S. Tanaka et al., Time-resolved x-ray spectroscopies: Nonlinear response functions and Liouville-space pathways - art. no. 063405, PHYS REV A, 6306(6), 2001, pp. 3405
A systematic description of coherent ultrafast x-ray spectroscopies in term
s of nonlinear response functions and susceptibilities is developed. Correl
ation-function expressions of charge and current densities provide a unifie
d treatment and classification of information content of the various possib
le techniques and connect them with their optical counterparts. Application
s to pump-probe and four-wave mixing measurements are discussed.