Narrow-band, narrow-field-of-view Raman lidar with combined day and night capability for tropospheric water-vapor profile measurements.

We describe a high-performance Raman lidar system with combined day and night capability for tropospheric water-vapor profile measurements. The system incorporates high-performance UV interference filters and a narrow-band, dual-field-of-view receiver for rejection of background sunlight. Daytime performance has been demonstrated up to 5 km with 150-m vertical and 5-min temporal averaging. The nighttime performance is significantly better with measurements routinely extending from 10 to 12 km with 75-m range resolution and a 5-min temporal average. We describe design issues for daytime operation and a novel daytime calibration technique.

Turn-key Raman lidar for profiling atmospheric water vapor, clouds, and aerosols.

We describe an operational, self-contained, fully autonomous Raman lidar system that has been developed for unattended, around-the-clock atmospheric profiling of water vapor, aerosols, and clouds. During a 1996 three-week intensive observational period, the system operated during all periods of good weather (339 out of 504 h), including one continuous five-day period. The system is based on a dual-field-of-view design that provides excellent daytime capability without sacrificing nighttime performance. It is fully computer automated and runs unattended following a simple, brief (~5-min) start-up period. We discuss the theory and design of the system and present detailed analyses of the derivation of water-vapor profiles from the lidar measurements.

Detection of atomic hydrogen by two-color laser-induced grating spectroscopy.

We report a novel multiphoton scheme for the detection of atomic hydrogen. Interference of two laser beams near 243 nm crossed at a small angle induces a spatially modulated two-photon excitation (i.e., a grating of excited atoms), which diffracts a third laser beam tuned to either 486 or 656 nm to generate a coherent signal beam. We demonstrate the technique by making H-atom concentration measurements that compare quantitatively with those made using laser-induced fluorescence in low-pressure H(2)/O(2) flames.

Time-resolved two-photon-excited fluorescence detection of atomic hydrogen in flames.

We describe time-resolved fluorescence decay measurements of atomic hydrogen in 20-Torr flames using twophoton, 205-nm excitation with 50-psec laser pulses. These studies provide measurements of the variations in collisional quenching rates of hydrogen atoms measured directly in combustion environments.

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

We describe a new single-laser, two-step fluorecence technique for detecting atomic hydrogen and demonstrate its application to flame measurements. This method provides the advantages of a previously demonstrated two-step method (two-photon 243-nm n = 1 ? n = 2 excitation and subsequent single-photon 656-nm n = 2 ? n = 3 excitation, by using two beams produced with two dye lasers) but with a much simpler experimental implementation (two-photon 243-nm n = 1 ? n = 2 excitation and subsequent single-photon 486-nm n = 2 ? 4 excitation, by using the fundamental and frequency-doubled beams from a single 486-nm dye laser).

Doppler-free two-photon-excited fluorescence spectroscopy of atomic hydrogen in flames.

We describe Doppler-free 205-nm two-photon excitation studies of atomic hydrogen performed using a single-mode pulsed dye-laser system in low-pressure flames. Excitation spectra confirm calculated line-strength ratios for transitions to the 3S and 3D states. Collisional broadening rates are obtained in several flames and compared with quenching rates measured in the same flames, using time-resolved fluorescence measurements.

Laser applications to chemical analysis: introduction by the feature editors.

This introduction briefly describes the motivation behind this feature issue on laser applications to chemical analysis.

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.

Two-photon-excited stimulated emission from atomic oxygen in flames and cold gases.

We report detection of stimulated emission from the atomic-oxygen 3 (3)P-3 (3)S transition at 845 nm after two-photon excitation of the 2 (3)P-3 (3)P transition using 226-nm laser radiation. We study this stimulated emission process in flames and in room-temperature flows of O(2) and N(2)O and compare its behavior with that of fluorescence signals acquired simultaneously. Rapid depletion of the laser-excited state by the stimulated emission process may have an impact on the use of diagnostic techniques based on multiphoton excitation in oxygen and in other species. The strength of the stimulated emission signal suggests that it may have applications as a diagnostic technique.

Photochemical effects in 243-nm two-photon excitation of atomic hydrogen in flames.

