shown that for the present discharge conditions, OH is mainly produced by charge transfer reactions to water followed by dissociative recombination of the water ion. In addition, two often encountered issues with the calibration of OH LIF are highlighted with an example.

On OH Density of an Atmospheric Pressure Plasma Jet by Laser-Induced Fluorescence

For room-temperature atmospheric pressure plasma jets (RT-APPJs), because of the gas flow, the OH density at a given position is different from the case without the gas flow. In this paper, an OH density decay model, which includes the main loss processes of OH and gas flow effect, is present. Based on the model, the simulation results have a very good agreement with the OH decay behavior measured by laser-induced fluorescence. The absolute OH density of the RT-APPJ is obtained when the best fit is achieved, which is about $2.4\times 10^{13}~{\rm cm}^{-3}$ at 5 mm away from the plasma jet nozzle and 1 $\mu{\rm s}$ after the discharge. In addition, to control the OH density, the effect of voltage polarity, applied voltage magnitude, pulse frequency, pulsewidth on the OH density is also investigated and discussed.

Ignition of hydrogen–air mixtures using pulsed nanosecond dielectric barrier plasma discharges in plane-to-plane geometry

Ignition of preheated (400–500K) H2–air mixtures at low pressures (80–100torr) excited by pulsed nanosecond dielectric barrier discharges is investigated through experiments and simulations. Time resolved absolute OH concentration and temperature data are obtained using Laser Induced Fluorescence (LIF) technique. Ignition is achieved in the decaying plasma after a burst of discharge pulses (repetition rate 10–40kHz), with the time delay inferred from sudden rise in OH* emission. One-dimensional simulations are performed to obtain information about the plasma generated radicals and heat release across the discharge gap. A plasma fluid formulation is used with ions and neutral species at gas temperature, and electrons in non-equilibrium. An accurate reduced chemistry mechanism is developed through sensitivity analysis to expedite the plasma simulations. The model predictions show excellent agreement with experimental measurements, validating the numerical framework and chemistry data. The input pulse energy and ignition characteristics are found to be highly sensitive to uncertainties in dielectric properties. Ignition delay exhibits a threshold-like dependence on input plasma energy, and increases steeply as the number of pulses in the burst is reduced. The nanosecond plasma assisted ignition is achieved through a two-step process. Firstly, the burst of discharge pulses produce a large pool of radicals and provide an average temperature rise of ∼1–2K/pulse. In the next step, if the temperature exceeds a threshold value of ∼700K, significant heat release from partial fuel oxidation is triggered. The process becomes self-sustaining and the temperature continues to rise even after the plasma source is switched off, accelerating the conventional H2–O2 chain branching pathways and leading to ignition. We provide conclusive evidence of large volume ignition with nanosecond plasma as opposed to thermal ignition at a hot-spot. Ignition is first observed at the center of the discharge gap, but the kernel expands rapidly to the entire volume, except near walls where heat losses keep the temperature low. It is demonstrated that the ignition occurs independently at different locations due to local plasma chemistry effects and heat transport does not play a significant role.

Absolute OH density measurements in the effluent of a cold atmospheric-pressure Ar–H2O RF plasma jet in air

Absolute OH densities are obtained in a radio-frequency-driven Ar–H2O atmospheric-pressure plasma jet by laser-induced fluorescence (LIF), calibrated by Rayleigh scattering and by UV broadband absorption. The measurements are carried out in ambient air and the effect of air entrainment into the Ar jet is measured by analyzing the time-resolved fluorescence signals. The OH densities are obtained for different water vapor concentrations admixed to the Ar and as a function of the axial distance from the nozzle. A sensitivity analysis to deduce the accuracy of the model-calculated OH density from the LIF measurement is reported. It is found that the UV absorption and the LIF results correspond within experimental accuracy close to the nozzle and deviate in the far effluent. The possible reasons are discussed. The OH densities found in the plasma jet are in the range (0.1–2.5) × 1021 m−3 depending on the water concentration and plasma conditions.

