  Using extreme-ultraviolet attosecond-pulse-trains, we investigate the
photoionization dynamics of a Helium atom in the presence of moderately-strong
(~10^12 W/cm^2) femtosecond laser pulses. The electronic structure of a
laser-dressed atom is traced in real-time through precision measurements of
ion-yields and photo-electron angular distributions. Quantum interferences
between photo-excitation paths are interpreted using the Floquet formalism. As
the laser pulse intensity ramps on femtosecond timescales, we observe
transitions between ionization channels mediated by different atomic
resonances. The quantum phase of interfering paths is extracted for each
channel and compared with simulations. Our results elucidate photoionization
mechanisms in strong-fields and open the doors for photo-absorption/ionization
control schemes.

Atomic Physics;Optics