This paper describes photochemical effects observed during two-photon 1S-2S excitation of atomic hydrogen in flames using 243-nm laser radiation. An I(4) intensity dependence is observed in regions of the flame where the natural atomic concentration is low, suggesting an I(2) photochemical production mechanism, which we believe is due to two-photon excitation of water molecules, which then predissociate to form H and OH fragments. In a measurement of OH created in the flame by the 243-nm beam, we observe the same I(2) intensity dependence with the laser detuned from the atomic hydrogen 1S-2S resonance, but an apparent I(3.4) dependence is observed when the laser is tuned to the resonance. We believe that a second photochemical mechanism contributes at the resonance, namely, two-photon excitation of H, followed by collisional energy transfer to water molecules, which then fall apart into H and OH fragments. We model this process and show that a combination of I(2) and I(4) dependences can lead to an intensity dependence that mimics a single I(3.4) dependence over a limited range of intensities.

Laser-enhanced flame ionization detector.

A laser-enhanced flame ionization detector is proposed which promises to retain the desirable properties of conventional flame ionization detectors while providing significantly lower detection limits. Experiments are described which demonstrate laser enhancement of the flame ionization detector response. Realization of the full potential sensitivity of the laser-enhanced flame ionization detector requires significant modifications to permit effective saturation of the laser absorption over a major fraction of the flame ionization zone.

Photochemical effects in two-photon-excited fluorescence detection of atomic oxygen in flames.

This paper describes photochemical effects observed using 226-nm two-photon-excited fluorescence detection to measure the atomic oxygen concentration in hydrogen-oxygen flames. In a study of a lean atmospheric-pressure flame, we observed artificially high atomic-oxygen concentration levels in the postflame gases using all but the most gentle excitation conditions (intensities greater than ~0.1 GW/cm(2)). A similar study of a lean low-pressure (72-Torr) flame showed little evidence of photochemical production of atomic oxygen. Using a second laser system in a pump-probe configuration, with the probe laser monitoring the atomic oxygen concentration in a very lean flame while the pump laser was scanned across molecular-oxygen Schumann-Runge bands at 221 nm, we demonstrated that excess atomic oxygen concentrations can be produced by single-photon excitation of these bands in vibrationally excited oxygen molecules present in the flame. This production mechanism explains at least part of the artificially high concentration levels observed in the atmospheric-pressure flame.

Photochemical effects in 205-nm, two-photon-excited fluorescence detection of atomic hydrogen in flames.

This Letter describes photochemical effects observed by using 205-nm, two-photon-excited fluorescence spectroscopy to measure the atomic-hydrogen concentration in atmospheric-pressure hydrogen-oxygen flames. We found that the 205-nm radiation photolyzed water produced in the flame, forming atomic hydrogen in concentrations higher than those naturally present in lean flames and leading to erroneous concentration measurements. UV excitation of molecular oxygen transitions near 205 nm is also discussed; such excitation, followed by rapid predissociation to produce atomic oxygen, may also affect some flame measurements.

Two-step saturated fluorescence detection of atomic hydrogen in flames.

We report a new variation on fluorescence detection, two-step saturated fluorescence spectroscopy, which can provide significant advantages over existing methods for detecting some atomic and molecular species. The method uses multiple-photon excitation of the selected atom or molecule, followed by saturated excitation of an optical transition from the initially excited state and subsequent detection of fluorescence emitted from the final state populated by the saturated excitation. The technique is demonstrated with the detection of atomic hydrogen in flames.

Spatially resolved optical Stark-modulation spectroscopy in flames.

This Letter describes the application of spatially resolved optical Stark-modulation spectroscopy for making point-absorption measurements in combustion environments by monitoring transitions between electronic energy levels. The technique is demonstrated in a study of atomic sodium aspirated into a flame.

Resonant multiphoton optogalvanic detection of atomic hydrogen in flames.

Atomic hydrogen has been studied in an atmospheric-pressure hydrogen-air flame by using resonant multiphoton optogalvanic spectroscopy. This technique offers excellent spatial and temporal resolution, with an estimated sensitivity at the 1-part-in-10(6) level. This experiment represents the first reported direct, in situ optical detection of the extremely important hydrogen radical in a combustion environment.