Measurements of temperature and hydroxyl radical generation/decay in lean fuel–air mixtures excited by a repetitively pulsed nanosecond discharge

OH Laser Induced Fluorescence (LIF) and picosecond (ps), broadband Coherent Anti-Stokes Raman Spectroscopy (CARS) are used for time-resolved temperature and time-resolved, absolute OH number density measurements in lean H2-air, CH4-air, C2H4-air, and C3H8-air mixtures in a nanosecond (ns) pulse discharge cell/plasma flow reactor. The premixed fuel–air flow in the reactor, initially at T0=500K and P=100torr, is excited by a repetitive ns pulse discharge in a plane-to-plane geometry (peak voltage 28kV, discharge gap 10mm, estimated pulse energy 1.25mJ/pulse), operated in burst mode at 10kHz pulse repetition rate. In most measurements, burst duration is limited to 50 pulses, to preclude plasma-assisted ignition. The discharge uniformity in air and fuel–air flows is verified using sub-ns-gated images (employing an intensified charge-coupled device camera). Temperatures measured at the end of the discharge burst are in the range of T=550–600K, using both OH LIF and CARS, and remain essentially unchanged for up to 10ms after the burst. Time-resolved temperature measured by CARS during plasma-assisted ignition of H2-air is in good agreement with kinetic model predictions. Based on CARS measurement, vibrational nonequilibrium is not a significant factor at the present conditions.Time-resolved, absolute OH number density, measured after the discharge burst, demonstrates that OH concentration in C2H4-air, C3H8-air, and CH4 is highest in lean mixtures. In H2-air, OH concentration is nearly independent of the equivalence ratio. In C2H4-air and C3H8-air, unlike in CH4-air and in H2-air, transient OH-concentration overshoot after the discharge is detected. In C2H4-air and C3H8-air, OH decays after the discharge on the time scale of ∼0.02–0.1ms, suggesting little accumulation during the burst of pulses repeated at 10kHz. In CH4-air and H2-air, OH concentration decays within ∼0.1–1.0ms and 0.5–1.0ms, respectively, showing that it may accumulate during the burst.The experimental results are compared with kinetic modeling calculations using plasma/fuel chemistry model employing several H2-air and hydrocarbon-air chemistry mechanisms. Kinetic mechanisms for H2-air, CH4-air, and C2H4-air developed by A. Konnov provide the best overall agreement with OH measurements. In C3H8-air, none of the hydrocarbon chemistry mechanisms agrees well with the data. The results show the need for development of an accurate, predictive low-temperature plasma chemistry/fuel chemistry kinetic model applicable to fuels C3 and higher.

Absolute OH density determination by laser induced fluorescence spectroscopy in an atmospheric pressure RF plasma jet

In this paper, the ground state OH density is measured in high pressure plasma by laser-induced fluorescence (LIF) spectroscopy. The OH density determination is based on the simulation of the intensity fraction of fluorescence from the laser-excited level of OH (A) in the total detected LIF signal. The validity of this approach is verified in an atmospheric pressure Ar + H2O plasma jet sustained by a 13.56 MHz power supply. The transition line P1 (4) from OH (A,v′ = 1,J′ = 3) → OH (X,v′′ = 0,J′′ = 4) is used for the LIF excitation. The absolute OH density is determined to be 2.5 × 1019 m-3 at 1 mm away from the jet nozzle. It corresponds to a dissociation of 0.06% of the water vapor in the working gas. Different mechanisms of OH (X) production in the core of the plasma jet are discussed and analyzed.

OH radical and temperature measurements during ignition of H2-air mixtures excited by a repetitively pulsed nanosecond discharge

Ignition delay time, time-resolved temperature, and time-resolved absolute OH concentration are measured in mildly preheated H2–air mixtures (T=473–500K), excited by a repetitive nanosecond pulse discharge in a plasma flow reactor. The measurements are done in decaying plasma after the discharge pulse burst (pulse repetition rate 10–40kHz). Ignition delay increases steeply as the number of pulses in the burst is reduced, exhibiting threshold-like behavior. Temperature and OH concentration during the ignition process are measured by two-line OH LIF, as functions of time delay after the discharge burst. Absolute calibration is done using a near-adiabatic flame. The results show that the temperature at the end of the discharge burst is 700–750K, rising to 1500–1600K within a few ms after the burst, indicative of ignition. During ignition, OH concentration increases by a factor of 20–50, from (0.5–1.1)×1014cm−3 at the end of the burst to peak value of (2.3–2.6)×1015cm−3. Kinetic modeling calculations show good agreement with the experimental results, demonstrating quantitative insight into kinetics of plasma-assisted ignition. Modeling calculations demonstrate that ignition is induced primarily by accumulation of H atoms and gradual temperature rise in the discharge, at temperatures significantly lower than autoignition temperature, by up to 200K.

