Impulsive Stimulated X-ray Raman Scattering
Still Under Construction
(Text is from the Results of the recent Ultrafast Roundtable Discussion: PRO 1: Probing and controlling electron motion within a molecule: The attosecond frontier; which involved member of the ATO task.)
X-ray free-electron lasers are important tools for studying matter on its natural time and length scales due to their broad coherent bandwidth and high pulse energy. When these extremely bright beams are focused, the x-ray photon density (which is related to the field strength) can become so high that a single molecular target may interact with multiple x-ray photons. Initially these non-linear interactions were observed through sequential ionization of the target species. The interaction of a target system with multiple laser photons has led to the development of non-linear spectroscopy, which encodes a great deal of information compared to the linear response. In the optical regime, development of non-linear spectroscopy has led to sophisticated probes of molecular excited states. Similar developments will be made in the x-ray domain.
As an example, consider, impulsive stimulated x-ray Raman scattering (SXRS) as a nonlinear technique for preparing valence electronic wavepackets at a desired atomic site. As shown in the cartoon to the left, under SXRS, an inner-shell electron is excited near to an unoccupied valence state or to the continuum. Before this core-excited state can decay, a second interaction with the x-ray field stimulates emission of a photon and the core-vacancy is re-filled. In the impulsive limit of SXRS both the pump and Stokes (stimulating) frequencies are contained in the same laser pulse. Therefore when the exciting laser pulse has a sufficient bandwidth, spanning the energy separation between the ground and excited electronic states, the refilling is similarly probable from multiple valence excited and/or ground states. Moreover, if the excitation borrows largely from the oscillator strength of a particular atomic site, then the wavepacket has a definite and reproducible starting location. This approach provides a way to control the time-dependent electronic distribution in a molecule and to observe how various deformations affect the persistence of charge motion.
In addition to such nonlinear excitation mechanisms, there are powerful nonlinear probing techniques that have been developed theoretically to extend multidimensional spectroscopy to atom-specific measurements in the x-ray domain. For example to probe non-adiabatic dynamics near conincal intersecrtions, and to characterize the coupling of valence electrons near different atomic sites.