Atomic Resonance Absorption Spectrophotometry Research Articles

A High Temperature Chemical Kinetics Study of the Reaction: OH+Ar = H+O+Ar by Atomic Resonance Absorption Spectrophotometry

The OH radical dissociation has been studied behind shock waves in the temperature range of 2950-3700 K at total pressures of about 220-310 kPa by Atomic Resonance Absorption Spectroscopy using mixtures of H2 and O2 highly diluted in Ar. The OH decomposition was followed by monitoring the time dependent O concentration in the post shock reaction zone. In our experimental conditions this reaction is very sensitive to O-atom profiles. The rate constant k−2 of the termolecular recombination reaction between H, O and Ar as collision partner was calculated from OH dissociation rate constant k2 measurements and the equilibrium constant. The results expressed in the simple Arrhenius form are: with an uncertainty of ± 30 %, which are in good agreement with the estimate of Tsang and Hampson in 1986. No experimental results were reported by earlier investigators.

Rate Constants for the Thermal Dissociation of N2O and the O(3P) + N2O Reaction

The thermal dissociation of N2O in argon was investigated by monitoring the formation of O(3P) atoms in the reflected shock regime using atomic resonance absorption spectrophotometry (ARAS). The to...

Rate constants for the reactions H+O2→OH+O and D+O2→OD+O over the temperature range 1085–2278 K by the laser photolysis–shock tube technique

Rate constants for the reactions (1) H+O2→OH+O and (2) D+O2→OD+O have been measured over the temperature ranges 1103–2055 K and 1085–2278 K, respectively. The experimental method that has been used is the laser-photolysis–shock-tube technique. This technique utilizes atomic resonance absorption spectrophotometry (ARAS) to monitor H- or D-atom depletion in the presence of a large excess of reactant, O2. The results can be well represented by the Arrhenius expressions k1(T)=(1.15±0.16)×10−10 exp(−6917±193 K/T) cm3 molecule−1 s−1, and k2(T)=(1.09±0.20)×10−10 exp(−6937±247 K/T) cm3 molecule−1 s−1. Over the experimental temperature range, the present results show that the isotope effect is unity within experimental uncertainty. The Arrhenius equations, k−1(T)=(8.75±1.24) ×10−12 exp(1121±193 K/T) cm3 molecule−1 s−1 and k−2 (T)=(9.73±1.79)×10−12 exp(526±247 K/T) cm3 molecule−1 s−1, for the rate constants of the reverse reactions were calculated from the experimentally measured forward rate constants and expressions for the equilibrium constants that have been derived from the JANAF thermochemical database. The theoretical implications of the present results are also discussed.

High temperature reaction rate coefficients derived from N‐atom ARAS measurements and excimer photolysis of NO

AbstractMixtures of NO and NO/H2 in Ar were shock‐heated and photolyzed with an ArF excimer laser. Measurements in these experiments of N‐atom profiles using atomic resonance absorption spectrophotometry (ARAS) permitted the determination of two rate coefficients. The rate coefficient for the reaction equation image was found to be 4.29 × 1013 exp(−787/T) cm3 mol−1 sec−1 (±20% at 1400 K to ±10% at 3500 K). This is the first direct high temperature measurement of this rate coefficient in the exothermic direction. The rate coefficient for the reaction equation image was found to be 1.60 × 1014 exp(−12650/T) (±35% from 1950 to 2850 K). To our knowledge, this is the first direct measurement of this rate coefficient.A study of the N‐atom ARAS absorption behavior revealed a noticeable pressure dependence, as well as a weak temperature dependence, in the Beer‐Lambert law absorption coefficient. Proper consideration of these effects is important when the N‐atom ARAS diagnostic is used for absolute concentration measurements.

Stoßtwellenuntersuchungen zum Start der Reaktion CO + O2

AbstractBy use of the atomic resonance absorption spectrophotometry (ARAS) the rate coefficient of the reaction k1 = 8.4 · 10−11 · exp (‐31800 K/T)cm3 s−1has been determined by direct measurements of the time dependent concentrations of O atoms in the reaction zone behind reflected shock waves. With additional measurements of H atom concentrations the influence of catalysing impurity reactions can be determined. The experiments were done under very clean conditions in mixtures of argon with small relative concentrations of CO (500 to 10000 ppm) and of O2 (100 to 20000 ppm) in the temperature range 1700 K ≤ T ≤ 3500 K at total pressures of about 1.8 bar.

