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Real-time time-dependent density functional theory, in conjunction with theEhrenfest molecular dynamics scheme, is becoming a popular methodology toinvestigate ultrafast phenomena on the nanoscale. Thanks to recentdevelopments, it is also possible to explicitly include in the simulations atime-dependent laser pulse, thereby accessing the transient excitation regime.However, the complexity entailed in these calculations calls for in-depthanalysis of the accessible and yet approximate (either "dressed" or "bare")quantities, in order to evaluate their ability to provide us with a realisticpicture of the simulated processes. In this work, we analyze the ultrafastdynamics of three small molecules (ethylene, benzene, and thiophene) excited bya resonant laser pulse in the framework of the adiabatic local-densityapproximation. The electronic response to the laser perturbation in terms ofinduced dipole moment and excited-state population is compared to the resultsgiven by an exactly solvable two-level model. In this way, we can interpret thecharge-carrier dynamics in terms of simple estimators, such as the number ofexcited electrons. From the computed transient absorption spectra we unravelthe appearance of nonlinear effects such as excited-state absorption andvibronic coupling. In this way, we observe that the laser excitation affectsthe vibrational spectrum by enhancing the anharmonicities therein while thecoherent vibrational motion contributes to stabilize the electronic excitationalready within a few tens of femtoseconds.

Understanding real-time time-dependent density-functional theory simulations of ultrafast laser-induced dynamics in organic molecules