Validation of THz absorption spectroscopy by a comparison with ps-TALIF measurements of atomic oxygen densities

Abstract

Terahertz (THz) absorption spectroscopy has recently been developed as a diagnostic technique for measuring absolute ground-state atomic oxygen densities in plasmas. To demonstrate the validity of this approach, we present in this Letter a benchmark against a more established method. Atomic oxygen densities were measured with THz absorption spectroscopy and compared to those obtained from picosecond (ps) two-photon absorption laser induced fluorescence (TALIF) measurements on the same capacitively coupled radio frequency oxygen discharge. Similar changes in the atomic oxygen density were observed with both diagnostics when varying the applied power (20–100 W) and the gas pressure (0.7–1.3 mbar). Quantitatively, the results are in good agreement as well, especially when considering the total margin of error of the two diagnostics. For example, for a gas pressure of 1.3 mbar and an applied power of 30 W, atomic oxygen densities measured with THz absorption spectroscopy and TALIF were (7.0 ± 1.7)×1014 cm−3 and (5.3 ± 3.2)×1014 cm−3, respectively. This shows that THz absorption spectroscopy is an accurate technique that can be reliably used for real-world applications to determine atomic oxygen densities in plasmas.

Absolute calibration of the ratio of Xe/O two-photon absorption cross-sections for O-TALIF applications

“…O obtained in this work is similar to the data obtained in [13] and differs markedly from the ratio used in [27] were it was suggested, following [25,26] that the ratio should be reduced by a factor slightly greater than 2. The question of how the ratio of cross sections relates to the absolute value of the cross-section of the two-photon excitation of Xe measured in [25,26] remains open.…”

“…The ratio of the cross-sections, based on the measurements [25,26] and providing almost two times smaller atomic oxygen density, was recently used [27] to compare O-TALIF with THz absorption spectroscopy on the same capacitively coupled radio frequency oxygen discharge. Similar behaviour of the atomic oxygen density was observed for both techniques when varying the applied power (20-100 W) and the gas pressure (0.7-1.3 mbar).…”

“…in equation(27). Uncertainty δα/α was estimated by the error of Abel transform (radial profile) of N 2 SPS emission, δn O /n O and δn Xe /n Xe were taken from the accuracy of the flow rate controller and the pressure sensor, δτ Xe 0 /τ Xe 0 and δτ O 0 /τ O 0 were measured in[13].…”

Performance of a High-Speed Pyroelectric Receiver as Cryogen-Free Detector for Terahertz Absorption Spectroscopy Measurements

“…In particular, the detection of fine structure transitions in atoms is of high interest, as this could provide a means of developing absolute detection methods for atoms based on direct absorption spectroscopy. Recent examples of this are the detection of atomic oxygen by heterodyne spectroscopy in astronomy and atmospheric research [12][13][14] and the determination of the absolute densities of oxygen atoms in a plasma reactor [15][16][17]. There have been a few reports on the application of THz QCLs for absorption spectroscopy in the gas phase as well [18][19][20][21].…”

“…In addition to the requirement of cryogenic cooling, they typically have a limited bandwidth in the order of up to a few hundred Hz for the 3 dB roll-off frequency, which significantly reduces the laser tuning frequencies that can be applied and thus leads to longer measurement times when recording an absorption spectrum. For example, in previous work [15,16], laser tuning frequencies of 10 Hz or 11 Hz had to be used to avoid any asymmetric deformation of the observed absorption features, which, in turn, also limited the spectral range that could be probed.…”