N2 Buffer Gas Research Articles

Kinetics of the Reaction NH2(X̃2B1, v2 = 0 and 1) + NO

The rate constants for reactions NH2(X̃2B1, v2 = 0 and 1) + NO have been determined at 298 ± 1 K using the pulsed UV photolysis/pulsed laser-induced fluorescence (LIF) technique. NH3 was photolyzed with an ArF excimer laser (193 nm), and NH2(X̃2B1, v2 = 0 and 1) were detected by LIF in the Ã2A1−X̃2B1 system. It has been found that CF4 is efficient at vibrational relaxation of NH2, and the rate constant for deactivation of NH2(X̃2B1, v2 = 1) by CF4 has been determined to be [3.2 ± 0.5(2σ)] × 10-11 cm3 molecule-1 s-1. Fast vibrational relaxation of NH2 by CF4 prior to the reaction with NO permits us to obtain an accurate rate constant for NH2(X̃2B1, v2 = 0) + NO. The rate constant measured in N2 buffer gas at 1 Torr is [2.05 ± 0.1(2σ)] × 10-11 cm3 molecule-1 s-1. The sources of discrepancy in previously reported room-temperature rate constants have been discussed on the basis of a survey of the experimental conditions and the methods of analysis. The overall rate constant for NH2(X̃2B1, v2 = 1) + NO has been also obtained (without CF4) to be [2.6 ± 0.4(2σ)] × 10-11 cm3 molecule-1 s-1.

Line Broadening and Shifting Studies of the J=5←4 Transition of Carbon Monoxide Perturbed by CO, N2, and O2

The collisional relaxation of the J=5←4 rotational transition of CO induced by carbon monoxide, nitrogen, and oxygen has been studied at room temperature. Pressure-broadening parameters were determined as 3.29(2), 2.61(2), and 2.30(2) MHz/Torr for CO, N2, and O2 buffer gases, respectively. Experimental deviations from the Voigt line shape profile have been observed which are mostly the effect of a narrowing in the spectral line core. The difference between the model profile and the experimental profile is less than 0.5% of the maximum line amplitude in the investigated pressure range 0.2–5 Torr. In addition, a small positive collision-induced shift of the line center frequency was observed for the pure gas, corresponding to a pressure self-shift parameter of 6(3) kHz/Torr.

Carbon- and CxN-coated iron nanocrystals synthesized by a modified AC arc method in N2 buffer gas

Carbon- and CxN-coated iron nanocrystals synthesized by a modified AC arc method in N2 buffer gas

Rb-Xe spin relaxation in dilute Xe mixtures

We present measurements of spin-relaxation of Rb atoms in He(98%)-N2(1%)-Xe(1%) mixtures similar to those used in magnetic resonance imaging polarizers. The pressure dependence allows us to separate out contributions from Rb-Xe van der Waals molecules and binary collisions. For the first time, we observe the predicted increase in the molecular contribution at high pressure. Our data suggest that the deduced molecular breakup rate has a strong temperature dependence. The realization of magnetic resonance imaging of human subjects using hyperpolarized noble gases @1# has stimulated the intense development of Xe-Rb spin-exchange optical pumping @2# in high pressure buffer gases @3‐5#. The need to minimize the mismatch between the Rb absorption linewidth and the linewidth of inexpensive, high power but broadband diode laser sources requires pressure broadening the Rb resonance lines with many atmospheres of He gas, chosen for its weak spin-relaxation properties @6#. Despite the widespread use of high pressure He for Xe-Rb spin exchange, there exist no systematic studies of spin relaxation and spin exchange rates under such conditions, as most of the pioneering work in Xe spin-relaxation and spin-exchange either had no buffer gas @7# or used N2 buffer gas @8#. Two recent experiments report relaxation rates measured at a single pressure and temperature @4,5#. In this paper we report measurements of spin-relaxation rates at 80 and 150 °C, in

