Absolute calibration of hydrogen atoms measurement by femtosecond two-photon laser induced fluorescence

A variable pressure high voltage plasma discharge test cell was built for analysis of H-fs-TALIF capabilities for application in fusion divertors and other relevant environments. The discharge cell allows operation in the pressure range from a few mPa to tens of Pa with a gas flow through the cell that ensures a permanent gas replacement, which guarantees the absence of impurities. The plasma parameters in the discharge cell were estimated using current-voltage characteristics of the discharge. Based on the analysis of detailed kinetics, it is shown that in Xe-H₂ mixtures with low hydrogen content, complete dissociation of molecular hydrogen in the plasma is achieved in the absence of significant ionization and excitation of hydrogen atoms. This fact makes it possible to use such a system as a calibrated hydrogen atom source for various applications. Based on this H-atom source a new calibration method for H-fs-TALIF was proposed and the ratio of two-photon absorption cross-sections for H and Kr was reconstructed. The obtained estimate of the ratio of two-photon absorption cross-sections for H and Kr for broadband femtosecond laser excitation is 0.034±0.006, which is almost twenty times lower than the values obtained with the narrowband nanosecond lasers. This difference is explained by the significantly different spectral width of the excitation line and shows the need for independent calibration of the TALIF measurements in the femtosecond range.