Time-resolved X-ray absorption spectroscopy with a water-window high-harmonic source

Summary form only given. Time-resolved spectroscopy at element-specific absorption edges provides a powerful tool to study ultrafast phenomena of matter on the atomic scale in the gas, solid or liquid phase. Combining element sensitivity of core-to-valence transitions due to the very localized initial core state and femtosecond/attosecond time resolution of new soft X-ray sources based on high-harmonic generation from long-wavelength drivers, accurate electronic and structural information is now accessible.In this work [1], we show the first time-resolved absorption spectroscopy at the K-edge of carbon in CF4 (298 eV) and the L-edges of sulfur in SF6 (L2,3 at 181 eV, L1 at 240 eV). Strong-field ionization of CF4 by a 800 nm, 1 mJ, 45 fs pulse induces ionization to CF4+, which spontaneously dissociates to CF3+ + F from each of its first three lowest electronic states. Evidence of symmetry lowering (line splitting) and valence-Rydberg orbital mixing (intensity changes) arises directly from experimental data (see Fig. 1). Quantitative analysis with TDDFT and reaction-path calculations allows us to reconstruct the first 100 fs of the time evolution of the C-F bond length along the reaction coordinate. Similar experimental data from the dissociative ionization of SF6 to SF5+ + F leads to comparable conclusions with the advantage of simultaneous access to two different edges (L1 and L2,3). This has the advantage of allowing us to probe final states of different parities (g vs. u), thereby increasing the amount of information accessible. Qualitative reasoning and additional numerical calculations help us to understand the complex electronic changes during the dissociation process.