  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.
