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Free keywords:
Physics, Atomic Physics, physics.atom-ph,Quantum Physics, quant-ph
MPINP:
Research group Z. Harman – Division C. H. Keitel
Abstract:
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 2$p$--3$d$ 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
$(2.4\pm0.3)\,\text{fs}$. 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.
