Fig. 2: Femtosecond \({\cal{E}}\)(τ) field undressing of the surface bands.

IFT spectra of the 2ω_l- a, b and 3ω_l-polarization c, d responses at k_|| = 0 Å⁻¹ for excitation with high (top) and low (bottom) optical fluence. At high fluence and τ = 0 fs, in a and c, the AT doublet and the MT triplet structures are visible, respectively. As the instantaneous \({\cal{E}}\)(τ) decreases with increasing τ; however, the dressed bands converge to undressed ones within ~15 fs (cf. inset). At low fluence, \({\cal{E}}\)(τ) is sufficient to drive the 4PP process, but its dressing is minimal. Energy line profiles taken for selected delay points (color coded) are shown for each measurement in the right. The inset shows how the dressing depends on the instantaneous excitation fields, as τ is increased.