Monitoring molecular nonadiabatic dynamics with femtosecond X-ray diffraction

Abstract

Ultrafast time-resolved X-ray scattering, made possible by free-electron laser sources, provides a wealth of information about electronic and nuclear dynamical processes in molecules. The technique provides stroboscopic snapshots of the time-dependent electronic charge density traditionally used in structure determination and reflects the interplay of elastic and inelastic processes, nonadiabatic dynamics, and electronic populations and coherences. The various contributions to ultrafast off-resonant diffraction from populations and coherences of molecules in crystals, in the gas phase, or from single molecules are surveyed for core-resonant and off-resonant diffraction. Single-molecule [Formula: see text] scaling and two-molecule [Formula: see text] scaling contributions, where N is the number of active molecules, are compared. Simulations are presented for the excited-state nonadiabatic dynamics of the electron harpooning at the avoided crossing in NaF. We show how a class of multiple diffraction signals from a single molecule can reveal charge-density fluctuations through multidimensional correlation functions of the charge density.

Excited Electronic States in Total Isotropic Scattering from Molecules

“…The I J scattering terms play a critical role in coherent mixed scattering, which appears when there is a coherence between two electronic states. 35,40,41 This concludes the presentation of the fundamental theory.…”

Simulation of ultrafast excited-state dynamics and elastic x-ray scattering by quantum wavepacket dynamics

“…Theoretical frameworks for the simulation of TR X-ray scattering of molecules 19,20 are under continuous development; most recent contributions were achieved by Kirrander et al [21][22][23] Their ab initio method is based on the evaluation of matrix elements of the scattering operator, utilizing high-level multiconfigurational electronic wave functions and nuclear wavepackets (WPs) obtained from quantum dynamics (QD) simulations. Their approach did not only yield high-accuracy results, but also accessed the inelastic and coherent mixed components 20,[24][25][26] of the scattering signal, in addition to elastic X-ray scattering. However, due to its high computational cost, the method has been applied so far to very small molecules (H 2 and CO 2 ).…”

Theory of ultrafast x-ray scattering by molecules in the gas phase

“…Remarkably, their approach led to equations very similar to those obtained by more recent and fundamental derivations. Most notably, Cao and Wilson 25 could distinguish the same three components to the scattering signal identified in more recent work 28,[35][36][37] : elastic scattering, inelastic scattering, and scattering related to electronic coherences.…”