When a molecule absorbs a photon, one of that molecule’s electrons is elevated to an orbital with more potential energy. The electron moves from its ground state to an excited state. The only photons that a molecule can absorb are those whose energy matches exactly the energy difference between the ground state and excited state of this electron. Because this energy difference varies among atoms and molecules, a particular compound absorbs only photons corresponding to specific wavelengths. Thus, each pigment has a unique absorption spectrum. Excited electrons are unstable. Generally, they drop to their ground state in a billionth of a second, releasing heat energy. Some pigments, including chlorophyll, can also release a photon of light in a process called fluorescence. If a solution of chlorophyll isolated from chloroplasts is illuminated, it will fluoresce and
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