Absorbed light can re-emit at lower frequency {fluorescence}|.
purpose
Re-emission can measure concentration. Fluorescent dyes can bind to molecules to trace them. Fluorescence is 100 times more accurate than UV-visible methods and is more sensitive. Fluorescence has no interference, because instrument can choose band.
cause
Molecules with rigid co-planar structures, like anthracene and naphthalene, have fluorescence. Electrons can jump to high orbital, fall back to lower excited orbital by short vibration-induced jumps, and then fall to lowest band by spontaneous emission, giving visible light. Lowest band can have vibrational levels. Fluorescence is fast.
compounds
Substances, such as amino acids tyrosine, phenylalanine, and tryptophan, can absorb ultraviolet light and emit visible light.
concentration
If concentration is less than 0.01 M, Beer's law applies.
factors
Solvent, pH, molecule interactions, and temperature affect fluorescence. Higher temperatures cause less intensity, because more jump types are possible. Nitrates quench fluorescence.
phosphorescence
Excited electrons can fall from excited singlet to triplet and then from triplet to singlet ground state if heavy atom collision is available to change angular momentum. In solids, process is slow, so phosphorescence lasts several seconds.
Physical Sciences>Chemistry>Analytical Chemistry>Light Intensity
5-Chemistry-Analytical Chemistry-Light Intensity
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Date Modified: 2022.0224