Kinetics of OH reactions with N2H4, CH3NHNH2and (CH3)2NNH2in the gas phase

AbstractThe gas phase reaction kinetics of OH with three di‐amine rocket fuels—N2H4, CH3NHNH2, and (CH3)2NNH2—was studied in a discharge flow tube apparatus and a pulsed photolysis reactor under pseudo‐first‐order conditions in [OH]. Direct laser‐induced fluorescence monitoring of the [OH] temporal profiles in a known excess of the [diamine] yielded the following absolute second‐order OH rate coefficient expressions;k1= (2.17 ± 0.39) × 10−11e(160±30)/T,k2= (4.59 ± 0.83) × 10−11e(85±35)/Tandk3= (3.35 ± 0.60) × 10−11e(175±25)/Tcm3molec−1s−1, respectively, for reactions with N2H4, CH3NHNH2and (CH3)2NNH2in the temperature range 232–637 K. All three reactions did not show any discernable pressure dependence on He or N2buffer gas pressure of up to 530 torr. The magnitude of the weak temperature and the lack of pressure effects of the OH + N2H4reaction rate coefficient suggest that a simple direct metathesis of H‐atom may not be important compared to addition of the OH to one of the N‐centers of the diamine skeleton, followed by rapid dissociation of the intermediate into products. Our findings on this reaction are qualitatively consistent with a previousab initiostudy [3]. However, in the alkylated diamines, direct H‐abstraction from the methyl moiety cannot be completely ruled out. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 354–362, 2001

Gas Phase Reaction Kinetics of O Atoms with (CH3)2NNH2, CH3NHNH2, and N2H4, and Branching Ratios of the OH Product

The gas-phase reaction kinetics of O atoms with the two alkylated diamine rocket fuels, (CH3)2NNH2 and CH3NHNH2, was studied in a discharge flow-tube apparatus under pseudo-first-order conditions in [O atom]. Direct vuv cw-resonance fluorescence monitoring of the [O atom] temporal profiles in a known excess of the [diamine] yielded the following absolute second-order O atom rate coefficient expressions: k1 = (1.94 ± 0.34) × 10-11e(25±25)/T and k2 = (2.29 ± 0.40) × 10-11 e(-145±40)/T cm3 molecule-1 s-1, respectively, for reactions with (CH3)2NNH2 and CH3NHNH2 in the temperature range 232−644 K and in He pressure of 2.0 Torr. The total yields of OH in the reactions were measured to be (0.12 ± 0.09) and (0.14 ± 0.10) at 298 K and in 2.0 Torr He pressure. Close to ∼ 53% and ∼ 59% of the OH produced was estimated to be vibrationally excited. A pulsed-photolysis reactor was used to extend our measurements on the O atom reaction kinetics with the unsubstituted rocket fuel, N2H4 that we had previously studied in the flow-tube apparatus. At 298 K, both the rate coefficient, k3 = (0.59 ± 0.12) × 10-11 cm3 molecule-1 s-1 and the total OH yield = (0.35 ± 0.14) did not show any discernible dependence on He or N2 buffer gas pressures of up to 404 Torr. The magnitude of, the weak temperature dependence and the lack of pressure effect in the O + N2H4 reaction rate coefficient suggests that simple direct metathesis of H atom may not be important compared to initial addition of the O atom to the diamine, followed by rapid dissociation of the intermediate into a variety of products.

Theoretical Study on NH3 Superradiant Miniature Optically Pumped Far-Infrared Laser with Buffer Gas

A miniature superradiant NH3 optically pumped far-infrared laser (OPFIRL) with buffer gas N2 was theoretically studied. Base on the mechanism model of buffer gas, synthetical relaxation time was calculated. By solving the semi-classic density matrix equations, spectra of NH3 superradiant OPFIRL with buffer gas N2 was calculated, also the operating gas pressure. It was found that by adding buffer gas, FIR power could be raised, and there existed an optimum ratio of gases mixture and an optimum operating gas pressure which was higher than that of pure NH3 operating.

Kinetics of the OH‐initiated oxidation of isoprene

Absolute rate coefficients have been determined for the reaction OH + C5H8 (1) and two of its isotopomeric variants. Reaction (1) was studied as a function of temperature and pressure in N2, N2/O2 and He buffer gases. At room temperature, a rate coefficient, k1, of (8.56±0.26)*10−11 cm³ s−1 independent of pressure and buffer gas identity was obtained. An Arrhenius fit to data over the temperature range 250 – 340 K gave a negative activation energy of −666±150 cal/mol. HO2 production was inferred from observations of radical regeneration in the presence of NO. The absence of a kinetic isotope effect, together with the specificity of radical regeneration, indicates that reaction proceeds via an addition channel with no significant abstraction component. Our rate coefficient for reaction (1) is 15% lower than the value currently recommenced for atmospheric modeling.

Effects of Buffer Gas on Spectra of Optically Pumped Far-Infrared Cavity Laser

By developing a mechanism model of buffer gas, the comprehensive relaxation time of laser operation substance was calculated. By means of solving the density matrix equations, the effects of buffer gas N2 on the spectrum characteristics of NH3 FIR laser line with buffer gas N2 were studied. It was found that by adding proper buffer gas, FIR laser output power could be increased, and spectra widened because of the molecular collision. This work made it possible to develop high output and wide range tunable FIRL.

