Photo-Induced Phase Transition of Diamond—A Nonadiabatic Quantum Molecular Dynamics Study

Irradiation of matter with a femtosecond laser pulse can commonly induce structural transition to a disordered state, such as amorphization, or creation of defects. Graphitization of diamond is a counterexample, as it is an order-to-order (solid-to-solid) phase transition. Graphitization was observed by applying soft x-ray in experiments by Tavella <i>et al.</i> [<i>High Energy Density Phys</i>ics <b>24</b>, 22 (2017)]. While the graphitization mechanism was considered to be interatomic bond breaking that forms <i>sp</i>² instead of <i>sp</i>³ bonds, how photoexcitation causes such bond change remains elusive. To elucidate the atomic mechanism of photo-induced graphitization of diamond, we performed nonadiabatic quantum molecular dynamics (NAQMD) simulations. The simulations were performed using a system consisting of 64 carbon atoms under microcanonical ensemble. Collapse of electronic band gap was observed by exciting 3% of valence electrons including <i>s</i> electrons by 40 eV, inducing the graphitization. This excitation condition corresponds to the experiments by Tavella <i>et al.</i> We will discuss detailed analysis of photo-induced changes in electronic structures and how they lead to graphitization.