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Sub-7-femtosecond conical-intersection dynamics probed at the carbon K-edge

January 29, 2021

Conical intersections allow electronically excited molecules to return to their electronic ground state. Researchers in the group of Hans Jakob Wörner (ETH Zürich) observe the fastest electronic relaxation dynamics measured to date by extending attosecond transient-absorption spectroscopy (ATAS) to the carbon K-edge. They selectively launch wave packets in the two lowest electronic states (D0 and D1) of C2H4+. The electronic D1 → D0 relaxation takes place with a short time constant of 6.8 ± 0.2 femtoseconds. The electronic-state switching is directly visualized in ATAS owing to a spectral separation of the D1 and D0 bands caused by electron correlation. Multidimensional structural dynamics of the molecule are simultaneously observed. The results demonstrate the capability to resolve the fastest electronic and structural dynamics in the broad class of organic molecules. They show that electronic relaxation in the prototypical organic chromophore can take place within less than a single vibrational period.

Fig. 1 Attosecond transient-absorption spectroscopy of ethylene at the carbon K-edge. (A) Experimental setup. CCD, charge-coupled device. (B and C) Overview of measured data showing (B) the optical density (OD) of the unexcited target and (C) its change under the action of the pump pulse (ΔOD). (D to G) The (D) experimental and (E) calculated (complete simulation) ΔOD as a function of pump-probe delay. The dashed red boxes indicate the signal assigned to the D1 state of C2H4+. Shown at the bottom are the calculated x-ray absorption spectra of C2H4+ in its (F) electronic ground state (D0) and (G) excited state (D1). (H and I) Illustration of the conical intersection between the D1 and D0 states of C2H4+ that mediates the electronic relaxation.

The authors demonstrate attosecond transient-absorption spectroscopy at the carbon K-edge and have used it to reveal the fastest electronic relaxation process in molecular systems measured to date. They show that the technique is particularly powerful in revealing electronic-state switching at conical intersections, a property that we traced back to electron correlation. Moreover, the technique simultaneously displays a pronounced sensitivity to multidimensional structural dynamics, including electronic-structure encoding.
This study shows that electronic relaxation can take place on time scales below the shortest vibrational periods, extending previous results from frequency-domain spectroscopy (43). This finding supports the feasibility of electronic coherent control over conical-intersection dynamics (44) because electronic coherence might be sufficiently long lived (45). It further reveals the crucial importance of subfemtosecond spectroscopies for fully understanding excited-state dynamics in molecules through a clean separation of electronic and structural dynamics. The extension of ATAS from the extreme-ultraviolet domain to the carbon K-edge generalizes the considerable potential of this method to the broad class of organic molecules and thereby opens a wide range of opportunities, such as access to the ultimate time scale of the retinal isomerization reaction (46, 47). It will also enable studies of solvated molecules (48) that can now be extended from the femtosecond (49) to the attosecond time scale, giving access to the effects of solvation on the time scales of conical intersection dynamics and electronic decoherence.
Reference: Zinchenko, K.S., Ardana-Lamas, F., Seidu, I., Neville, S.P., van der Veen, J., Lanfaloni, V.U., Schuurman, M.S., and Wörner, H.J. (2021). Sub-7-femtosecond conical-intersection dynamics probed at the carbon K-edge. Science 371, 489 (10.1126/science.abf1656)

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