Interstellar radiation field intensity I_ν at high Galactic latitude from Table 3 and Eqs. (8)–(14) as a function of the wavelength. The wavelengths of the 1725 allowed pure rotational, rovibrational, and rovibronic radiative transitions of CH⁺ are also displayed in blue, purple, and red, taking into account the first two, five, and thirteen electronic, vibrational, and rotational levels of CH⁺.