Normalized spectra of a single hydrogen Rydberg atom with n = 20 in a high density background gas of hydrogen at ρ = 1015 cm⁻³. Only detunings higher than 25 MHz, where the quasistatic approximation is accurate, are shown. The quasistatic results for the line shape is depicted by the solid blue line, whereas the red line represents the line shape assuming a two-body quasistatic approach (see equation (10) in the text for details). Each of the peaks of the spectra correlates with the existence of local extrema in the Rydberg-perturber interaction potential, from Jesús Pérez-Ríos et al 2016 J. Phys. B: At. Mol. Opt. Phys. 49 14LT01 © IOP Publishing, All Rights Reserved.