Femtosecond x-ray absorption spectroscopy with a laser-driven high-harmonic
source is used to map ultrafast changes of x-ray absorption by femtometer-
scale coherent phonon displacements. In LiBH4, displacements along an Ag
phonon mode at 10 THz are induced by impulsive Raman excitation and give rise
to oscillatory changes of x-ray absorption at the Li K edge. Electron density
maps from femtosecond x-ray diffraction data show that the electric field of
the pump pulse induces a charge transfer from the BH4− to neighboring Li+
ions, resulting in a differential Coulomb force that drives lattice vibrations
in this virtual transition state
