Nanoscale Depth-Resolved Coherent Femtosecond Motion in Laser-Excited Bismuth

We employ grazing-incidence femtosecond x-ray diffraction to characterize the coherent, femtosecond laser-induced lattice motion of a bismuth crystal as a function of depth from the surface with a temporal resolution of 193±8 fs. The data show direct consequences on the lattice motion from carrier diffusion and electron-hole interaction, allowing us to estimate an effective diffusion rate of D=2.3±0.3 cm²/s for the highly excited carriers and an electron-hole interaction time of 260±20 fs.