XFEL 펄스 시분해 공명 X-선 산란을 이용한 Ge의 초고속 용융과정 연구

Abstract

Photoinduced nonequilibrium phase transitions stimulate the interest on dynamic interactions between electrons and crystalline ions, which has long been overlooked within the Born-Oppenheimer approximation. Ultrafast melting ranging from picosecond to sub-picosecond timescale before lattice thermalization prompts researchers to revisit this issue to understand ultrafast photoinduced weakening of the crystal bonding. However, absence of direct evidence manifesting the role of orbital dynamics in lattice disorder leaves it elusive. Recently, the advent of X-ray Free Electron Laser (XFEL) facility enabled us to tackle this subject by adapting ultrafast time-resolved investigation to the conventional resonant x-ray scattering technique. In this thesis, I directly monitor ultrafast dynamics of bonding orbitals of germanium crystal, driving the photoinduced melting, by performing time-resolved resonant X- ray scattering with X-ray free-electron laser (XFEL) pulses. From the temporal evolution of forbidden reflection in germanium, it is verified that increased photoexcitation of bonding electrons amplifies the orbital disturbance to expedite the lattice disorder approaching the sub-picosecond scale of the nonthermal regime. The lattice disorder time is estimated and it shows strong nonlinear dependence on the laser fluence with a crossover behavior from thermal-driven to nonthermal-dominant kinetics, 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 interpretations on photoinduced melting and addresses the fundamental science issue on the impart of bonding electron on solid-liquid transition. It is shown that single-particle diffraction patterns, collected from Au sphere nanoparticles, using XFEL pulses can provide information about the incident photon flux and coherence property simultaneously of single XFEL pulse, and the overall X-ray beam profile is inferred. The proposed scheme is highly adaptable to most experimental configurations, and will become an essential approach to understanding single X-ray pulses.