May 6 2014
Fluorescence is the emission of light by an atom, or molecule, following the absorption of light, or other radiation, by the molecule. The emitted light arises due to the transition of the excited electrons from the first singlet level to ground level.
The emitted light has a longer wavelength, and thus lower energy, when compared to that of the absorbed light. However, one electron can absorb two photons upon absorbing a large amount of electromagnetic radiation, thereby emitting radiation of a shorter wavelength when compared to that of absorbed radiation.
Resonance fluorescence is another phenomenon, where the wavelength of emitted radiation is similar to that of absorbed radiation.
Basic Principle of Fluorescence
Fluorescence is a three-stage process that takes place in molecules known as fluorescent dyes, or fluorophores. These fluorophores are sensitive to certain types of stimulus, and are confined to a specific region of a molecule.
During absorption, the electrons in the molecule are excited by a high energy light, which leads to the transition of electrons from the ground state to the excited state. Following this, the electrons in the excited state rapidly relax to the lower energy level within a few pico-seconds. After a lag period of a few nano-seconds, which is known as the fluorescence lifetime, the electrons in the lower level drop to the ground state, thereby releasing all the stored energy of emitted photon.
The difference between the energy required for excitation and emission is referred to as the Stokes shift. Thus, the excited molecule will be capable of transferring the energy to a second molecule, which releases fluorescence upon excitation.
Quantum Yield
The fluorescence quantum yield is the ratio of the number of emitted photons to the number of absorbed photons. It indicates the efficiency of fluorescent phenomenon, and measurement is carried out by comparison with a standard.
Φ = Number of emitted photons/Number of absorbed photons
The maximum fluorescence quantum yield was estimated to be 1. However, compounds with quantum yields of 0.10 are also said to be less fluorescent.
Applications of Fluorescence
Fluorescence processes find application in the following:
- Fluorescent lamps
- Biological detectors
- Spectroscopy/chemical sensors
- Fluorescent labeling
- Mineralogy
- Forensic applications
Sources and Further Reading
- Fluorescence Fundamentals – Life Technologies