Thermal decomposition of hydrogen sulfide at low concentrations

The initial step of the thermal decomposition of hydrogen sulfide has been investigated by the direct measurement of the time-dependent H atom concentration behind reflected shock waves in the temperature range 1965°K ≤ T ≤ 2560°K, using the atomic resonance absorption spectrophotometry (ARAS). The experiments have been performed in mixtures of Ar with small concentrations of H 2S (25 to 200 ppm), at total pressures of about 1.8–2.0 bar, corresponding to concentrations of the order of (5–7) × 10 18 molecules cm −3. The rate constants for reactions (1) and (2). (1) H 2S + M → HS + H + M, (2) H 2S + H → HS + H 2, were found to be given by k1 = 7.7 × 10 −10 exp (−41,500 K/T) molecule −1 cm3 s −1, k2 = 1.8 X 10 −11 exp (−1500 K/T) molecule −1 cm3 s −1,

High temperature thermal decomposition of acetylene and diacetylene at low relative concentrations

The initial steps involved in the thermal decomposition of acetylene and diacetylene in the temperature range 1850 to 3000 K have been investigated by monitoring the hydrogen-atom production behind reflected shock waves in highly diluted mixtures of argon (5–400 ppm, total densities 6 × 10 −6 to 2.7 × 10 −5 mol/cm 3). The technique of atomic resonance absorption spectrophotometry (ARAS) was used to measure the H-atom concentration. In the low pressure region for unimolecular decomposition the initial step of the reaction can be characterized as: C 2 H 2+M → k 1 C 2 H+ H+M for which the rate coefficient k 1=(3.6±0.6)10 16 exp ( −E 1 RT ) cm 3 mol −1 s −1 E 1 R = 53600±400 K T:2100−3000 K M:8×10 −6−2.6×10 −5 mol cm −3 was obtained. The measurements also favor the recent value of 127 kcal/mol for the formation enthalpy of C 2H published by Okabe and Dibeler. In addition, it was possible to derive for the reaction C 4 H 2 → k 3 C 4 H+ H a rate coefficient k 3=(2.2±0.6)10 14 exp ( −E 3 RT ) s −1 E 3 R = 58700±700 K T:1850−2300 K M:1×10 −5−2.4×10 −5 mol cm −3 which is not far from the high pressure limit. It was also possible to derive rate coefficients in certain regimes of the investigated temperature interval at total densities around 10 −5 mol cm −3 for the following reactions: C 2 H 2+ C 2 H → k 2 C 4 H 2+ H k 2=3−4·10 13 cm 3 mol −1 s −1 T:2300−2700 K C 4 H → k 4 C 4+ H k 4=0.5−2.2·10 14 exp ( E 3 RT ) s −1 T:2100−2300 K C 2 H 2+ C 2 H 2 → k 6 C 4 H 2+2 H k 6=(3±1)·10 8 cm 3 mol −1 s −1 T:1850−2000 K C 4 H 2+ C 4 H → k 7 C 8 H 2+ H k 7=1.5−5.5·10 13 cm 3 mol −1 s −1 T:1850−2300 K C 4 H 2+ C 2 H → k 9 C 6 H 2+ H k 9=1−4·10 13 cm 3 mol −1 s −1 T:2400−2700 K

Production of hydrogen atoms during the thermal dissociation of ethylene between 1700 and 2200°K

The technique of atomic resonance absorption spectrophotometry (ARAS) was used to study the generation of H atoms during the thermal decomposition of C 2 H 4 . Highly diluted mixtures with relative concentrations between 20 and 800 ppm of C 2 H 4 in Ar were used. The experiments were performed behind reflected shock waves at temperatures between 1700 and 2200°K and densities between 6·10 18 to 1.6·10 19 particles/cm 3 . For reaction (1) C 2 H 4 + M →C 2 H 3 +H+ M we obtained the rate coefficient expression: k 1 =(3.8±1.3)·10 17 exp(− E 1 / R·T ) cm 3 /mol·sec E I / R =49 400±900°K Together with previous measurements for reaction (1a) C 2 H 4 + M →C 2 H 2 +H 2 + M which gave k 1a =(2.6±0.5)·10 17 ·exp(− E 1a / R·T ) cm 3 /mol·sec E 1a / R =39 900±500°K it became apparent that C 2 H 4 decays simultaneously via two reaction channels under the chosen conditions. In addition it was possible to derive approximate rate coefficients for reaction (3) H+C 2 H 4 →C 2 H 3 +H 2 k 3 ≅1.5·10 13 cm 3 /mol·sec; T =2000°K k 3 ≅5·10 12 cm 3 /mol·sec, T =1700°K

