Doppler velocity offset between the I₂ model and observations after zero-point correction. Top: individual Doppler offsets in chunks of 200 km s⁻¹ width (black circles). Gray dashed lines show uncertainty limits as estimated in Knöckel et al. (2004). The red line shows a spline fit to the Doppler offsets showing a systematic pattern in the offsets. Middle: relative I₂ line intensities as determined from the fitting procedure (see text). Bottom: observed spectrum of I₂ for comparison.