  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.
