Now that, in bound state, the particles have quantized energy. So the system can only absorb certain kinds of photons. But why when I see the absorption graphic in books(x-axis is wavelength; y-axis is intensity, transmission percentage or sth), they are all continuous? They do have peaks, though, why at other wavelengths, there is still some amount of intensity? Why they are not 0 or nearly equal to 0? E.g. the CO O2 rotation-absorption spectrum. https://www.mecheng.osu.edu/netl/vibrational-energy-storage-high-pressure-gas-mixtures Is there existing other mechanism that let the system absorb photons continuously? I meant generally, why in quantum world it is quantized, but the in real life it is still continuous?