Absorption of a photon causes a transition of the chlorophyll molecule from its ground state to its excited state. The photon boosts an electron to an orbital where it has more potential energy. If the illuminated molecule exists in isolation, its excited electron immediately drops back down to the ground-state orbital, and its excess energy is given off as heat and fluorescence. A chlorophyll solution excited with ultraviolet light fluoresces with a red-orange glow.
Urry, Lisa A.. Campbell Biology (p. 195). Pearson Education. Kindle Edition.