Population Trap in X-ray-induced Ultrafast Nonadiabatic Dynamics of Tropone Probed at the O(1\textit{s}) pre-edge

Nonadiabatic transition (NAT) drives a variety of x-ray-induced photochemistry and photophysics used in nature and various fields. To clarify the x-ray-induced NAT dynamics, we performed nonadiabatic molecular dynamics simulations on electronically excited tropone (Tr) dications created by the carbon $KLL$ normal Auger decay. The Tr$^{2+}$ undergoes the NAT cascade via 10-10$^2$ states with time constants of 200-400 fs. We observed population traps in the highly excited states in 100 fs during the NAT cascade. The fingerprint of this population trap can be extracted from C($1s$) edge pump O($1s$) pre-edge probe femtosecond transient x-ray absorption spectra measured by the O($1s$) Auger electron yield method (TR-AEYS) using intense narrow band femtosecond x-ray free electron laser pulses. Our coupled ionization rate equation model demonstrates that selective and saturable C($1s$) core-ionization of Tr realizes background-free measurement. These results indicate that the importance of NAT in x-ray photochemistry and photophysics in large molecules. The real-time tracking of the NAT dynamics using TR-AEYS shall be a powerful approach for deeper insight.