Fluorescence
Fluorescence can be thought of as the opposite of absorption spectroscopy. Instead of the radiation being absorbed and an electron being promoted, a photon (the unit of energy which travels at the speed of light) is emitted.
Fluorescence is the property of some atoms or molecules to absorb radiation of a particular wavelength, then after an interval known as the fluorescence lifetime, the radiation is emitted at a longer wavelength.
The absorption of radiation to promote an electron to the excited state occurs in about 10-15 seconds. Then relaxation to the lowest level of the excited state happens in around 10-11 secs. In this period, energy is transferred to the surroundings as heat. Relaxation from this state back to the ground state causes emissions of a photon and this is the process which is termed fluorescence.
The whole process is described in this diagram (a Jablonski diagram):
The emitted photon is lower in energy than the original photon, hv1. When this occurs, the molecule is said to fluoresce.
When fluorescence is used as a means of determining structures, it is always used alongside UV-vis spectroscopy. Fluorescence and absorbance peaks are related. Absorbance spectra show the vibrational characteristics of the excited state as absorbance occurs when an electron is promoted from the ground state to the excited state. Fluorescence shows the structural characteristics of the ground state as fluorescence occurs when the electron undergoes relaxation from the lowest level of the excited state to the ground state. Therefore, by using both absorbance and fluorescence, we can obtain a complete picture.
Fluorescence has some very important applications such as in the detection of DNA. A molecule known as fluorosceine is a chemical tag for DNA so we can see if DNA is present in a sample and if it is hybridizing. The are some dyes that are more specific and will only fluoresce when DNA is present.
