Attosecond-resolved evolution of a laser-dressed helium atom: interfering excitation paths and quantum phases.

Using high-order harmonic attosecond pulse trains, we investigate the photoionization dynamics and transient electronic structure of a helium atom in the presence of moderately strong (∼10(12)  W cm(-2)) femtosecond laser pulses. We observe quantum interferences between photoexcitation paths from the ground state to different laser-dressed Floquet state components. As the intensity ramps on femtosecond time scales, we observe switching between ionization channels mediated by different atomic resonances. Using precision measurements of ion yields and photoelectron distributions, the quantum phase difference between interfering paths is extracted for each ionization channel and compared with simulations. Our results elucidate photoionization mechanisms in strong fields and open the doors for photoabsorption or photoionization control schemes.