Ultrafast resonant X-ray scattering investigation of non-thermal melting using XFELs*

Ultrafast melting transitions in noble metals and semiconductors irradiated by femtosecond IR laser have been reported with signatures of lattice disorder in less than a picosecond timescale, which is faster than electron-phonon energy transfer rates. With this mysteriously fast reaction of crystal lattice, the interest in unveiling the fundamental energy transfer mechanism has been intense with extensive research effort using ultrafast electron diffraction, time resolved X-ray diffraction, etc. By employing time-resolved resonant X-ray scattering, we have investigated the ultrafast lattice melting with femtosecond IR laser irradiation synchronized to the X-ray laser pulses from PAL-XFEL. Experimental evidence on the electron transition driven lattice disorder has been obtained, supporting the bond softening explanation on the nonthermal melting in covalent bonding crystals. IR laser fluence dependence of the melting time suggests essential role of electrons in bonding orbitals in causing the crystal melting.