Measurement of the two-photon excitation cross-section of the 6p′[3/2]₂ and 6p′[1/2]₀ levels of Xe I at the wavelengths 224.3 and 222.6 nm

Abstract

The two-photon excitation cross-section is a key parameter for the two-photon absorption laser induced fluorescence (TALIF) method, which is commonly used to measure atomic densities in gaseous media, especially for plasma diagnostics. The method consists in recording the fluorescence signal that follows the resonant absorption of two photons of UV light. Calibration often relies on comparing the signal recorded in the studied sample with the fluorescence produced, at a similar wavelength, in a noble gas vapor, the density of which can be easily known. The ratio of the involved cross-sections however plays an essential role for the accuracy of such measurements. Yet the two-photon excitation cross-section of atomic xenon, which is often used as the reference for oxygen density measurements, was measured only once, at the wavelengths of interest. The aim of the present study has been to consolidate the experimental value of that key parameter. The cross-section is found equal to 1.36 +0.46 −0.34 and 1.88 +0.75 −0.54 × 10 −43 m 4 for the 6p [3/2] 2 and 6p [1/2] 0 levels, respectively. For the 6p [3/2] 2 level this is more than twice smaller than previously admitted. Even though the necessarily large relative uncertainty of a non-linear crosssection attaches a relatively large uncertainty to this factor of one half, the result suggests that atomic densities already measured by Xe-calibrated TALIF may have to be revised to significantly lower values. The experiments performed also provide an opportunity to revisit the validity of the approximations used for quantitative TALIF measurements and the collisional broadening and pressure shift of the two-photon 6p [1/2] 0 line. A new formula has been used to describe the two-photon absorption of a Gaussian beam in a long gas cell, which makes the decrease of the beam intensity a simple analytic expression even in strong absorption regime, based on a polylogarithmic function of the absorption rate variable.

Two-photon absorption laser induced fluorescence (TALIF) detection of atomic iodine in low-temperature plasmas and a revision of the energy levels of I I

“…This may suggest that the larger intensity observed in iodine for the excitation of the ( 3 P 2 )6p 2 [1] o 3/2 , when compared to excitation of the ( 3 P 2 )6p 2 [3] o 7/2 level as shown in figure 5, follows from a general rule, according to which ∆J = 0 twophoton transitions would be especially intense ones. The hypothesis may seem substantiated by our finding that the excitation cross-section from the ground J = 0 state to a J = 0 level was larger than to a J = 2 level of the same configuration in xenon (Drag et al 2021). Yet in this latter case the situation was more clear-cut, for starting from a J g = 0 state even made the transition to a J e = 0 state a pure isotropic one.…”

State enhanced actinometry in the COST microplasma jet

“…The accuracy of the TALIF measurements is based on the reproducibility of the data, as well as the noble gas calibration with xenon, which is described in more detail elsewhere [30]. Systematic errors that could affect the absolute densities, such as the choice of the two-photon excitation cross section ratio are not included in the confidence intervals [32].…”

Concentration measurements of atomic nitrogen in an atmospheric-pressure RF plasma jet using a picosecond TALIF

“…In the end, the uncertainty of measured concentrations was estimated. A detailed study was conducted in the cited work [60] to address the challenges associated with quantifying the concentration of atomic species (H, N, O) using TALIF. The primary sources of uncertainty in the TALIF technique stem from uncertain values of cross-sections (in our case, σKr σN , the ratio of the cross-section [21] has 50% uncertainty ) used in the calculation.…”