Vibrational energy levels of the CO₂ molecule (right) together with the rotational ladder of the ground state (left). We note that for the ground state the rotational ladder increases with ΔJ = 2. Lines connecting the vibrational levels denote the strongest absorption and emission pathways. The colour indicates the wavelength range of the transition: blue, 4–6 μm, green, 8–12 μm and red, 12–20 μm (spectrum in Fig. 2). More information on the rotational ladders is given in Sect. 2.2.