Measurement of OH concentration profiles by laser diagnostics and modeling in high-pressure counterflow premixed methane/air and biogas/air flames

Detailed kinetic mechanisms validated under atmospheric or low-pressure flame conditions cannot generally be directly extrapolated toward high pressure, due to the lack of experimental data or to uncertainties concerning the rate constants of elementary reactions. It is thus necessary to complete the experimental database and to extend the validation domain of combustion mechanisms at high pressure. In this work, OH concentration profiles were measured in high-pressure methane/air and biogas/air laminar premixed counterflow flames by linear laser-induced fluorescence at different equivalence ratios (0.7–1.2) and pressures (0.1–0.7MPa). Each flame was calibrated in absolute OH concentration by a combination of laser absorption and planar laser-induced fluorescence measurements. Experimental results were compared with simulations using the OPPDIF code and three detailed kinetic mechanisms: two versions of the Gas Research Institute Mechanism, GRI-Mech 2.11 and GRI-Mech 3.0, and the recently updated GDFkin®3.0_NCN mechanisms. Results show that the flame front positions are very well predicted by modeling with the three mechanisms for lean and stoichiometric CH4/air flames and stoichiometric CH4/CO2/air flames. However, discrepancies appear at a higher equivalence ratio (Φ=1.2) for the CH4/air flames and at a lower equivalence ratio (Φ=0.7) for the CH4/CO2/air flames. OH mole fractions are quantitatively well predicted at high pressure in all cases, while systematic overestimation by modeling is observed at atmospheric pressure. A kinetic analysis of the results is also presented.

Absolute OH density measurements by broadband UV absorption in diffuse atmospheric-pressure He–H2O RF glow discharges

The measurement of radical densities in atmospheric-pressure plasmas has gained increasing importance in recent years in view of their crucial role in many applications. In this paper we present absolute OH density measurements by broadband UV absorption in diffuse atmospheric-pressure RF glow discharges in mixtures of He and H2O. The use of a 310 nm light-emitting diode as a light source and a very high resolution spectrometer (2.6 pm resolution) made the estimation of the total OH density possible by simultaneously measuring the absorption rates of different spectrally resolved rotational lines of the OH(A–X) transition. For different RF powers and water concentrations, OH densities and gas temperatures ranging between 6 × 1019and 4 × 1020 m−3 and 345 and 410 K, respectively, were obtained. The gas temperature Tg was also measured by three different methods. Tg deduced from the rotational temperature of N2(C–B) emission, nitrogen being present as a trace impurity, provided the most reliable value. The rotational temperature Tr of the ground state OH(X) presented values with a maximum deviation of 25 K compared with Tg. To obtain the gas temperature from the emission intensities of OH(A–X) rotational lines, the recorded intensities of different lines must be corrected for the effect of self-absorption inside the plasma.

Absolute OH concentration profiles measurements in high pressure counterflow flames by coupling LIF, PLIF, and absorption techniques

A high-pressure combustion chamber enclosing counterflow burners was set-up at ICARE-CNRS laboratory. It allows the stabilization of flat twin premixed flames at atmospheric and high pressure. In this study, lean and stoichiometric methane/air counterflow premixed flames were studied at various pressures (0.1 MPa to 0.7 MPa). Relative OH concentration profiles were measured by Laser Induced Fluorescence. Great care was attached to the determination of the fluorescence signal by taking into account the line broadening and deexcitation by quenching which both arise at high pressure. Subsequently, OH profiles were calibrated in concentration by laser absorption technique associated with planar laser induced fluorescence. Results are successfully compared with literature. The good quality of the results attests of the experimental set-up ability to allow the study of flame structure at high pressure.