Lynne M. Hitchcock

Identification and imaging of hot O2 (v″=2, 3, or 4) in hydrogen flames using 193 nm- and 210 nm-range light

Some simple molecules can be excited by light within the tuning ranges of ArF (193–194 nm) and KrF (247.8–248.8 nm) excimer lasers and their fluorescence has been previously used for imaging. Additional wavelength ranges should become available by Raman shifting. As a demonstration, we present excitation and fluorescence spectra from hot O2 obtained (a) with tunable 193 nm-range light and (b) with that light shifted into the 210 nm range. All measurements are via predissociative upper states. In the 193 nm range, results are compared with those of Andresens group. In the 210 nm range, the light is tuned to various excitation wavelengths in the (5 ← 3), (8 ← 4), and (7 ← 4) bands of the B–X transition. The (7 ← 4) excitations are well separated and the (7 → v″) fluoresence spectrum is in accord with Franck-Condon calculations. The wavelengths tend to overlap for the (5 ← 3) and (8 ← 4) excitations and the fluorescence is weaker. State-specific two-dimensional fluorescence images of an H2-O2 flame are obtained in both wavelength ranges to illustrate the use of the method.

Stimulated raman pumping to H2 (v″=1, J″=1) and resonance-enhanced multiphoton ionization from it using a single laser

Light pulses from one ArF laser are used both a) to produce H2 in v″=1, J″=1 of the X-state by means of stimulated Raman pumping (SRP) and b) to analyze for that state via (2+1) resonance-enhanced multiphoton ionization (REMPI). Both SRP and REMPI have been previously measured in H2, but with a different laser for each process. Some of our laser light is Raman shifted in H2. The resulting mixture of fundamental (≈193 nm) and first Stokes (≈210 nm) light is focused into low-pressure H2 where the SRP and REMPI both occur. The SRP is efficient and it produces only v″=1, J″=1. As the laser is tuned, a REMPI spectrum occurs from excitation by two photons of 193 nm, by two photons of 210 nm, or by one photon each of 210 and 193 nm. The features of this approach are a) that the necessary temporal and spatial overlaps are automatically achieved, b) that the frequency difference generated in the Raman shifter is precisely that needed for SRP, c) that large pulse energies are available for the REMPI, and d) that only one laser is needed.

Absorption of laser light in air in the 193 nm range : analysis of laser locking

Abstract Light from a tunable ArF-excimer laser is absorbed by air at 300 K and at higher temperatures. The data are compared with calculations of Lee and Hanson and they also lead to values of the fraction of the laser light that is narrow-band.

Planar imaging of rayleigh and fluorescence light from H2-air combustion inside a bomb using tunable 193 nm light

Rayleigh scattering and planar laser induced predissociative fluorescence are used to obtain 2-D images of total and specific densities inside a combustion bomb. The experimental arrangement is the same for both methods except for the selection of laser wavelength and the filtering of the radiated light. The former method yields a distribution of total densities while the latter provides densities of hot O2, i.e., those with v″=2, J″=17 and 33. Hydrogen-air mixtures of various compositions are used. Because the thermodynamics are well known, a bomb may serve as a reference device for diagnostics for high temperature species, and the results are in accord with calculations. Additional Rayleigh experiments are described which yield a) scattering cross sections at 193 nm, b) a 2-D temperature distribution in a hot air stream, and c) a 2-D temperature distribution, of limited precision, inside the bomb.

Rayleigh and predissociative fluorescence imaging of densities from an internal combustion engine using a tunable KrF laser

We apply light scattering to the production of species-, space-, and time-resolved images from a Sandia transparent engine, which operates with propane and air. By appropriate tuning of our KrF laser, and by treating the scattered light with an appropriate filter, we can image (a) the total density (and thus the temperature), via Rayleigh scattering, and (b) densities of OH, and of hot O2 (i.e., vibrational states 6 and 7) via laser-induced fluorescence.

Graphitization of synthetic diamond by 193 nm laser light: Comparison of 12C-enriched diamonds with those of natural isotopic composition

Abstract Polished surfaces of diamonds of natural isotopic composition are transformed to graphite upon irradiation by ArF excimer laser light. The surface of a similar diamond made of enriched 12 C is not. A possible explanation involves a competition between graphitization and ablation in which the former is more strongly affected by the recently determined higher thermal diffusivity of 12 C diamond.

Diagnostics In H 2 Discharges Using A Tunable 193nm Excimer Laser

An H- source for high brightness H° beams should have the lowest possible internal temperature T, because the beam-power density at a distance scales as 1/T. We determine local T inside a discharge with the use of a tunable ArF excimer laser that measures the populations of the lowest four rotational states of H2 by means of (2+1) resonance enhanced multiphoton ionization (REMPI). The number of laser-created ions causes a proportional change of the discharges electrical impedance. A calibration in room temperature H2 (without a discharge) showed that the REMPI works well. Another objective is to develop a monitor for the vibrationally excited molecules, H2(v), that are believed to be the primary origin of H- within volume-type ion sources. We use the same apparatus and approach, but add a Raman shifter to change the laser wavelengths to those appropriate for (2+1) REMPI of H2(v).