Donald W. Sweeney

Laser-saturated fluorescence measurements of OH concentration in flames☆

Abstract A laser-saturated fluorescence technique which can be used to measure accurate OH radical concentrations over wide ranges of flame pressure, composition, and temperature has been developed. The balanced cross-rate model is used to analyze the fluorescence data. The basic premise of the model is that the population of the upper and lower rotational levels which are coupled by the laser is approximately constant during the laser pulse. Using the balanced cross-rate model to calculate concentrations, OH number densities calculated from saturated fluorescence agree to within 20% with number densities calculated from independent absorption measurements over a wide range of flame conditions. In addition, the ratio of number densities calculated from saturated fluorescence and absorption is constant over an order of magnitude range of flame pressure, demonstrating the insensitivity of the saturated fluorescence signal to collisional quenching rates. The estimated OH detection limit of the fluorescence system is 10 12 –10 13 molecules/cm 3 .

Two-photon-excited fluorescence measurement of hydrogen atoms in flames

We report the first two-photon-excited hydrogen-atom fluorescence measurements in flames made to our knowledge. The n = 3 level of the H atom was excited by 205.1-nm radiation generated by Raman shifting a 224-nm beam produced by frequency mixing. Fluorescence was observed at 656.3 nm as a result of radiative decay from n = 3 to n = 2, the Balmer-alpha transition. A novel technique, photoionization-controlled loss spectroscopy, is proposed to eliminate the quenching dependence of the fluorescence signal.

Balanced cross-rate model for saturated molecular fluorescence in flames using a nanosecond pulse length laser

The balanced cross-rate model is proposed to analyze laser-induced molecular fluorescence signals when the laser pulse length is of the order of nanoseconds. Nanosecond pulse length lasers, specifically Q-switched Nd:YAG-pumped dye lasers, are attractive for saturated molecular fluorescence spectroscopy because of their high peak power and because their short pulse length minimizes the risk of laser-induced chemistry. In the balanced cross-rate model, single upper and lower rotational levels are assumed to be directly coupled by the laser radiation. Because the laser-induced processes which couple these levels are so fast at saturation intensities, a steady state is established between the two levels within picoseconds. Provided that the total population of the two laser-coupled rotational levels is constant during the laser pulse, the total molecular population can be calculated from the observed upper rotational level population using a two-level saturation model and Boltzmann statistics. Numerical simulation of the laser excitation dynamics of OH in an atmospheric pressure H(2)/O(2)/N(2) flame indicates that the balanced cross-rate model will give accurate results provided that the rotational relaxation rates in the upper and lower sets of rotational levels are approximately equal.

Time-resolved fluorescence investigation of rotational transfer in A 2 ∑ + (v = 0) OH

Rotational transfer cross sections for the OH A2∑+ (v = 0) rovibronic levels were determined for collisions with water vapor at flame temperatures. OH fluorescence was induced by a 10-ns pulse from a Nd:YAG pumped dye laser, and the resulting fluorescence spectrum was recorded at selected times during the laser pulse. The measurements were performed in the postflame region of 30-Torr stoichiometric hydrogen/oxygen/nitrogen flames. Four different upper rotational levels were excited. State-to-state and total rotational transfer rates were calculated by optimizing the agreement between experimental spectra and synthetic spectra generated by a time-dependent numerical code. For the F1(1), F1(4), F2(5), and F1(10) upper rotational levels in the A2∑+ (v = 0) manifold, total rotational transfer cross sections were found to be (110 ± 30), (170 ± 60), (190 ± 60), and (110 ± 30) × 10−16 cm2, respectively, for collisions of OH with water vapor.

Temperature measurement by two-line laser-saturated OH fluorescence in flames

A technique is proposed and demonstrated for measuring combustion temperatures using two-line laser-saturated fluorescence. The rotational temperature of OH is determined by saturating two different rotational transitions in the (0,0) band of the A(2)Sigma(+)-X(2)II electronic system and detecting fluorescence emission which originates from the laser-pumped upper rotational levels. Temperature is calculated from the ratio of the fluorescence intensities for the two different excitation-emission pairs. The method is demonstrated by measuring temperature profiles in subatmospheric H(2)/O(2)/Ar flat flames. Temperatures measured by two-line saturated fluorescence are compared with temperatures measured by coated thermocouples and OH absorption and with predictions from an elementary chemical kinetics code. The temperatures measured by the two-line fluorescence technique are accurate to 3-5% and exhibit low random error.

