  The strong coupling between two autoionizing states in helium is studied
theoretically with the pump-probe scheme. An isolated 100-as XUV pulse is used
to excite helium near the 2s2p(1P) resonance state in the presence of an
intense infrared (IR) laser. The laser field introduces strong coupling between
2s2p(1P) and 2p^2(1S) states. The IR also can ionize helium from both
autoionizing states. By changing the time delay between the XUV and the IR
pulses, we investigated the photoelectron spectra near the two resonances. The
results are used to explain the recent experiment by Gilbertson et al. [Phys.
Rev. Lett. 105, 263003 (2010)]. Using the same isolated attosecond pulse and a
540-nm laser, we also investigate the strong coupling between 2s2p(1P) and
2s^2(1S) by examining how the photoelectron spectra are modified versus the
time delay and the possibility of observing Autler-Townes doublet in such
experiments.
