Influence of Coherent Adiabatic Excitation on Femtosecond Transient Signals

Femtosecond laser pulses are the tools of choice for inducing and tracking the temporal evolution of electronic excitation in molecular systems. To obtain this information, a proper theoretical modeling of the observables monitored in these experiments is essential. Herein, we present a coherent approach to simulate the time-dependent signals that result from femtosecond pump-probe experiments with ionization detection. Thus, the transient signals derived from femtosecond pump-probe experiments are analyzed in terms of the coherent evolution of the energy levels perturbed by the excitation pulse. The model system is treated as the sum of independent two-level subsystems that evolve adiabatically or are permanently excited, depending on the detuning from the central wavelength of the excitation laser. This approach will allow us to explain numerically and analytically the convergence between the coherent and incoherent (rate equations) treatments for complex multi-level systems. It will be also shown that the parameter that determines the validity of the incoherent treatment is the distribution of states outside and inside the laser bandwidth, rather than the density of states as it is commonly accepted.