We study theoretically the photoabsorption spectra of an attosecond extreme ultraviolet (XUV) pulse by a laser-dressed atomic system. A weak XUV excites an autoionizing state which is strongly coupled to another autoionizing state by a laser. The theory was applied to explain two recent experiments [Loh, Greene, and Leone, Chem. Phys. 350, 7 (2008); Wang, Chini, Chen, Zhang, Cheng, He, Cheng, Wu, Thumm, and Chang, Phys. Rev. Lett. 105, 143002 (2010)] where the absorption spectra of the XUV lights were measured against the time delay between the laser and the XUV. In another example, we study an attosecond pulse exciting the
2s2p(1P) resonance of helium which is resonantly coupled to the
2s2(1S) resonance by a moderately intense 540-nm laser. The relation between the photoabsorption spectra and the photoelectron spectra and the modification of the transmitted lights in such an experiment are analyzed. The role of Rabi flopping between the two autoionizing states within their lifetimes is investigated with respect to the laser intensity and detuning.
We review the theoretical investigations of the autoionzing wave packet excited by an isolated attosecond pulse and dressed by a time-delayed intense laser pulse. The few-level model is described and the applications in photoemission and photoabsorption are given. For the three-level, resonantly coupled system, the main features are explained by the Rabi oscillation modulated in the dressing field. For such a system, by precisely controlling the intensity and the time delay of the dressing pulse, we show the shaping of the attosecond pulse when propagating in a gas medium. A more sophisticated multi-level system with coupling terms involving continuum states is also developed, in which the importance of the continuum-continuum coupling is evaluated with the help of an ab initio calculation. DOI: PACS numbers: 32.80.Qk, 32.80.Zb, 42.50.Gy I.