# VUV absorption spectroscopy for ultrafast TALIF calibration and mw plasma jet characterisation

Alexandra Brisset, C. Pascual-Fort, Nelson de Oliveira, Gabi Daniel Stancu

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Alexandra Brisset, C. Pascual-Fort, Nelson de Oliveira, Gabi Daniel Stancu. VUV absorption spectroscopy for ultrafast TALIF calibration and mw plasma jet characterisation. XXXVI ICPIG, Jul 2025, Aix-en-Procence, France. hal-05392481

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Submitted on 2 Dec 2025

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# VUV absorption spectroscopy for ultrafast TALIF calibration and mw plasma jet characterisation

A. Brisset¹, C. Pascual-Fort¹, N. de Oliveira², G.D. Stancu¹

¹Laboratoire EM2C, CNRS, CentraleSupélec, Université Paris-Saclay,
3 rue Joliot Curie, 91192, Gif-sur-Yvette, France

²Soleil, l'Orme des Merisiers, St. Aubin BP 48, 91192 Gif-sur-Yvette, France

This study is aimed at performing VUV-absorption to i) improve the calibration accuracy of the ultrafast Two-photon Absorption Laser Induced Fluorescence (TALIF) diagnostic under high laser intensity conditions (i.e. TWcm⁻²), ii) investigate the generation of atomic radicals in microwave (mw) plasma jets. Traditional rate equations are invalid in the high-intensity TALIF regime, requiring calibration with a source of known atomic density. A DC plasma was employed as a reference source for atomic O, H and N, where absolute densities were measured by Vacuum Ultraviolet (VUV) absorption spectroscopy at the SOLEIL synchrotron. The accuracy was improved by choosing operating conditions with non-saturated absorption lines and negligible molecular spectral overlap. VUV absorption was applied to characterize a low-pressure mw plasma jet in Ar:O₂ mixtures. Resonant broadening and hysteresis effects were observed in these experiments.

## 1 Context of the study

The extreme laser intensity and mode-lock features of ultrafast lasers make the fluorescence signal in TALIF dependent on the ground and excited state populations, as well as novel phenomena like Stark detuning and coherent excitation, and therefore, not correctly described by the classical rate equations. To overcome these challenges, it was suggested in [1] to perform the calibration of the fluorescence signal using a source of known density of the same reactive atom as the probed one. A DC discharge characterised by high resolution VUV absorption spectroscopy [2] is used to serve as the calibration cell. Since the fluorescence becomes dependent only on the laser characteristics (e.g. rates of laser induced processes are much larger than the plasma quenching rates), the atomic spectroscopic properties and their ground state densities, the calibration reduces to a factor that only depends on the ratio of fluorescence signals in the cell, $S_F^{calib}$ and in the plasma of interest, $S_F$. Using the same laser beam, the density of the probed species X in the plasma of interest ($n_X$) is related to the density of X in the calibration cell ($n_X^{calib}$) through: $n_X = C \times \frac{n_X^{calib}}{S_F^{TALIF}} \times S_F$ with C, a constant accounting for potential changes, such as different transmission coefficients of fluorescence collection windows of the two plasma reactors.

## 2 Results

Means to improve the accuracy of the VUV-calibration method (e.g. source design, gas dilution, increase spectral resolution) were obtained. For example, N density in plasma was evaluated with a

total uncertainty of ~20% which is appreciably better compared to traditional nanosecond TALIF. Among findings, we can cite the importance of laser dispersion and photon statistic effects on the TALIF signals.

VUV absorption was also performed for H and O generated in the calibration cell (see Figure 1) and in a low-pressure mw plasma jet (1-100 mbar).

<figure><img src="image_2.png" /><figcaption>Figure 1: VUV absorption spectrum of O in the DC plasma cell at 6.5 mbar, Ar:O₂ = 5000:1 and 7 mA.</figcaption></figure>

Mw plasma jet measurements revealed, for instance, a complex broadening of O lines at 130 nm and a hysteresis power dependence for oxygen radical production.

## Acknowledgments

This research was funded by ANR, ULTRAMAP project (ANR-22-CE51-0027), Soleil, project 20231659 and IdF SESAME, project DIAGPLAS.

## References

[1] Dumitrache C. et al. Plasma Sources Sci. Technol. 31 015004 (2022)

[2] de Oliveira N. et al. J. Synchrotron Radiation, 23 887 (2016)