Resonance fluorescence in ultrafast and intense x-ray free-electron-laser pulses

TL;DR

This paper models the resonance fluorescence spectrum of neon cations driven by intense, ultrashort x-ray pulses, predicting observable signatures of Rabi oscillations and comparing chaotic and Gaussian pulse scenarios.

Contribution

It introduces a two-level model incorporating decay and ionization effects to predict resonance fluorescence spectra under ultrafast x-ray excitation, highlighting the potential to observe Rabi flopping signatures.

Findings

Resonance fluorescence spectra show clear Rabi oscillation signatures.

Chaotic and Gaussian pulse scenarios produce distinguishable spectral features.

Seeding techniques may enable observation of Rabi flopping at x-ray frequencies.

Abstract

The spectrum of resonance fluorescence is calculated for a two-level system excited by an intense, ultrashort x-ray pulse made available for instance by free-electron lasers such as the Linac Coherent Light Source. We allow for inner-shell hole decay widths and destruction of the system by further photoionization. This two-level description is employed to model neon cations strongly driven by x rays tuned to the 1s 2p-1 --> 1s-1 2p transition at 848 eV; the x rays induce Rabi oscillations which are so fast that they compete with Ne 1s-hole decay. We predict resonance fluorescence spectra for two different scenarios: first, chaotic pulses based on the self-amplified spontaneous emission principle, like those presently generated at x-ray free-electron-laser facilities and, second, Gaussian pulses which will become available in the foreseeable future with self-seeding techniques. As an example of the exciting opportunities derived from the use of seeding methods, we predict, in spite of above obstacles, the possibility to distinguish at x-ray frequencies a clear signature of Rabi flopping in the spectrum of resonance fluorescence.