excited state
/ ik-SY-tid stayt /
An excited state is a molecule charged up with extra energy, like a ball you have just kicked up onto a high ledge. It does not stay there comfortably: sooner or later it tumbles back down to its resting position, releasing the energy it borrowed. For molecules, that energy usually arrives as a photon of light and leaves as heat, light, or chemistry.
More precisely, an excited state is any energy level of a molecule above its lowest, most stable ground state. When a molecule absorbs a photon, an electron jumps to a higher orbital, lifting the molecule into an excited state; it later relaxes back, sometimes emitting a photon as fluorescence or phosphorescence along the way.
It matters because every light-based method in this field is, at heart, a story of molecules being pushed into excited states and what happens as they fall back. The honest caveat is that excited states are fleeting and reactive — they may glow, dump their energy as heat, hand it to a neighbor, or even break the molecule apart, and which path wins shapes the whole measurement.
When ultraviolet light strikes a quinine molecule, an electron leaps to a higher orbital, putting the molecule in an excited state. As it relaxes, it sheds a little energy as heat and the rest as a blue fluorescent photon.
Absorbing light lifts a molecule to an excited state; relaxing back releases the energy.
An excited state's brief lifetime matters: fluorescence comes from a short-lived excited state and dies in nanoseconds, whereas phosphorescence comes from a longer-lived one and lingers far longer.