Carbon K-edge

Femtosecond Symmetry Breaking and Coherent Relaxation of Methane Cations via X-Ray Spectroscopy

Science 380, 713-717 (2023) 

E. Ridente, D. Hait, E. A. Haugen, A. D. Ross, D. M. Neumark, M. Head-Gordon, S. R. Leone

Understanding the relaxation pathways of photoexcited molecules is essential to gain atomistic-level insight into photochemistry. We performed a time-resolved study of ultrafast molecular symmetry breaking through geometric relaxation (Jahn-Teller distortion) on the methane cation. Attosecond transient absorption spectroscopy with soft x-rays at the carbon K-edge revealed that the distortion occurred within 10 ± 2 femtoseconds after few-femtosecond strong-field ionization of methane. The distortion activated coherent oscillations in the asymmetric scissoring vibrational mode of the symmetry-broken cation, which were detected in the x-ray signal. These oscillations were damped within 58 ± 13 femtoseconds because vibrational coherence was lost with the energy redistributing into lower-frequency vibrational modes. This study completely reconstructs the molecular relaxation dynamics of this prototypical example and opens avenues for exploring complex systems. 

X-Ray Transient Absorption Reveals the 1Au (nπ*) State of Pyrazine in Electronic Relaxation

Nat. Comm. 12, 5003 (2021)

Valeriu Scutelnic, Shota Tsuru, Mátyás Pápai, Zheyue Yang, Michael Epshtein, Tian Xue, Eric Haugen, Yuki Kobayashi, Anna I. Krylov, Klaus B. Møller, Sonia Coriani and Stephen R. Leone

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 characterized 1B2u (ππ*) (S2) and 1B3u (nπ*) (S1) states, the participation of the optically dark 1Au (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. Despite 1Au (nπ*) and 1B3u (nπ*) states having similar energies at relaxed geometry, their X-ray absorption spectra differ largely in transition energy and oscillator strength. The 1Au (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.