Study On Attosecond Transient Absorption Spectra By Atoms

With the HHG technique grows mature gradually, more phenomena of attosecond scale behavior are probed by the attosecond pulse in atomic or molecular systems, such as the Autler-Townes splitting, Fano autoionization, and electromagnetically induced transparency. Recently, based on pump-probe technique, many experiments utilize the transient absorption spectra to analyze the dynamical processes of the atomic or molecular systems. The ACstark shifts of the states energies show attosecond scale changes with respect to the delay between the two pulses which could be hardly elucidated with the theories like the rotating wave approximation. This paper mainly studies about the attosecond transient absorption dynamical processes of the three-level system in the presence of the femtosecond pulses, and the adiabatic approximation resolution of the two-level system predigested from the three-level system with the adiabatic approximation. We analyze the interference fringes phenomena appeared in the transient absorptions spectra by He that have been probed by pump-probe experiments. For the coupling of the bounds states by the dressing field, the states change into Floquet states combined with the two-photos states of the dressing field. we find the interference fringes result from the overlap of the spectrums between the two Floquet states closed with each other where the two components interfere with respect to delay. The two-level system can give a clear picture of the interference fringes in the attosecond transient absorption spectra.Further more, we utilize the single-active-electron approximation TDSE equation to simulate the attosecond transient absorption processes by He in the presence of weaker and stronger femtosecond pulses. It is shown that the transient absorption in the presence of the stronger field is more complex than the weaker one. And we discuss the probability of the Rydberg states at the end of the evolvement with each delay. It is could be found that the probabilities are influenced by the multi-photos transition processes which are controlled by the delay. That provides a reference for increasing the probabilities of the Rydberg states. In the end, we attempt to utilize the MCTDHF to calculate the transient absorption spectra by He.