Original spectra when applying the procedure described in Sect, 2.2 with a zoom factor of 6.4 instead of 5.5 as suggested. Two distinct peaks appear whose location depends on the time of acquisition (warm colour, t < t_shift; cold colours, t > t_shift) and therefore are attributed to the change of the post-acceleration. In particular, the red colour is right before the shift and the green colour right afterwards according to the colour scale. Even if we applied a different zoom factor, more likely to be 5.5 here and therefore lower than assumed for the figure, both peaks for each spectrum would move to the left and be closer to each other and m₀.