Rate Coefficients for the Propargyl Radical Self-Reaction and Oxygen Addition Reaction Measured Using Ultraviolet Cavity Ring-down Spectroscopy

By using 193 nm laser photolysis and cavity ring-down spectroscopy to produce and monitor the propargyl radical (CH2CCH), the self-reaction and oxygen termolecular association rate coefficients for the propargyl radical were measured at 295 K between total pressures of 300 Pa and 13300 Pa (2.25 and 100 Torr) in Ar, He, and N2 buffer gases. The rate coefficients obtained by simple second-order fits to the decay data were observed to vary with the photolytic precursors: allene, propargyl chloride, and propargyl bromide. By using a numerical fitting routine and more comprehensive mechanisms, a distinct rate coefficient for the self-reaction was determined, k∞(C3H3+C3H3) = (4.3 ± 0.6) × 10-11 cm3 molecule-1 s-1 at 295 K. This rate coefficient, which is a factor of 2.8 times slower than reported previously, was independent of total pressure and buffer choice over the entire pressure range. Other rate coefficients derived during the modeling included k(C3H3+H 665 Pa He) = (2.5 ± 1.1) × 10-10 cm3 molecule-1 s-1...

The Optimum Operation of an Unified Mini-Optically Pumped NH 3 Submillimeter Wave Laser

An unified miniature optically pumped NH3 submillimeter wave laser (OPSMMWL), including a mini TEA-CO2 pump laser, was developed. It lased successfully with coherent emission at 67.2μm, 90.4μm and 151.5μm. The optimum operation of the unified mini-OPSMMWL was studied experimentally and the relations among SMMW laser output power, operating gas pressure, length and coupling condition of the cavity were measured. It has been found that buffer gas N2 has significant effect on 67.2μm and 151.5μm emissions and very wide band SMMW laser emission was a common feature of the mini-OPSMMW cavity laser.

Effects of buffer gas N2 on miniature optically pumped NH3 submillimeter wave cavity laser emission at 67.2 μm

The effect of buffer gas N2 on miniature optically pumped NH3 submillimeter wave (SMMW) cavity laser emission at 67.2μm was studied theoretically and experimentally. It has been found that: 1. N2 had a positive effect on laser output of 67.2μm, 2. sharp absorption in laser spectrum under certain gases mixture pressure corresponded to the threshold of 'bottleneck effect”.

Laser Flash Photolysis Studies of Radical−Radical Reaction Kinetics: The HO2 + BrO Reaction

Laser flash photolysis of Cl2/CH3OH/O2/Br2/O3/N2 mixtures at 308 nm has been coupled with simultaneous time-resolved detection of HO2 (by infrared tunable diode laser absorption spectroscopy) and BrO (by ultraviolet absorption spectroscopy) to investigate the kinetics of the important stratospheric reaction HO2 + BrO → products at 296 ± 3 K in N2 buffer gas at pressures of 12 and 25 Torr. All experiments were performed under near pseudo-first-order conditions with HO2 in excess over BrO. The HO2 + BrO rate coefficient is found to be k1 = (2.0 ± 0.6) × 10-11 cm3 molecule-1 s-1, with the primary source of uncertainty being knowledge of the infrared line strength(s) required to convert measured HO2 absorbances to absolute concentrations. The rate coefficient for the reaction HO2 + HO2 → H2O2 + O2 derived based on infrared absorption measurements of the HO2 concentration is consistent with the currently accepted value. The results reported in this study are compared with other recent studies of HO2 + BrO kinetics, and their implications for our understanding of stratospheric chemistry are discussed.

Kinetic Studies of Negative Ion Reactions in a Quadrupole Ion Trap: Absolute Rate Coefficients and Ion Energies

A new apparatus consisting of an ion−molecule reactor coupled to a quadrupole ion trap is described. The kinetics and products of the rapid reactions of SF6- with SO2, HNO3, and HCl in the ion trap are reported. There is excellent agreement between the ion trap rate coefficients and yields, and flowing afterglow results. Rate coefficients for reactions of products of the SF6- reactions are also reported. Measurements of the equilibrium constant for the reaction NO3-·HNO3 + HNO3 ⇄ NO3-·(HNO3)2 in the ion trap provide a very sensitive measure of the energy of the trapped ions. These measurements indicate that the effective temperature of the trapped NO3-·(HNO3)2 ion is 320 ± 30 K for an ion trap at 298 K (1−2 mTorr He, qz = 0.074). The presence of 0.2 mTorr N2 buffer gas increases the internal temperature of the trapped ions by about 10 K. The internal temperature is also a weak function of the trapping potential, increasing by about 10 K over the range 0.074 < qz < 0.286.

