We propose a general technique to retrieve the information of
dipole-forbidden resonances in the autoionizing region. In the simulation, a
helium atom is pumped by an isolated attosecond pulse in the extreme
ultraviolet (EUV) combined with a few-femtosecond laser pulse. The excited wave
packet consists of the 1S, 1P, and 1D states, including the background
continua, near the 2s2p(1P) doubly excited state. The resultant electron
spectra with various laser intensities and time delays between the EUV and
laser pulses are obtained by a multilevel model and an ab initio time-dependent
Schr\"odinger equation calculation. By taking the ab initio calculation as a
"virtual measurement", the dipole-forbidden resonances are characterized by the
multilevel model. We found that in contrast to the common assumption, the
nonresonant coupling between the continua plays a significant role in the
time-delayed electron spectra, which shows the correlation effect between
photoelectrons before they leave the core. This technique takes the advantages
of ultrashort pulses uniquely and would be a timely test for the current
attosecond technology.Comment: 10 pages, 6 figure
