Title: Calibration of hydrogen atoms measurement by femtosecond two-photon laser induced fluorescence

Abstract: A new calibration method for H-fs-TALIF is proposed, and the ratio of two-photon absorption cross-sections $\sigma^{(2)}$ for atomic hydrogen (H) and krypton (Kr) is determined for the broadband emission of a femtosecond laser system. The obtained estimate of the ratio of two-photon absorption cross-sections $\sigma^{(2)}$ for H and Kr $\sigma^{(2)}$(Kr)/$\sigma^{(2)}$(H) = $0.034\pm 0.006$, which is almost twenty times lower than the values previously obtained using narrowband nanosecond lasers. This difference is explained by the significantly different spectral width of the excitation line, and demonstrates the need for independent calibration of the TALIF measurements in the femtosecond range.