Photoelectron spectra of hydrogen atoms exposed to pulses of τ₀ = 6 fs with carrier frequency ω₀ = 53.605 eV and chirp parameters for ξ = 0, ±1. All curves are calculated by velocity gauge [(a) and (d)], length gauge [(b) and (e)], and Kramers–Henneberger theory [(c) and (f)], respectively.