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Free keywords:
ELECTROMAGNETICALLY INDUCED TRANSPARENCY; LASER-INDUCED TRANSITIONS;
DOUBLE-EXCITATION STATES; PHASE-SHIFTS; HELIUM; ABSORPTION; SHAPES; HEOptics; Physics;
Abstract:
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 S-1, P-1, and D-1 states, including the background continua, near the 2s2p(P-1) 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 Schrodinger 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.
