Observing the Real-Time Evolution of Helium Atoms in a Strong Laser Field

Abstract

The interaction of a strong laser field with an atom significantly modifies its atomic structure. Such an atom can be modeled using the Floquet theory in which the atomic states are described by Floquet states composed of several Fourier components. We use high-order harmonics present in extreme-ultraviolet (XUV) attosecond pulse trains (APTs) to create excited states in infra-red(IR) laser dressed He atoms which are ionized by the dressing laser field itself. The quantum interference between different components of the Floquet states leads to oscillation in the ion yield as a function of XUV-IR time delay. We measure the phase of this quantum interference process through the phase of the ion yield signal which allows us to follow the evolution of the dressed atom, in real-time, as the intensity of the IR field is varied. We observe a transition from a 5p Floquet state dominated ionization to a 2p Floquet state dominated ionization with increasing IR intensity.