Normand M. Laurendeau

Pump/probe method for fast analysis of visible spectral signatures utilizing asynchronous optical sampling.

We report the results from a new pump/probe spectrometer for potential use in combustion diagnostics that employs asynchronous optical sampling. The instrument consists of two frequency-doubled mode-locked Nd:YAG lasers operating at slightly different repetition rates, synchronously pumping two dye lasers (rhodamine 6G) to generate the pump and probe beams. The spectral and temporal capabilities of the instrument are examined by obtaining a spectrum and an excited state decay of rhodamine B. The instrument response is shown to be proportional to pump power, probe power, and sample absorptance. Different frequency synthesizers and different modes of triggering are used to study their effect on signal stability.

Temperature measurements by light-scattering methods

Abstract In the last fifteen years, a variety of laser-based, light-scattering techniques have been developed for the measurement of local temperature in gaseous flows. The principal application has been to laminar and turbulent flames, though some of the methods have been extended to nonreactive flows at temperatures of 100–1000 K. The techniques considered in this review include spontaneous vibrational and rotational Raman scattering, Rayleigh scattering, and laser-induced fluorescence, both two-line and monochromatic. Consideration is also given to more recent work which extends the Rayleigh and fluorescence methods to planar thermometry.

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.

Laser-saturated fluorescence measurements of nitric oxide in laminar, flat, C2H6/O2/N2 flames at atmospheric pressure

Abstract We have performed both laser-saturated fluorescence (LSF) and linear laser-induced fluorescence (LIF) measurements of NO in lean and rich atmospheric-pressure C2H6/O2/N2 flames. Unlike previous LSF measurements of OH, NH, and CH, the LSF measurements of NO require a broadband detection scheme, and thus we include a comprehensive theory for broadband LSF. Saturation of NO is found to be easily attainable at atmospheric pressure. When high laser energies are used to insure saturation of NO, background fluorescence often occurs from additional flame species; hence, a subtraction technique is introduced to eliminate this fluorescence from the NO signal. Calibration of both the LSF and LIF techniques was accomplished by doping lean flames with known quantities of NO. A comparison of the LSF and LIF signals from postflame gases at ~ 1700 K as a function of equivalence ratio suggests that the influence of stoichiometry on fluorescence quenching is nearly negligible. Finally, we discuss the relative meri...

Statistical Thermodynamics: Fundamentals and Applications

Preface 1. Introduction Part I. Fundamentals of Statistical Thermodynamics: 2. Probability and statistics Problem set I. Probability theory and statistical mathematics 3. The statistics of independent particles 4. Thermodynamics properties in the dilute limit Problem set II. Statistical modeling for thermodynamics Part II. Quantum Mechanics and Spectroscopy: 5. Basics of quantum mechanics 6. Quantum analysis of internal energy modes 7. The spectroscopy of diatomic molecules Problem set III. Quantum mechanics and spectroscopy Part III. Statistical Thermodynamics in the Dilute Limit: 8. Interlude: from particle to assembly 9. Thermodynamic properties of the ideal gas Problem set IV. Thermodynamic properties of the ideal gas 10. Statistical thermodynamics for ideal gas mixtures 11. Concentration and temperature measurements Problem set V. Chemical equilibrium and diagnostics Part IV. Statistical Thermodynamics beyond the Dilute Limit: 12. Thermodynamics and information 13. Elements of the solid state 14. Equilibrium radiation Problem set VI. The solid state and radiation Part V. Non-Equilibrium Statistical Thermodynamics: 15. Elementary kinetic theory 16. Kinetics of molecular transport 17. Chemical kinetics Problem set VII. Kinetic theory and molecular transport Part VI. The Ensemble Method of Statistical Thermodynamics: 18. The canonical and grand canonical ensembles 19. Applications of ensemble theory to real gases Problem set VIII. Ensemble theory and the non-ideal gas 20. Whence and whither Part VII. Appendices Index.

Calibration of laser-saturated fluorescence measurements using Rayleigh scattering

Calibration of laser-saturated fluorescence measurements using Rayleigh scattering is presented as an alternative to absorption. This new procedure is advantageous when measuring radical species at concentrations well below the corresponding detection limit for absorption. The calibration accounts for nonuniform laser irradiation by extracting the local fluorescence emission along the laser axis and works equally well for both saturated and near-saturated center-line conditions. The predicted error due to misfocusing of the collection optics is nearly negligible when the measured fluorescence is within 10% of its peak value. Number densities obtained using this method are within 15% of those obtained from absorption measurements.

Two-level model for near saturated fluorescence in diatomic molecules

The near saturated behavior of diatomic molecules is modeled by considering the interactions of two sets of rotational levels. The molecules are assumed to be subjected to high intensity laser radiation tuned to one particular rotational transition between the sets of levels. Previous two-level models for atomic fluorescence are modified to account for the strong coupling of rotational energy levels in molecules. The steadystate rate equations for the system are manipulated to give a simple expression for the total fluorescence power in terms of the electronic quenching rate Q(21), the spontaneous emission rate A(21), the stimulated emission rate B(21)I(21), and the population fractions of the lower and upper levels which are directly excited by the laser, F(l)(e) and F(2) (e), respectively. When the ratio of the rotational relaxation rates Q(22) and Q(ll) to Q(21) is on the order of 100 or less, the distribution of molecules amg rotational levels is found to be markedly non- Boltzmann. The influence of the non-Boltzmann distribution on data interpretation is discussed.

Einstein coefficients for rotational lines of the (0, 0) band of the NO A2Σ+-X2π system

Abstract We present a summary of the spectroscopic equations necessary for prediction of the molecular transition energies and the Einstein A and B coefficients for rovibronic lines of the γ(0, 0) band of nitric oxide (NO). The calculated molecular transition energies are all within 0.57 cm-1 of published experimental values; in addition, over 95% of the calculated energies give agreement with measured results within 0.25 cm-1. Einstein AJ′J″ and BJ′J″ coefficients are calculated from the band A00 value and the known Honl-London factors and are tabulated for individual rovibronic transitions in the NO A2Σ+-X2π(0, 0) band.