Michael J. Kurylo

Detection of free radicals by an intracavity dye laser technique

A new technique for the detection of free radicals inside the cavity of a dye laser is described. This intracavity absorption phenomenon has two important advantages: (1) It has the potential for quantitative detection suitable for kinetic studies of transient chemical species and (2) It has a high degree of sensitivity. In the present work, the technique is shown to be at least as sensitive as, and most probably several orders of magnitude more sensitive than, previous methods for the detection of transients. It is presently a powerful tool for obtaining high resolution absorption spectra as well as obtaining precise information about the energy distribution of transient species produced photolytically or kinetically. Spectra for both NH2 and HCO (produced flash photolytically) are presented.

Absolute Rate Constants for the Reaction of Atomic Oxygen with Ethylene over the Temperature Range 232–500°K

Rate constants for the reaction of atomic oxygen with ethylene were measured over a temperature range of 232–500°K using the flash photolysis‐resonance fluorescence technique. The rate constant at room temperature was also determined using a flash photolysis‐kinetic absorption spectroscopy system and a discharge‐flow system coupled to a mass spectrometer. Within the experimental errors of the three techniques, good agreement was found for the rate constant at 298°K. The bimolecular rate constant was also found invariant to changes in both total pressure and reactant concentration. Over the temperature range of the experiments, the rate data could be fitted by a simple Arrhenius expression of the form, k=5.42± 0.30× 10−12 exp[(−1130± 32 cal mole−1)/RT]cm3molecule−1· sec−1.

Absolute Rates of the Reactions H+C2H4 and H+C2H5

The hydrogen atom–ethylene system was studied at 298°K employing the methods of resonance fluorescence and absorption by hydrogen atoms of Lyman α radiation at 1216 A. The contribution of hydrogen atom–radical reactions was evaluated under varying experimental conditions, and the rate of disappearance of H atoms in ethylene was measured under conditions where stoichiometric corrections became significant. Measurements in the literature of reaction rates for H+C2H4 at low total pressure are now in good agreement; however the limiting high‐pressure absolute rate constants thus far reported differ depending on the assignment of stoichiometric factors. Our results indicate that stoichiometric factors obtained under low‐pressure conditions may not be applicable to high pressure. Furthermore, extrapolations based on plots of inverse rate constant vs inverse pressure may be in error due to significant curvature in such plots. Our high‐pressure limiting rate constant for H+C2H4, extrapolated from data at pressure...

Flash photolysis resonance fluorescence investigation of the reactions of OH radicals with OCS and CS2

Abstract Rate constants for the reaction of OH radicals with OCS and CS2 have been determined at 296 K using the flash photolysis resonance fluorescence technique. The values derived from this study are kOH + OCS = (5.66 ± 1.21) × 10−14 cm3 molecule−1 s−1 and kOH + CS2 = (1.85 ± 0.34) × 10−13 cm3 molecule−1 s−1, where the uncertainties are 95% confidence limits making allowance for possible systematic errors.

Absolute Rate of the Reaction H+H2S

Flash photolysis coupled with resonance fluorescence of Lyman‐α radiation at 121.6 nm has been used to investigate the rate of reaction of H atoms with H2S over the temperature range 190–464°K. Conditions were chosen under which atom–radical and radical–radical reactions were unimportant and only the H‐atom–H2S reaction occurred. The rate constant thus obtained can be expressed as k1 = (1.29 ± 0.15) × 10−11exp[− (1709 ± 60) / 1.987T] cm3molecule−1·sec−1. Comparison of the Arrhenius A factor with that predicted by entropy considerations suggests a somewhat loose activated complex, but not as loose as expected on the basis of the exothermicity of the H+H2S reaction.

