Quantitative two-photon laser-induced fluorescence measurements of atomic hydrogen densities, temperatures, and velocities in an expanding thermal plasma

Abstract

We report on quantitative, spatially resolved density, temperature, and velocity measurements on ground-state atomic hydrogen in an expanding thermal Ar-H plasma using two-photon excitation laser-induced fluorescence ͑LIF͒. The method's diagnostic value for application in this plasma is assessed by identifying and evaluating the possibly disturbing factors on the interpretation of the LIF signal in terms of density, temperature, and velocity. In order to obtain quantitative density numbers, the LIF setup is calibrated for H measurements using two different methods. A commonly applied calibration method, in which the LIF signal from a, by titration, known amount of H generated by a flow-tube reactor is used as a reference, is compared to a rather new calibration method, in which the H density in the plasma jet is derived from a measurement of the two-photon LIF signal generated from krypton at a well-known pressure, using a known Kr to H detection sensitivity ratio. The two methods yield nearly the same result, which validates the new H density calibration. Gauging the new ''rare gas method'' by the ''flow-tube reactor method,'' we find a krypton to hydrogen two-photon excitation cross section ratio Kr ͑2͒ / H (2) of 0.56, close to the reported value of 0.62. Since the H density calibration via two-photon LIF of krypton is experimentally far more easy than the one using a flow-tube reactor, it is foreseen that the ''rare gas method'' will become the method of choice in two-photon LIF experiments. The current two-photon LIF detection limit for H in the Ar-H plasma jet is 10 15 m Ϫ3 . The accuracy of the density measurements depends on the accuracy of the calibration, which is currently limited to 33%. The reproducibility depends on the signal-to-noise ͑S/N͒ ratio in the LIF measurements and is orders of magnitude better. The accuracy in the temperature determination also depends on the S/N ratio of the LIF signal and on the ratio between the Doppler-width of the transition and the linewidth of the excitation laser. Due to the small H mass, the current linewidth of the UV laser radiation is never the accuracy limiting factor in the H temperature determination, even not at room temperature. Quantitative velocity numbers are obtained by measuring the Doppler shift in the H two-photon excitation spectrum. Both the radial and axial velocity components are obtained by applying a perpendicular and an antiparallel excitation configuration, respectively. The required laser frequency calibration is accomplished by simultaneously recording the I 2 absorption spectrum with the fundamental frequency component of the laser system. This method, which is well-established in spectroscopic applications, enables us to achieve a relative accuracy in the transition frequency measurement below 10 Ϫ6 , corresponding to an accuracy in the velocity of approximately 200 m/s. This accuracy is nearly laser linewidth limited.

Inflow and shock formation in supersonic, rarefied plasma expansions

“…The measurements are furthermore corrected for all of the experimental parameters ͑laser power, optical transmission, detector efficiency͒, the linearity is checked, and the densities are calibrated using a two-photon transition in a known amount of krypton. 35 Finally, Rayleigh scattering is used to measure the neutral densities of both argon atoms and hydrogen molecules. 15,27 The Rayleigh scattering is performed using a 230 nm laser beam, of which the details are given in Ref.…”

“…Hydrogen and nitrogen atoms are probed using the two-photon absorption laser-induced fluorescence technique ͑TALIF͒. 29,35 A Nd:YAG ͑yttrium aluminum garnet͒ laser pumped dye laser is operated at a wavelength of around 615 nm for hydrogen and 621 nm for nitrogen. The laser beam is frequency-tripled using a potassium dihydrogen phosphate ͑KDP͒ and a beta-barium borate ͑BBO͒ crystal, resulting in tunable, 8 ns pulses at a wavelength of around 205 or 207 nm.…”

Fluorescence (TALIF) measurement of atomic hydrogen concentration in a coplanar surface dielectric barrier discharge

“…In this respect, a suitable method is twophoton absorption laser-induced fluorescence (TALIF), which has become a powerful tool for the detection of reactive species in plasma [9][10][11][12][13][14]. When TALIF is used for the detection of free hydrogen atoms, these atoms are usually excited by the simultaneous absorption of two laser photons with wavelength of 205.08 nm from the ground state to the state n = 3 [15,16]. This leads to the generation of fluorescent Hα radiation with an intensity proportional to the atomic hydrogen concentration.…”

Capacitively Coupled Hydrogen Discharges: Modeling vs. Experiment

“…The resulting induced fluorescent radiation was collected through the reactor grid by a lens, was imaged on the entrance slit of a monochromator, and it was detected by a photomultiplier connected to a BOXCAR averager. The absolute H-atom density was derived from the measurements of the generated TALIF signal, on the axis of the discharge at 12 mm from the driven electrode, using a well-known Kr:H detection sensitivity ratio [34]. (showing negligible variations of n H with p), although both experiment and simulations yield the same order of magnitude for the n H density.…”