  The ultrafast nonadiabatic internal conversion in azomethane is explored by
the on-the-fly trajectory surface-hopping simulations of photoinduced dynamics
and femtosecond transient absorption (TA) pump-probe (PP) spectra at three
electronic-structure theory levels, OM2/MRCI, SA-CASSCF, and XMS-CASPT2. All
these dynamics simulations predict ultrafast internal conversion. On the one
hand, the OM2/MRCI and SA-CASSCF methods yield similar excited-state dynamics,
while the XMS-CASPT2 method predicts a much slower population decay. On the
other hand, the TA PP signals simulated at the SA-CASSCF and XMS-CASPT2 levels
show the similar spectral features, particularly for the similar stimulated
emission contributions, while the OM2/MRCI signals are quite different. This
demonstrates that the nonadiabatic population dynamics and time-resolved
stimulated emission signals may reflect different aspects of photoinduced
processes. The combination of the dynamical and spectral simulations definitely
provides more accurate and detailed information which sheds light on the
microscopic mechanisms of photophysical and photochemical processes.