Multinozzle gas-dynamic molecular-beam source

The results of an experimental investigation of a multinozzle gas-dynamic molecular-beam source, in which the working material is introduced directly into a supersonic jet of a buffer gas, are presented. The source is designed to create slow cold beams of atoms and molecules. Time-of-flight profiles of the beams of the N2 buffer gas and the working material SF6 formed by a source with six nozzles are presented. The measured parameters of the flow field of the buffer gas are compared with the results of calculations for an axisymmetric nozzle with an inner body having an annular critical cross section. The results obtained show that multinozzle designs can be used, in principle, in molecular-beam sources instead of axisymmetric nozzles with an inner body. This permits relaxation of the requirements placed on the accuracy needed in fabricating such sources, reduction of the buffer gas flow rate, and the employment of fairly simple schemes for recycling the buffer gas.

Optimized operation of optically pumped NH3 laser emission at 12.08 µm and 12.81 µm

Based on the semi-classical density matrix equations, optimized operation of optically pumped NH3 MIR laser emission at 12.08µm and 12.81µm was studied theoretically and experimentally. The effect of pump power, gas pressure and buffer gas N2 on MIR output power were discussed in detail.

Reduced angular dependence for degenerate four-wave mixing in potassium vapor by including nitrogen buffer gas

Degenerate four-wave mixing in alkali vapors exhibits a strong dependence on the angle between the forward pump and probe beams, due to atomic motion. We show that this angular dependence is dramatically decreased by including several hundred Torr of N2 buffer gas in a potassium vapor cell. The angle at which the signal has decreased by half is 250 mrad, an improvement of over 50 times the angle for potassium vapor without buffer gases.

Laser flash photolysis studies of radical–radical reaction kinetics: The O(3P J)+BrO reaction

A novel dual laser flash photolysis-long path absorption-resonance fluorescence technique has been employed to study the kinetics of the important stratospheric reaction O(3PJ)+BrO→k1Br(2PJ)+O2 as a function of temperature (231–328 K) and pressure (25–150 Torr) in N2 buffer gas. The experimental approach preserves the principal advantages of the flash photolysis method, i.e., complete absence of surface reactions and a wide range of accessible pressures, but also employs techniques which are characteristic of the discharge flow method, namely chemical titration as a means for deducing the absolute concentration of a radical reactant and use of multiple detection axes. We find that k1 is independent of pressure, and that the temperature dependence of k1 is adequately described by the Arrhenius expression k1(T)=1.91×10−11 exp(230/T) cm3 molecule−1 s−1; the absolute accuracy of measured values for k1 is estimated to vary from ±20% at T∼230 K to ±30% at T∼330 K. Our results demonstrate that the O(3PJ)+BrO rate coefficient is significantly faster than previously ‘‘guesstimated,’’ and suggest that the catalytic cycle with the O(3PJ)+BrO reaction as its rate-limiting step is the dominant stratospheric BrOx odd-oxygen destruction cycle at altitudes above 24 km.

Thermal electron attachment to C6F5X and C6H5X (X=I, Br, Cl, and F)

Rate constants have been measured for thermal electron attachment to C6F5X (X=I, Br, Cl, F, and H) and C6H5X (X=I, Br, Cl, and F) at room temperature in N2 buffer gas (1–100 Torr) using the pulse-radiolysis microwave cavity method. For all the compounds studied, the rate constants are of the two-body type. Unexpectedly, the values for C6F5X except C6F5H are all the same (∼2×10−7 cm3 molecule−1 s−1), which are higher than most of the previous values, while that for C6F5H, measured in Xe and Ar buffer gases, is very low (7×10−12 cm3 molecule−1 s−1). For C6H5X, the value decreases dramatically with varying X from I to Br to Cl as 1.0×10−8 to 6.5×10−12 to 3×10−14 cm3 molecule−1 s−1, and that for C6H5F must be much lower than 10−13 cm3 molecule−1 s−1. These results for the magnitude of the rate constant are rationalized by the variation in the energy of a transient negative-ion state of each molecule, which results from a combination of the electron affinities of constituents (halogen atom X and C6F5 radical) and the strength of the C6F5–X (or C6H5–X) bond.

Diode laser spectroscopy of methane overtone transitions

With the aid of commercial room-temperature AlGaAs diode lasers, frequency modulation absorption spectroscopy was performed on the 7900 A and 8600 A rovibrational combination overtone bands of methane. Three weak transitions are reported in the range around 8610 A that, to our knowledge, have not yet been observed and measured. Self-broadening and pressure-broadening coefficients of one ofthese new absorption features (at 8608.93 A) were derived from CH(4) and for CH(4) immersed in N(2) and He buffer gases. An evaluation of the methane detection sensitivity is given for favorable laboratory conditions as well as for an open-path situation.