High temporal and spectral resolution of stimulated x-ray Raman signals with stochastic free-electron-laser pulses

Abstract

The chaotic nature of x-ray free-electron-laser pulses is a major bottleneck that has limited the joint temporal and spectral resolution of spectroscopic measurements. We show how to use the stochastic x-ray field statistics to overcome this difficulty by correlation signals averaged over independent pulse realizations. No control is required over the spectral phase of the pulse, enabling immediate application with existing, noisy x-ray free-electron-laser pulses. The proposed stimulated Raman technique provides the broad observation bandwidth and high time-frequency resolution needed for the observation of elementary molecular events. A model is used to simulate chaotic freeelectron-laser pulses and calculate their correlation properties. The resulting joint temporal/spectral resolution is exemplified for a molecular model system with time-dependent frequencies and for the RNA base Uracil passing through a conical intersection. Ultrafast coherences, which represent a direct signature of the nonadiabatic dynamics, are resolved. The detail and depth of physical information accessed by the proposed stochastic signal are virtually identical to those obtained by phase-controlled pulses. I. INTRODUCTIONRecent advances in the generation of sub-femtosecond extreme-ultraviolet (XUV) and x-ray pulses are enabling the control of electron dynamics on their natural time scales [1][2][3][4]. This is essential for the direct manipulation of the ensuing electronic and nuclear dynamics and for the control of chemical reactions with light, with broad applications to photochemistry and photobiology [5][6][7].Free-electron lasers (FELs) provide intense pulses at frequencies ranging from the XUV to the hard-x-ray domain [2] suitable for nonlinear x-ray spectroscopy [8]. While XUV seeded FELs offer stable coherent XUV pulses [9] with the possibility of pulse shaping [10] and control [11], soft-and hard-x-ray FELs based on the selfamplified spontaneous emission (SASE) mechanism [12] provide stochastic pulses with limited longitudinal coherence, and noisy spikes in their temporal and spectral profiles. Stimulated x-ray Raman scattering, a fundamental building block of nonlinear spectroscopy [13], was recently demonstrated using hard-x-ray FEL pulses [14], but future multidimensional nonlinear x-ray spectroscopy protocols [15-17] require coherent and reproducible pulses. Self-or laser-seeding methods have been implemented to improve the coherence of hard-x-ray FEL pulses [18]. Novel techniques have demonstrated highintensity few-femtosecond pulses [19] with reduced intensity spikes [20], but with an underlying SASE structure which renders them not reproducible from shot to shot. Recently, transient redistribution of ultrafast electronic coherences in attosecond Raman signals (TRUE-CARS) [21] was proposed as a suitable technique to achieve the demanding time-frequency resolution nec-