Far-infrared (top), near-infrared (middle), and optical (bottom) radiation fields (in units of the local ISRF) at which the pure rotational levels of CH⁺ are equally populated by collisions and radiative pumping, depending on the kinetic temperature and the density of the gas. Below these curves, excitations are primarily driven by reactive collisions and chemical pumping. Above these curves, excitations are primarily driven by direct radiative pumping (top) and by the radiative pumping of the vibrational levels of the X¹Σ⁺ state (middle) and of the A¹Π electronic state (bottom).