Nonlinear coherence effects in transient-absorption ion spectroscopy with stochastic extreme-ultraviolet free-electron laser pulses

• Main Authors: Ding, Thomas, Rebholz, Marc, Aufleger, Lennart, Hartmann, Maximilian, Meyer, Kristina, Stooß, Veit, Magunia, Alexander, Wachs, David, Birk, Paul, Mi, Yonghao, Borisova, Gergana Dimitrova, da Costa Castanheira, Carina, Rupprecht, Patrick, Loh, Zhi Heng, Attar, Andrew R., Gaumnitz, Thomas, Roling, Sebastian, Butz, Marco, Zacharias, Helmut, Düsterer, Stefan, Treusch, Rolf, Cavaletto, Stefano Michele, Ott, Christian, Pfeifer, Thomas
• Other Authors: School of Physical and Mathematical Sciences
• Format: Article
• Language: English
• Published: 2020
• Subjects: Science::Chemistry; Strong Electromagnetic Field Effects; Ultrafast Phenomena
• Online Access: https://hdl.handle.net/10356/141913
• Institution: Nanyang Technological University

We demonstrate time-resolved nonlinear extreme-ultraviolet absorption spectroscopy on multiply charged ions, here applied to the doubly charged neon ion, driven by a phase-locked sequence of two intense free-electron laser pulses. Absorption signatures of resonance lines due to 2p-3d bound-bound transitions between the spin-orbit multiplets ^{3}P_{0,1,2} and ^{3}D_{1,2,3} of the transiently produced doubly charged Ne^{2+} ion are revealed, with time-dependent spectral changes over a time-delay range of (2.4±0.3) fs. Furthermore, we observe 10-meV-scale spectral shifts of these resonances owing to the ac Stark effect. We use a time-dependent quantum model to explain the observations by an enhanced coupling of the ionic quantum states with the partially coherent free-electron laser radiation when the phase-locked pump and probe pulses precisely overlap in time.