Energy diagram of HCN. The quantum numbers, υ and J, indicate the vibrational state and rotational state, respectively, l-type doubling causes the splitting of every rotational level into two sub-levels, le and 1f, which are also denoted in this figure. Pumping photons from the ground state to highly vibrationally excited states are indicated by cyan arrows. The J = 2−1 (0, l^le, 0) and J = 2−1 (0, l^lf, 0) transitions are indicated by the blue and purple arrows, respectively. The direct l-type transitions are marked by red arrows. We note that the separations of the l-type doublets are not drawn to scale.