Fig. 2. Pulse intensity dependence.

a Cartoon of the experiment: an XUV femtosecond pulse ionizes the Ne atom to a state in the continuum, which is dressed by an IR pulse, inducing a time periodicity. In frequency, this corresponds to the creation of a Floquet ladder whose states, (called sidebands, SBs) are spaced by almost an IR photon. b Photoelectron spectra collected at selected IR intensities using the quasi-monochromatic pulse (~146 fs). The shaded areas represent the spectral area and are drawn to guide the eye. c Behavior of $A_n^2$ as a function of the IR intensity. The experimental values (markers) nicely follow the prediction of Eq. (1) (solid lines). The shaded areas represent the uncertainty over the calibration of the TOF transfer function. The main band signal (MB) is divided by 2. The upper panels show the Fourier transform in the time domain of the normalized scattering amplitude for I_IR equal 10⁹, 5 × 10¹¹, and 2 × 10¹² W/cm².