Absolute atomic hydrogen density distribution in a hollow cathode discharge by two-photon polarization spectroscopy

Abstract

We report on quantitative measurements of ground-state atomic hydrogen densities in a stationary plasma far off thermodynamic equilibrium, generated in a hollow cathode discharge, by two-photon polarization spectroscopy via the 1S-2S transition. Absolute densities are obtained using a well established calibration method based on the non-resonant two-photon polarization signal of xenon gas at room temperature, which serves as the reference at the wavelength of the hydrogen transition. This study is dedicated to demonstrating the capability of two-photon polarization spectroscopy close to the detection limit. Therefore, it requires single-longitudinal mode UV-laser radiation provided by an advanced UV-laser spectrometer.

Production and Detection of H Atoms and Vibrationally Excited H2 Molecules in CVD Processes

“…These backgrounds can be reduced by using a monochromator or by setting a short detection time gate, since pulsed lasers are usually used; it is difficult, however, to eliminate them completely. These problems can be avoided by monitoring the amplified spontaneous emission (ASE)107, 114 or by employing two‐photon polarization spectroscopy 124…”

“…In two‐photon polarization spectroscopy, a circularly polarized pump beam is focused into the detection zone where it overlaps with a linearly polarized signal beam. In the arrangement of Gonzalo et al,124 the wavelength of both beams was 243.1 nm, two‐photon resonant to the H(2s 2 S‐1s 2 S) transition. In the presence of H atoms, the polarization of the signal beam is rotated and becomes elliptical.…”

Calculation of the spatial resolution in two-photon absorption spectroscopy applied to plasma diagnosis

“…(1)) for tiny volume elements (cells), all having the same size of Dx ¼ 6.7 lm  Dy ¼ 6.7 lm  Dz ¼ 500 lm. Accounting only for the laser induced changes, we start to resolve the rate equations with a fixed and homogenous ground state number density of N 1 ¼ 10 13 cm À3 , 7 hence all results are scaling linearly with variations of this ground state density. The integration finishes with the end of the laser pulse and reveals for each cell, the number of generated ions (Ion Yield ¼ YN Ion ), the number of already emitted laser induced photons YN f , and the remaining number of laser excited atoms YN 2 , which are expected to decay via fluorescence in absence of inelastic collisions.…”

“…Two-photon polarization spectroscopy 4 allows for direct measurement of the two-photon absorption and was used for measuring Stark broadening 1S-2S hydrogen, 5 for the determination of atomic hydrogen densities in a stationary wall stabilized cascaded arc plasma, 6 in a hollow cathode discharge (HCD) 7 and in atmospheric pressure flames, 8 and for the determination of local electric field (E-field) strength 9 as well. Two-photon induced Lyman-a fluorescence was employed in feasibility study for tokamak diagnostic 10 and Balmer-a fluorescence for local E-field strength determination in an optogalvanic cell.…”

Determination of Two-Photon Absorption Cross-Section of Noble Gases for Calibration of Laser Spectroscopic Techniques

“…It has been used e.g. to determine absolute atomic hydrogen density in quite different environments with an uncertainty as low as 10% [5][6][7]. This precision can be by achieved by relative measurements with respect to a standard of atomic hydrogen density.…”