Sketch of the excitation mechanisms contributing to the population of the (v, J) = (1, 0) and (1, 2) levels. Direct population is due to CRs (blue arrows), and indirect population (black arrows) occurs from the decay of electronic excited states previously populated by the radiative excitation of interstellar UV photons and by collisional excitation by CRs (magenta arrow) and from the decay of higher vibrational levels (v ≥ 2) formerly populated as a byproduct of the H₂ formation process (orange arrow). The four red arrows show the near-infrared (‘NIR’) transitions listed in Table 1. We have magnified the region of v = 0, 1 levels for clarity.