Intensity-dependent precision two-photon Doppler-free spectroscopy of Xe using narrow-bandwidth long-pulse deep-UV laser radiation

We report on a precision measurement of the ${(5p)}^{5}6p\phantom{\rule{0.16em}{0ex}}{[1/2]}_{0}\ensuremath{\leftarrow}{(5p)}^{6}\phantom{\rule{0.16em}{0ex}}^{1}S_{0}$ transition wave number of Xe by Doppler-free two-photon spectroscopy using near-Fourier-transform-limited long pulses of UV-laser radiation. The measurements led to absolute uncertainties of 750 kHz in the two-photon transition wave numbers for the five dominant isotopes of Xe, e.g., 80 118.982 918(27) ${\mathrm{cm}}^{\ensuremath{-}1}$ for $^{136}\mathrm{Xe}$. These values were combined with other precision measurements in Xe to resolve an $\ensuremath{\sim}1\text{\ensuremath{-}}\mathrm{GHz}$ discrepancy between the ionization energy of Xe obtained in recent measurements from the $(6s){[3/2]}_{2}$ metastable state [Herburger et al., Phys. Rev. A 109, 032816 (2024)] and the ionization energy listed in the NIST atomic database. The analysis indicates that the natural-abundance-weighted ionization energy of Xe should be revised by $\ensuremath{-}0.023\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ to 97 833.7641(20) ${\mathrm{cm}}^{\ensuremath{-}1}$. Large fluctuations in the UV-laser pulse intensities were exploited to characterize intensity-dependent shifts of the observed two-photon transition frequencies. Shifts of up to $\ensuremath{-}20\phantom{\rule{0.28em}{0ex}}\mathrm{MHz}$ were observed and attributed to frequency shifts arising from chirps in the amplification and up-conversion of the laser radiation rather than to the ac-Stark effect.