In general fluorescence is the emission of light which occurs within about one hundred nanoseconds of excitation. Longer lived emission is termed Phosphorescence. Many minerals and biological molecules fluoresce when irradiated with ultraviolet light. The term is derived from the mineral fluorspar which is naturally fluorescent and which was used as a flux (latin fluere-flow) in smelting.
In molecular photochemistry, fluorescence is defined as the emission of light associated with a transition between states of the same spin multiplicity. The radiative or natural lifetime of molecular fluorescence is typically between one and one hundred nanoseconds, but, as discussed by Turro (1991), competing decay processes often result in an observed lifetime less than this. Resonance fluorescence, where absorption and emission wavelengths are the same, occurs in low pressure gases where collisional deactivation is inefficient. In condensed phases, fluorescence usually occurs from the lowest vibrational level of the emitting state. Molecular fluorescence usually arises from the transition from the first excited singlet to the ground state. The emission spectrum is often a rough mirror image of the lowest energy absorption band but shifted up to a few tens of nanometers to higher wavelength (See Photoluminescence for details of absorption and emission processes and spectra).
Spectrofluorimetry is a very sensitive analytical technique and fluorescent compounds are used as molecular "probes" and "labels". Fluorescein and Rhodamine are examples of highly fluorescent dyes which emit in the green and orange-red spectral regions. Fluorescence from organic compounds is discussed by Krasovitskii and Bolotin (1988).
Turro. N. J. (1991) Modern Molecular Photochemistry, Univeresity Science Books, California.
Krasovitskii, B. M. and Bolotin, B. M. (1988) Organic Luminescent Materials, VCH, Weinheim. DOI: 10.1016/S0003-2670(00)83503-5