Time-resolved CH (A(2)Delta and B(2)Sigma(-)) laser-induced fluorescence in low pressure hydrocarbon flames

The effects of energy-level resonance on collision-induced electronic energy transfer: CD (A 2Δ ↔ B 2Σ−) coupling

Pulsed, time- and wavelength-resolved laser-induced fluorescence spectroscopy has been used to measure rate constants for collision-induced electronic energy transfer (EET) between the A (2)Delta and B (2)Sigma(-) states of the CD radical. EET rate constants in the exothermic direction from B (2)Sigma(-), v = 0 to the unresolved A (2)Delta, v = 0 and 1 levels span the range 0.1-2.4 x 10(-11) cm(3) s(-1) at room temperature (ca. 295 K) for the partners He, Ar, N(2), CO and CO(2). H(2) was also investigated, but was unsuitable for further study because of its rapid isotope exchange with CD(X (2)Pi). As expected, only CO results in a significant rate of removal on any distinct, unobserved channel, presumed to be chemical reaction. The efficient A (2)Delta, v = 1 --> 0 vibrational relaxation by CO(2) observed previously for CH was not found for CD. Despite the significant differences in their detailed rovibronic level structures, the overall efficiency of EET in CD was found to be very similar to that for CH. The positive correlation in a Parmenter-Seaver plot appears to confirm a role for long-range attractive forces in the EET process. However, the detailed deviations from this overall trend found reproducibly for CD and CH suggests that partner-specific interactions are also important.

Vibrational and rotational energy transfers involving the CH B 2Sigma(-) v=1 vibrational level in collisions with Ar, CO, and N2O

With photolysis-probe technique, we have studied vibrational and rotational energy transfers of CH involving the B (2)Sigma(-) (v=1, 0<or=N<or=6, F) state by collisions with Ar, CO, and N(2)O. For the vibrational energy transfer (VET) measurements, the time-resolved fluorescence of the B-X(0,0) band is monitored following the (1,0) band excitation. For the rotational energy transfer (RET) measurements, the laser-induced fluorescence of the initially populated state is dispersed using a step-scan Fourier transform spectrometer. The time-resolved spectra obtained in the nanosecond regime may yield the RET information under a single pressure of the collider. The rate constants of intramolecular energy transfers are evaluated with simulation of kinetic models. The VET lies in the range of 4x10(-12) to 4x10(-11) cm(3) molecule(-1) s(-1), with efficiency following the order of Ar<CO<N(2)O, reflecting the average over Boltzmann rotational distribution. The RET rates are more rapid by one to two orders of magnitude, comparable to the gas kinetic, with the trend of Ar<CO<N(2)O. The transfer rates decrease with increasing N and DeltaN, proceeding via the DeltaN=-1 transitions slightly larger than DeltaN=+1. With the fine-structure labels resolved up to N=6, the fine-structure-conserving collisions prevail increasingly with increasing N in DeltaN not equal 0. The rate constants for the F(2)-->F(1) transitions are larger than the reverse F(1)-->F(2) transitions in DeltaN=0 for the Ar and CO collisions. The trend of fine-structure conservation is along the order of N(2)O<CO approximately Ar. For the CH-Ar collisions, the fine-structure conservation is less pronounced as compared with the v=0 level reported previously. In general, the propensity rules obeyed in the v=0 collision with Ar are valid in v=1, but the latter case shows a weaker tendency. It might be caused by the anisotropy difference of interaction potential when vibrational excitation is considered. For the polyatomic collider, the strong long-range dipole-dipole interaction may have the chance to vary the rotational orientation to increase the fine-structure-changing transitions.

Quantum state-to-state rate constants for the rotationally inelastic collision of CH(B2Sigma(-), nu=0, N-->N') with Ar

We have calculated the state-to-state integral cross sections and rate constants for the rotationally inelastic collision of CH(B (2)Sigma(-), nu=0, N-->N') with Ar using the quantum coupled-state and close-coupling methods on an ab initio potential-energy surface constructed by Alexander et al. [J. Chem. Phys. 101, 4547 (1994)]. Overall the calculated rate constants are in good agreements with the three available experimental results. The rate constants are comparable to the usual gas kinetic and decrease with increasing N and DeltaN. For the multiquantum transition cases, the theory underestimates the experiment. We discuss some possible causes to the discrepancies among the theory and the experiments.

Optimization of CH fluorescence diagnostics in flames: range of applicability and improvements with hydrogen addition

This study quantifies the range of premixed flame conditions for which CH fluorescece diagnostics are applicable, and it shows that the CH fluorescence signal can be increased if some of the hydrocarbon fuel is replaced with hydrogen. The CH fluorescence signal is found to be adequate for fuel-air equivalence ratios (phi) as small as 0.85 for both methane-air and propane-air flames. The CH signal increases until a maximum at phi = 1.25 and phi = 1.35 for methane-air and propane-air flames, respectively, and then decreases for richer conditions. A strategy to increase the CH fluorescence signal and decrease interference from soot precursors is proposed by addition of the proper amount of hydrogen to the hydrocarbon fuel. Hydrogen addition reduces the background signal from soot precursors by as much as afactor of 10 and increases the CH fluorescence signal by as much as 80%. The normalized CH fluorescence measurements are compared with computations that utilize GRI-MECH 3.0 chemistry. Sources experimental uncertainties are discussed.

