Stimulated Electronic X-Ray Raman Scattering

Abstract

We demonstrate strong stimulated inelastic x-ray scattering by resonantly exciting a dense gas target of neon with femtosecond, high-intensity x-ray pulses from an x-ray free-electron laser (XFEL). A small number of lower energy XFEL seed photons drive an avalanche of stimulated resonant inelastic x-ray scattering processes that amplify the Raman scattering signal by several orders of magnitude until it reaches saturation. Despite the large overall spectral width, the internal spiky structure of the XFEL spectrum determines the energy resolution of the scattering process in a statistical sense. This is demonstrated by observing a stochastic line shift of the inelastically scattered x-ray radiation. In conjunction with statistical methods, XFELs can be used for stimulated resonant inelastic x-ray scattering, with spectral resolution smaller than the natural width of the core-excited, intermediate state. [5][6][7] with pulse durations in the fs range and unprecedented high intensities, hold the potential for transferring these techniques to the x-ray domain, to ultimately study the coherent interplay of electronic and vibrational degrees of freedom with high temporal and spatial resolution [8][9][10]. The high penetration depth of x rays, combined with the element and chemical sensitivity of inelastic x-ray scattering [11][12][13] could open pathways to temporally resolve complex dynamical processes such as energy transfer in light harvesting complexes or reaction dynamics of catalytic processes [14,15]. However, the cross section for x-ray Raman scattering is small compared to that in the visible spectral domain. Therefore, even at XFELs, single shot measurements are challenging, especially in dilute samples in the gas or liquid phase.This difficulty could be overcome by stimulating the Raman scattering process to produce a strong coherent amplification of the signal. We present the first demonstration of stimulated resonant electronic x-ray Raman scattering (SRXRS) in an atomic gas. Resonant x-ray Raman scattering, also referred to as resonant inelastic x-ray scattering [12], was discovered in 1974 [11] and since then has developed into a standard tool to study electronic and vibrational excitations in solids [16][17][18], liquids [19], gases [20], and on surfaces and interfaces [21,22]. Although several theoretical feasibility studies of SRXRS have been presented [23][24][25][26], no experiment has yet demonstrated this effect.SRXRS would ideally be realized by a narrow-band, tuneable two-color x-ray source: one frequency to excite an electronic inner-shell transition and the other, lower frequency to dump an electron into the short-lived core hole. Although the production of narrow-band x rays has been realized [27,28], and schemes for two-color self seeded sources have been proposed [29], there are currently no tunable, two-color, coherent sources available to perform such an experiment. Instead of this ideal, presently unrealizable approach, we report the demonstration of SRXRS in neon with a self-amplified...

The Atomic, Molecular and Optical Science instrument at the Linac Coherent Light Source

“…For detection of the lasing signal the outgoing beams are dispersed with a grating spectrometer where the atomic lasing line can be distinctly distinguished. In a conceptually similar experiment a stimulated X-ray Raman signal has been observed (Weninger et al, 2013). Here, the photon energy is tuned below the neon K-edge ionization threshold.…”

“…A femtosecond optical laser system is available for optical-X-ray pump-probe experiments. Typical experiments at the AMO instrument range from high-intensity X-ray spectroscopy (Young et al, 2010;Berrah et al, 2011;Doumy et al, 2011;Rudek et al, 2012) to timeresolved and pump-probe experiments (Cryan et al, 2010;Meyer et al, 2012;Schorb et al, 2012a;McFarland et al, 2014) as well as coherent diffractive imaging of biological objects (Seibert et al, 2011;Kassemeyer et al, 2012), aerosols (Loh et al, 2012), clusters (Gorkhover et al, 2012) and gas-phase laser-aligned molecules (Kü pper et al, 2014).…”

Reabsorption of Soft X-Ray Emission at High X-Ray Free-Electron Laser Fluences

“…Nonlinear spectroscopies, which are well established for optical wavelengths (e.g., stimulated Raman scattering), have been proposed [1][2][3] and recently pioneered in the soft x-ray regime [4][5][6]. In particular, stimulated effects in x-ray emission (XE) and resonant inelastic x-ray scattering (RIXS) promise to improve signal levels by orders of magnitude.…”

Atomistic three-dimensional coherent x-ray imaging of nonbiological systems

“…Unprecedented focused intensities at ultrashort wavelengths have led to the discovery of xray phemonena such as nonlinear multiphoton absorption in atoms, molecules, and clusters [6][7][8][9][10], atomic x-ray lasing [11,12], induced transparency or saturable absorption [6,9,10,[13][14][15], stimulated emission [16][17][18], and second harmonic generation [19]. Understanding these fundamental processes underpins the use of ultraintense XFEL pulses and fuels the dream of 3D imaging of single biomolecules using the "diffract-before-destroy" method initially envisioned at the dawn of the XFEL-enabled era of x-ray science [20,21].…”