³⁵Molecules typically have much more complex interactions with light than individual atoms. The optical signatures of atoms are principally defined by electron states, light absorbed when electrons are boosted into higher-energy states and light emitted when electrons fall into lower-energy states. Molecules, on the other hand, can absorb and release energy in the inter-atomic bonds as well as in the states of individual electrons. Since molecules have more degrees of freedom with respect to optical interactions, their optical signatures tend to be much broader. This is why molecular absorption spectra consist of broad bands of wavelengths (each band comprised of many discrete lines), while atomic absorption spectra consist of relatively few lines.