High-sensitivity detection of CH radicals in flames by use of a diode-laser-based near-ultraviolet light source

CH radical distributions in ethylene-air and methane-air diffusion flames are mapped by wavelength-modulation absorption spectroscopy (WMS). Tunable, wavelength-modulated 426-nm light is generated by frequency doubling of a modulated 852-nm distributed Bragg reflector diode laser. Absorbances of 5 x 10(-5) are measured with second-harmonic (2f) WMS with a signal-to-noise ratio of 3:1 in a 3-Hz measurement bandwidth. The feasibility of simultaneous line-of-sight absorption and spatially resolved laser-induced-fluorescence detection with a single excitation beam is also demonstrated. This near-UV source is suitable for microgravity drop-tower experiments and other applications in which compact, rugged, energy-efficient instrumentation is required.

Radiative, collisional, and predissociative effects in CH laser-induced-fluorescence flame thermometry

Laser-induced fluorescence of the CH radical is used to determine the flame-front temperature of an 8-Torr premixed CH(4)/O(2) flame. The A (2)D-X (2)Delta (0, 0) and B (2)Sigma- - X (2)II (0, 0) bands give values of 1960 +/- 60 and 1920 +/- 70 K, respectively. This is an improvement over a previous study that found discrepancies up to 20% between these bands. New rotational-level-dependent transition probabilities are the main reason for this improvement. The weaker off-diagonal bands A-X (0, 1) and B-X (0, 1) yield temperatures of 1930 +/- 90 and 1830 +/- 100 K, respectively. The influence of rotational transfer on the predissociated levels that have N' > in B (2)Sigma-, v'= 0 was investigated with fluorescence scans and a statistical power-gap law model of the relaxation; deviations up to 8% in temperature can occur because of this process.

Absolute concentration measurements of CH radicals in a diamond-depositing dc-arcjet reactor

Laser-induced fluorescence in the CH (B - X) and CH (A - X) electronic transitions is used to measure absolute number density versus position for CH radicals in the plume of a 25-Torr hydrogen/argon/methane (0.8:1:0.005) dc arcjet during the chemical vapor deposition of diamond film. The laser-induced-fluorescence signal is calibrated with argon Rayleigh scattering, and the resultant concentration of the CH radical in the center of the arcjet plume is found to be (3.5 +/- 0.8) x 10(12) molecules/cm(3). The characterization of the plasma plume shows three different regions in the reacting gas: nozzle, plume, and boundary layer. We observe substantial differences in spatial distribution of gas temperature, collisional quenching, and CH number density among these regions.

Measurements of atomic sodium in flames by asynchronous optical sampling: theory and experiment

Asynchronous optical sampling (ASOPS) is a pump-probe method for the measurement of species concentrations in turbulent high-pressure flames. We show that rapid measurement of species number density can be achieved in a highly quenched environment by maintaining a constant beat frequency between the mode-locking frequencies of the pump and the probe lasers. A model for the ASOPS method based on rate equation theory for three- and four-level atoms is presented. A number of improvements are made to the basic ASOPS instrument, which result in a greatly enhanced signal-to-noise ratio. Atomic sodium is aspirated into an atmospheric pressure C(2)H(4)/O(2)/N(2) flame and detected with the ASOPS instrument. When excited-state lifetimes are fitted by using the ASOPS theory, a 3P((1/2),3/2) ? 3S((1/2)) quenching-rate coefficient of 1.72 x 10(9) s(-1) and a 3P(3/2) ? 3P((1/2)) doublet-mixing rate coefficient of 3.66 x 109 s(-1) are obtained, in excellent agreement with literature values. ASOPS signals obtained over a wide range of pump and probe beam powers validate the rate equation theory. Improvements are suggested to improve the signal-to-noise ratio, since the present results are limited to laminar flows.

Laser-induced fluorescence determination of temperatures in low pressure flames

Spatially resolved temperatures in a variety of low pressure flames of hydrogen and hydrocarbons burning with oxygen and nitrous oxide are determined from OH, NH, CH, and CN laser-induced fluorescence rotational excitation spectra. Systematic errors arising from spectral bias, time delay, and temporal sampling gate of the fluorescence detector are considered. In addition, we evaluate the errors arising from the influences of the optical depth and the rotational level dependence of the fluorescence quantum yield for each radical. These systematic errors cannot be determined through goodness-of-fit criteria and they are much larger than the statistical precision of the measurement. The severity of these problems is different for each radical; careful attention to the experimental design details for each species is necessary to obtain accurate LIF temperature measurements.

Time resolved laser induced fluorescence of the NH radical in low pressure N(2)O flames

Total removal rate constants from the NH(A(3)II(i)) electronic state have been obtained in low pressure (~14-Torr) N(2)O flames. The fluorescence decay constants and quantum yields Phi depend on the temperature and composition at each position interrogated in the flame. In similar conditions, Phi for NH(A(3)II(u)) is significantly greater than that for other radicals CH(A(2)Delta and B(2)Sigma(-)) and OH(A(2)Sigma(+)). A small decrease (~5%) in the electronic removal is observed with rotational excitation increasing from N' = 2 to 12 in the A state. Estimates of collisional quenching cross sections at flame temperatures are made for the important combustion species, H(2)O, C(3)H(8), C(2)H(4), and C(2)H(2).