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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.