A Femtosecond Time-Resolved Transient X-ray Absorption Study of Light-Induced Coupling and Transparency in Xenon

We have performed a femtosecond time-resolved transient x-ray absorption spectroscopy study to monitor the light induced coupling between bright and dark states in xenon exposed to infrared (780 nm) laser pulses with intensities up to 5$\times$10$^{13}$ W/cm$^{2}$. Significant transient variations in the inner-shell absorption spectra between $\sim $63 eV and $\sim $69 eV photon energy are observed. Near time zero, the transmission at 67 eV increases from 6(1) {\%} (field-free) to 14(3) {\%} (laser dressed). Transient absorption and transient transparency effects are interpreted within a picture of strong-field induced coupling between core-excited 4$d^{-1}(^{2}$D$_{5/2})$6$p(^{2}$P$_{3/2})$ and 4$d^{-1}(^{2}$D$_{3/2})$6$p(^{2}$P$_{1/2})$ states at 65.1 and 67 eV, respectively, and nearby dark states with \textit{ns} and \textit{nd} characters. The major features of the transient absorption spectra can be well described within a three-level (Autler-Townes) coupling scheme. Employing this model, a dipole-moment for the 4$d^{-1}(^{2}$D$_{3/2})$6$p(^{2}$P$_{1/2})$ to 4$d^{-1}(^{2}$D$_{3/2})$6$s(^{2}$S$_{1/2})$ transition of 0.4(0.1) Debye is derived.