where E k is the position of the k-th ATI peak (k = 1, 2, 3, 4, 5), n min = 6 is the minimal number of IR photons with energy hν = 1.5 eV to produce ionization of FLU, IP = 7.9 eV is the ionization potential of FLU, and U p is the fitted pondermotive energy, which was found to be U p = 2.0 eV. Based on this value, we estimated the peak intensity of the laser pulses to be 3 × 10 13 W/cm 2 , which agrees with the estimated value from the laser parameters. Details of the fitting procedure and laser parameter estimations are provided in the ESI.