Simulated population of the Rydberg states of n ≥ 6 as a function of the π step position for pulse durations of 48 fs, 35 fs, 24 fs, and 12 fs, respectively. In each panel there are two curves corresponding to two different situations, taking the temporal phase θ(t) in Eq. (5) into account or not. The laser intensity is 1×10¹⁴ W/cm² and the central wavelength is 800 nm.