Shock tube studies of deuterium dissociation and oxidation by atomic resonance absorption spectrophotometry

The technique of Atomic Resonance Absorption Spectrophotometry (ARAS) was used to measure deuterium atom concentration profiles in the reaction zones behind both primary and reflected shock waves. Calibration of the ARAS technique over a wide range of D atom concentrations, 3×1011 to 3×1014 atom cm−3, enabled accurate rate measurements to be made of the dissociation reaction: D2+Ar→D+D+Ar, over the temperature range 1800 to 4000°K and yielded the Arrhenius expression kd(D2+Ar)=2.4×10−10exp (−93,400/RT) cm3 molecule−1 sec−1. The rates of other reactions which included the isotope exchange reactions: H2+D→HD+H and D2+H→HD+D, and the sequence of oxidation reactions: O2+D→ OD+O, D2+O→OD+D, and D2+OD→D2O+D, were also studied at temperatures in the range 1700 to 3100°K.

Shock-tube atom kinetics of nitric oxide decomposition

The rate of formation of ground-state atomic oxygen, O(3P), has been measured behind shock waves in NO−Ar mixtures by atomic resonance absorption spectrophotometry. After experimental calibration of the atomic absorption coefficient, rate measurements were made over a temperature range of 2600°–6300°K, for 0.1%, 1.0%, and 10.0% mixtures of NO in Ar. The specific reaction-rate constants were first calculated, assuming that the dissociation proceeds by a simple bimolecular NO−Ar collision process. On this basis, Arrhenius plots of the data for the mixtures containing the higher concentrations of NO exhibit, at lower temperatures, rate constants which are much greater than the constants corresponding to the lower concentrations. At the higher temperatures, the rate constants converge. A chemical mechanism was then developed and employed to reproduce, by means of a computer calculation, the results obtained experimentally. The experimental data have been explained in terms of six reactions, of which two have been verified as the initial, rate-limiting steps of the dissociation process. These are: NO+NO→N2O+O(3P) and NO+Ar→N+O(3P)+Ar, for which the specific rate-constant expressions k=2.35×1010 exp(−29,000/RT) and k=1.37×1014 exp(−148,400/RT), respectively, have been obtained. The first of these two reactions is dominant at low temperatures and high NO concentrations, while the second predominates at high temperatures and low concentrations.

Shock-Tube Kinetics by Atomic Absorption in the Vacuum Ultraviolet

Shock tube chemical kinetics of H dissociation by atomic resonance absorption spectrophotometry in far UV

Atom-Formation Rates behind Shock Waves in Hydrogen and the Effect of Added Oxygen

A direct isothermal measurement has been made of the rate of formation of atomic hydrogen behind shock waves in hydrogen–argon mixtures. This has been accomplished by using atomic resonance absorption spectrophotometry at the Lyman-α line of both 1H and 2H. The observations were made in an ultrahighpurity shock tube. Absorption coefficients for the atoms were determined experimentally. The hydrogen–argon dissociation rate is given by the expression kH2−Ar = 2.23 × 1012T1 / 2exp(−92 600 / RT) cm3mole−1·sec−1 over the temperature range 2290°–3790°K. The hydrogen molecule was found to be five times more effective than argon as a collision partner. Because of the sensitivity of the technique, it was also possible to determine quantitatively the catalytic effect of the addition of small amounts of oxygen. From these observations an expression kH−O2 = 6.0 × 1012T1 / 2exp(−17 750/RT) cm3mole−1·sec−1 was obtained for the reaction H + O2 = OH + O over the temperature range 1700°–2700°K.

Resonance Absorption Spectrophotometry of the Hydrogen Atom Behind Shock Waves

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