Schematic illustration of the transitions between 2 rotational levels J and J′. Left: Molecule is excited to J′ level by collision (double arrow), then de-excited by photon emission (wavy arrow). Assuming an optically thin medium, the photon is not reabsorbed, and the medium is cooled. Right: Molecule is excited by absorption of a photon (single arrow), then de-excited by collisions. The transition energy injected by the photon is then transmitted to the medium in the form of kinetic energy and the medium is heated.