Top: Solar spectral energy density from ISS observations at 1 au (Meftah et al. 2018). Bottom: Relative rate of photoexcitation and spontaneous emission vs. wavelength for any energy level transition based on the energy density from the top curve (scaled to 1 R_⊙) as shown in the example in Section 3.2.2. For all visible (and shorter) wavelengths, the rate of spontaneous emission is at least 2 orders of magnitude smaller than photoexcitation.