Energy level diagram for the CO-paraH₂ (top panel) and CO-orthoH₂ (bottom panel) van der Waals complex. Top panel: the energy levels are labelled by quantum numbers J, j_CO and l, where J is the total angular momentum; j_CO refers to the rotation of the CO sub-unit; and l refers to the end-over-end rotation of the complex. K corresponds to the projection of J onto the intermolecular axis. The labels e and f indicate the parity of the J levels within a given stack. The parity of an even-J level is “+” for stacks labelled with e and “−” for f, while the parity of an odd-J level is “−” for e and “+” for stacks labelled f. The insert shows the approximate geometrical structure of the CO-H₂ complex (see Potapov et al. 2009b, for details). Bottom panel: the energy levels are labelled by quantum numbers J, parity P and n_J,P, a consecutive number of the state for the given values of J and P (see Jankowski et al. 2013 for details). In both panels, the targeted transitions are indicated by arrows.