Femtosecond orbital dynamics driving nonequilibrium melting transition revealed by ultrafast resonant X-ray scattering

Photo-induced nonequilibrium phase transitions stimulate the interest on dynamic interactions between electrons and crystalline ions, which has been largely overlooked within Born-Oppenheimer adiabatic approximations. Ultrafast melting before lattice thermalization prompts researchers to revisit this issue to propose physical models accounting for the instantaneous weakening of the crystal bonding from photo-excited electrons. However, the absence of direct evidence manifesting the role of orbital dynamics in lattice disorder leaves this interpretation speculative. In this study, by performing time-resolved resonant X-ray scattering with an X-ray free-electron laser, we directly monitored ultrafast dynamics of bonding orbitals that drive the photo-induced melting transition. Increased laser fluence amplifies the orbital disturbance to expedite the lattice disorder approaching the sub-picosecond scale of the nonthermal regime. The lattice disorder time displays strong nonlinear dependence on the laser fluence reflecting cross-over behavior from thermal-driven to nonthermal-dominant regime, which is also verified by ab initio and two-temperature molecular dynamics simulations. This study elucidates the impact of bonding orbitals on lattice stability with unifying interpretation on photo-induced melting.