  Real-time time-dependent density functional theory, in conjunction with the
Ehrenfest molecular dynamics scheme, is becoming a popular methodology to
investigate ultrafast phenomena on the nanoscale. Thanks to recent
developments, it is also possible to explicitly include in the simulations a
time-dependent laser pulse, thereby accessing the transient excitation regime.
However, the complexity entailed in these calculations calls for in-depth
analysis of the accessible and yet approximate (either "dressed" or "bare")
quantities, in order to evaluate their ability to provide us with a realistic
picture of the simulated processes. In this work, we analyze the ultrafast
dynamics of three small molecules (ethylene, benzene, and thiophene) excited by
a resonant laser pulse in the framework of the adiabatic local-density
approximation. The electronic response to the laser perturbation in terms of
induced dipole moment and excited-state population is compared to the results
given by an exactly solvable two-level model. In this way, we can interpret the
charge-carrier dynamics in terms of simple estimators, such as the number of
excited electrons. From the computed transient absorption spectra we unravel
the appearance of nonlinear effects such as excited-state absorption and
vibronic coupling. In this way, we observe that the laser excitation affects
the vibrational spectrum by enhancing the anharmonicities therein while the
coherent vibrational motion contributes to stabilize the electronic excitation
already within a few tens of femtoseconds.