Single-pulse, laser-saturated fluorescence measurements of OH in turbulent nonpremixed flames

A single-pulse, laser-saturated fluorescence technique has been developed for absolute OH concentration measurements with a temporal resolution of 2 nsec, a spatial resolution of <0.1 mm(3), and an estimated accuracy of +/-30%. It has been applied in laminar, transitional, and turbulent hydrogen-air diffusion flames, providing the first reported quantitative measurements of average values, rms fluctuations, and probability density functions of OH radical concentration in nonpremixed flames.

Measurements of superequilibrium hydroxyl concentrations in turbulent nonpremixed flames using staturated fluorescence

The first quantitative, time- and space-resolved measurements have been obtained for probability density functions of OH concentration in nonpremixed flames. Measurements using single-pulse, laser-saturated fluorescence in laminar, transitional and turbulent nonpremixed H 2 -air flames provide unambiguous evidence for substantial OH superequilibrium concentrations, in qualitative agreement with predictions of laminar and turbulent combustion models. The average degree of superequilibrium (¯OH/¯OH AE ) is typically 4–5 near the jet exit and approaches unity far downstream. The maximum instantaneous OH concentration measured in transitional and turbulent H 2 -air flames is ∼6×10 16 molecules/cc in accord with the maximum determined by partial equilibrium thermodynamic calculations and with the maximum OH concentrations measured in premixed H 2 -air flames.

CO 2 laser beam shaping with computer generated holograms

In this Letter we wish to report the production of computer generated holographic gratings for use with CO2 lasers. Although these holograms can be reconstructed in the conventional manner, we believe the most interesting configuration is to use the holographic gratings to output couple a CO2 laser; the basic configuration is shown in Fig. 1. The partially reflecting plane mirror of an otherwise standard CO2 laser is replaced with an appropriate reflective phase hologram so the output wavefront can have any desired shape. The first diffracted order of the holographic grating is the output beam; the zero-order reflected beam is returned to the cavity to provide the necessary feedback. The energy associated with the zero order is not lost because the holographic optical element is inside the laser cavity. Since the first diffracted order is the output beam, this configuration does not provide the wavelength selectivity usually associated with the use of reflective gratings in laser cavities; but it does have many other significant advantages and potential applications. For example, the holographic grating could serve to provide a correction for aberrations

Materials processing with CO(2)laser holographic scanner systems.

The complexity of laser material processing can be greatly reduced using computer-generated phase reflection holographic scanners. These scanners direct and focus the beam of a carbon dioxide laser into a spot on the workpiece and then translate this spot over some general 2-D pattern as the scanner undergoes a simple 1-D motion. Procedures for constructing these scanners are presented, and the first-order aberrations introduced by them are analyzed. The primary aberrations cause the diffracted beam to focus to an astigmatic spot on the work surface. The severity of the astigmatism is proportional to the scan rate, scan angle, and f/number. A technique is presented in which the design of the scanner is adjusted so that the astigmatic image is aligned with the scan direction. The resolution perpendicular to the scan direction is the same as that of a scanner without aberrations of the same f/number. Materials processed using these scanners are presented to show their capabilities for carbon dioxide laser material processing. Power densities on the order of 10(6)/cm(2) can be readily obtained using the proposed technique.

Laser-saturated fluorescence measurements of NH in a premixed subatmospheric CH4/N2O/AR flame

Laser-saturated fluorescence (LSF) measurements of NH have been performed in a premixed methane/nitrous oxide/argon flat flame at a pressure of 50 Torr and an equivalence ratio of 1.10. The degree of saturation of the NH fluorescence signal is approximately 96%. The balanced cross-rate model is used to calculate number densities from the measured fluorescence signals. Independent LSF and optical absorption measurements of NH number density agree to within 20%. The estimated NH detection limit of the fluorescence system is 10 12 molecules/cm 3 .