Single-laser two-step fluorescence detection of atomic hydrogen in flames

Two-photon-absorption line strengths for nitric oxide: Comparison of theory and sub-Doppler, laser-induced fluorescence measurements

We discuss the results of high-resolution, sub-Doppler two-photon-absorption laser-induced fluorescence (TPALIF) spectroscopy of nitric oxide at low pressure and room temperature. The measurements were performed using the single-longitudinal mode output of a diode-laser-seeded optical parametric generator (OPG) system with a measured frequency bandwidth of 220 MHz. The measurements were performed using a counter-propagating pump beam geometry, resulting in sub-Doppler TPALIF spectra of NO for various rotational transitions in the (0,0) vibrational band of the A²Σ⁺ - X²Π electronic transition. The experimental results are compared with the results of a perturbative treatment of the rotational line strengths for the 20 different rotational branches of the X²Π(v″ = 0) → A²Σ⁺(v' = 0) two-photon absorption band. In the derivation of the expressions for the two-photon transition absorption strength, the closure relation is used for rotational states in the intermediate levels of the two-photon transition in analogy with the Placzek treatment of Raman transitions. The theoretical treatment of the effect of angular momentum coupling on the two-photon rotational line strengths features the use of irreducible spherical tensors and 3j symbols. The final results are expressed in terms of the Hund's case (a) coupling coefficients a_J and b_J for the X²Π(v″ = 0) rotational level wavefunctions, which are intermediate between Hund's case (a) and case (b). Considerable physical insight is provided by this final form of the equations for the rotational line strengths. Corrections to the two-photon absorption rotational line strength for higher order effects such as centrifugal stretching can be included in a straightforward fashion in the analysis by incorporating higher order terms in these coupling coefficients a_J and b_J, although these corrections are essentially negligible for J < 50. The theoretical calculations of relative line intensities are in good agreement both with our experiment and with published experimental results. In addition, the calculated line shapes and relative intensities for closely spaced main branch and satellite transitions are in excellent agreement with our experimental measurements.

Development of two-color laser system for high-resolution polarization spectroscopy measurements of atomic hydrogen

We have developed a high-spectral-resolution laser system for two-photon pump, polarization spectroscopy probe (TPP-PSP) measurements of atomic hydrogen in flames. In the TPP-PSP technique, a 243-nm laser beam excites the two-photon 1S-2S transition, and excited n=2 atoms are then detected by polarization spectroscopy of the n=2 to n=3 transition using 656-nm laser radiation. The single-frequency-mode 243 and 656-nm beams are produced using injection-seeded optical parametric generators coupled with pulsed dye amplifiers. The use of single-mode lasers allows accurate measurement of signal line shapes and intensities even with significant pulse-to-pulse fluctuations in pulse energies. Use of single-mode lasers and introduction of a scheme to select nearly constant laser energies enable repeatable extraction of important spectral features in atomic hydrogen transitions.

Comparison of nanosecond and picosecond excitation for interference-free two-photon laser-induced fluorescence detection of atomic hydrogen in flames

Two-photon laser-induced fluorescence (TP-LIF) line imaging of atomic hydrogen was investigated in a series of premixed CH4/O2/N2, H2/O2, and H2/O2/N2 flames using excitation with either picosecond or nanosecond pulsed lasers operating at 205 nm. Radial TP-LIF profiles were measured for a range of pulse fluences to determine the maximum interference-free signal levels and the corresponding picosecond and nanosecond laser fluences in each of 12 flames. For an interference-free measurement, the shape of the TP-LIF profile is independent of laser fluence. For larger fluences, distortions in the profile are attributed to photodissociation of H2O, CH3, and/or other combustion intermediates, and stimulated emission. In comparison with the nanosecond laser, excitation with the picosecond laser can effectively reduce the photolytic interference and produces approximately an order of magnitude larger interference-free signal in CH4/O2/N2 flames with equivalence ratios in the range of 0.5< or =Phi< or =1.4, and in H2/O2 flames with 0.3< or =Phi< or =1.2. Although photolytic interference limits the nanosecond laser fluence in all flames, stimulated emission, occurring between the laser-excited level, H(n=3), and H(n=2), is the limiting factor for picosecond excitation in the flames with the highest H atom concentration. Nanosecond excitation is advantageous in the richest (Phi=1.64) CH4/O2/N2 flame and in H2/O2/N2 flames. The optimal excitation pulse width for interference-free H atom detection depends on the relative concentrations of hydrogen atoms and photolytic precursors, the flame temperature, and the laser path length within the flame.

Nonperturbative modeling of two-photon absorption in a three-state system

The physics of the two-photon absorption process is investigated for a three-state system. The density-matrix equations for the two-photon interaction are solved in the steady-state limit assuming that the pump laser radiation is monochromatic. Collisional broadening, saturation, and Stark shifting of the two-photon resonance are investigated in detail by numerical solution of the steady-state density-matrix equations. Analytical expressions for the saturation intensity and the Stark shift are derived for the case where the single-photon transitions between the intermediate state and the initial and final states are far from resonance with the pump laser. For this case, it is found that the direction of the Stark shift is dependent on the relative magnitudes of the dipole-moment matrix elements for the single-photon transitions that couple the intermediate state with the initial and final states. Saturation and Stark shifting are also investigated for the case where the single-photon transitions between the intermediate state and the initial and final states are close to resonance with the pump laser.

Photochemical effects in multiple species fluorescence imaging in hydrogen-nitrous oxide flames

We describe photochemical perturbations observed in flame studies using multiple species fluorescence imaging techniques. We model these perturbations using a sequential two-step mechanism that is specific to flames containing significant quantities of nitrous oxide: [equation], followed by O((1)D) + H(2)O ? 2OH.