X-ray absorption in solids promotes an electron from an atomic core level to the conduction band states, and near-edge spectra structure gives insight into local atomic geometry and chemical bonding effects. Based on a novel implementation of femtosecond x-ray absorption spectroscopy with a laser-driven high-harmonic source, we demonstrate the ultrafast modulation of x-ray absorption by the femtometer-scale coherent vibrational displacements. In LiBH4, displacements along an Ag phonon mode at 10 THz are induced by impulsive femtosecond Raman excitation and observed by changes in the x-ray absorption spectrum in the time domain. The Raman process involves a field-driven transfer of electronic charge from the BH4 units to neighboring Li+ ions. Our results reveal the subtle interplay of nuclear motions and electronic structure, a scenario relevant for a wide range of materials.
Citation: Physical Review B
Pub Type: Journals
coherent phonon, impulsive Raman, LiBH4, lithium borohydride, Raman, Raman tensor, x ray, x-ray absorption