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X-ray absorption spectroscopy on attosecond and femtosecond timescales is a frontier of modern ultrafast science. For the last two decades, ultrafast lasers produced ever growing powers that facilitate implementation of attosecond high-harmonic soft X-ray sources in compact equipment. This unique broad-band light source opens the door for numerous applications; in particular it is ideal for chemical dynamics studies due to its sensitivity to electronic and vibrational state changes. Here, we outline briefly the evolution of time-resolved technology and review several case studies in which extreme ultraviolet and soft X-ray spectroscopy prove its exclusive sensitivity to the electronic structure of molecular species. 

X-ray Transient Absorption Spectroscopy (XTAS) and theoretical calculations are used to study CCl 4 + prepared by 800 nm strong-field ionization. XTAS simultaneously probes atoms at the carbon K-edge (280–300 eV) and chlorine L-edge (195–220 eV). Comparison of experiment to X-ray spectra computed by orbital-optimized density functional theory (OO-DFT) indicates that after ionization, CCl 4 + undergoes symmetry breaking driven by Jahn–Teller distortion away from the initial tetrahedral structure (T d ) in 6 ± 2 fs. The resultant symmetry-broken covalently bonded form subsequently separates to a noncovalently bound complex between CCl 3 + and Cl over 90 ± 10 fs, which is again predicted by theory. Finally, after more than 800 fs, L-edge signals for atomic Cl are observed, indicating dissociation to free CCl 3 + and Cl. The results for Jahn–Teller distortion to the symmetry-broken form of CCl 4 + and formation of the Cl–CCl+3 complex characterize previously unobserved new species along the route to dissociation. 

Abstract Electronic relaxation in organic chromophores often proceeds via states not directly accessible by photoexcitation. We report on the photoinduced dynamics of pyrazine that involves such states, excited by a 267 nm laser and probed with X-ray transient absorption spectroscopy in a table-top setup. In addition to the previously characterized¹B_2u(ππ*) (S₂) and¹B_3u(nπ*) (S₁) states, the participation of the optically dark¹A_u(nπ*) state is assigned by a combination of experimental X-ray core-to-valence spectroscopy, electronic structure calculations, nonadiabatic dynamics simulations, and X-ray spectral computations. Despite¹A_u(nπ*) and¹B_3u(nπ*) states having similar energies at relaxed geometry, their X-ray absorption spectra differ largely in transition energy and oscillator strength. The¹A_u(nπ*) state is populated in 200 ± 50 femtoseconds after electronic excitation and plays a key role in the relaxation of pyrazine to the ground state.