Optical lasers are extensively used to control the populations of low-energy atomic and molecular states, i.e., coherent control of valence electronic states and molecular vibrational and rotational motions. We are exploring the use of optical lasers to control x-ray and inner-shell processes. Despite the short lifetime of a K-shell hole in Ne (2.4 fs), we have demonstrated theoretically and experimentally that an intense, 800-nm laser field can couple core-excited states, e.g., transfer population from the 1s^-13p to the 1s^-13s state. In further theoretical work, we have demonstrated that this population transfer will be manifested in the resonant Auger electron spectrum (see Fig. 1). The simulations also show that the angular anisotropies of the Auger electrons will be imprinted on the sidebands that arise when continuum electrons interact with an optical field.

As XFEL sources mature and control increases over pulse properties, it is expected that highly-coherent x-ray pulses will allow population control of core-excited resonant states, as illustrated in Fig. 9(a). Our simulations demonstrate how this capability can be combined with optical lasers for coherent control of x-ray and inner-shell transitions. Asymmetries in sidebands will appear due to interference between different multiphoton quantum paths taken by Auger electrons (see Fig. 9(b)). The asymmetries are very sensitive to the x-ray and optical parameters due to the intrinsic coherence of the process. The effects can be applied to future pump-probe experiments, optical control schemes of x-ray absorption, and x-ray pulse characterization in XFELs.