Infra-red laser enhanced reactions: chemistry of vibrationally excited O3 with NO and O2(1Δ)

Abstract Vibrationally excited ozone, produced by absorption of CO2 laser radiation, was found to react significantly faster with NO and O2(1Δ) than thermal ozone. Using a modulation technique, absolute and relative rate constants at 300K for the following reactions were calculated assuming rapid equilibration between the three closely spaced vibrationally excited levels of O3, and that only the lowest level of these, the ν2 bending mode, is active in reaction. k1′ + k2′ = 2.7 × 10−13 cm3 molecule−1 s−1; (k1′ + k2′)/(k1 + k2) = 16.2 ± 4.0; k1′/k1 = 4.1 ± 2.0; k2′/k2 = 17.1 ± 4.3; k7′/k7 = 38 ± 20. These rate constants must be modified if a different combination of vibrationally excited levels is involved. The fraction of vibrational energy usable in chemical reaction was found to be about 15, 50 and ∼ 100% respectively for processes 1′, 2′ and 7′. Our measurements clearly differentiate between the participation of vibrational energy and thermal energy but do not distinguish differences between the individual vibrationally excited states. Details of the modulation technique, involving chemiluminescence detection of NO2 and resonance fluorescence detection of oxygen atoms, are described. Comparison of our results with a previous measurement of the summation reaction (1′ + 2′) shows excellent agreement.

Flash photolysis resonance fluorescence investigation of the reaction of OH radicals with dimethyl sulfide

Abstract Rate constants for the reaction of OH radicals in a homogeneous gas phase reaction with dimethyl sulfide have been determined using the flash photolysis resonance fluorescence technique over the temperature range 273–400 K. The data (combined with the results of another recent study) can be fit to the Arrhenius expression k = (6.08 ± 2.54) × 10 −12 exp[(134 ± 135)/ T ] cm 3 molecule −1 s −1 applicable from 273–426 K. The results are discussed in terms of reaction mechanisms and in light of recent suggestions that dimethyl sulfide plays an important role in the transport of natural sulfur to the earths atmosphere.

Infrared laser enhanced reactions: Temperature resolution of the chemical dynamics of the O3† + NO reaction systen

The rate constant for the decay of vibrationally excited ozone, O3†, in the O3† + NO reaction system has been measured from 153 to 373 K. Vibrationally excited O3 was produced in the asymmetric stretch normal mode by absorption of square wave modulated emission from a CO2 laser tuned to the P (30) 9.6 μm transition. Under appropriate experimental conditions, a rapid V→V coupling process involving all three normal modes of O3 is believed to set up a Boltzmann population distribution among them. Reaction or relaxation of O3+ out of this subset of normal modes is observed to proceed through a weighted average of rate constants. From the effects of temperature and buffer gas pressures an assessment can be made as to the predominant loss mechanism for the various modes. While there are three separate convolution schemes which appear to fit our data, we are persuaded to emphasize one whereby all three modes contribute via a reaction channel described by kD = (2.0×10−11) exp(−1525/T) cm3 molecule−1⋅sec−1 while ν...

Flash photolysis resonance fluorescence study of the reaction Cl + O3 → ClO + O2 over the temperature range 213–298 K

Abstract Rate constants for the removal of Cl atoms in the reaction Cl + O3 → ClO + O2 were measured by the flash photolysis resonance fluorescence technique over the temperature range 213–298 K. The rate constant is given by the Arrhenius expression (2.94 ± 0.49) × 10−11 exp[−(298 ± 39)/T] in units of cm3 molecule−1 s−1. Comparison with recent results from other laboratories are presented.

Infrared laser enhanced reactions: Spectral distribution of the NO2 chemiluminescence produced in the reaction of vibrationally excited O3 with NO

Vibrationally excited ozone, produced by CO2 laser radiation, was found to react significantly faster with NO than does thermal O3. The emission spectrum of the laser enhanced chemiluminescence from this reaction was measured from 520 to 810 nm. The lowest lying 12B2 state was identified as the primary source of NO*2 emission in the NO+O3 reaction. One quantum of vibrational excitation in the reactant O3 was found to introduce one quantum of vibrational energy in the product NO2 (12B2). The rate enhancement of the reaction channel producing NO2(12B2) as a result of vibrational excitation of O3 was 5.6±1.0. Thus, only about 50% of the available vibrational energy is used to enhance this reaction.