A systematic description of coherent ultrafast x-ray spectroscopies in terms of nonlinear response functions and susceptibilities is developed. Correlation-function expressions of charge and current densities provide a unified treatment and classification of information content of the various possible techniques and connect them with their optical counterparts. Applications to pump-probe and four-wave mixing measurements are discussed.

• Received 14 December 2000

DOI:https://doi.org/10.1103/PhysRevA.63.063405