Fluorescence vs Phosphorescence
When a molecule or atom absorbs energy, it can undergo various changes. Fluorescence and phosphorescence are two such processes.
What is Fluorescence?
Electrons in an atom or a molecule can absorb the energy in the electromagnetic radiation and thereby excite to an upper energy state. This upper energy state is unstable; therefore, electron likes to come back to the ground state. When coming back, it emits the absorbed wavelength. In this relaxation process, they emit excess energy as photons. This relaxation process is known as fluorescence. Fluorescence takes place much more rapidly. Generally, it completes in about 10-5 s or less time from the time of excitation. In atomic fluorescence, gaseous atoms fluoresce when they are exposed to radiation with a wavelength that exactly matches one of the absorption lines of the element. For example, gaseous sodium atoms absorb and excite by absorbing 589 nm radiations. Relaxation takes place after this by reemission of fluorescent radiation of the identical wavelength. Because of this, we can use fluorescence to identify different elements. When excitation and reemission wavelengths are the same, the resulting emission is called resonance fluorescence. Other than fluorescence, there are other mechanisms by which an excited atom or molecule can give up its excess energy and relax to its ground state. Nonradiative relaxation and fluorescence emissions are two such important mechanisms. Because of many mechanisms, the lifetime of an excited state is brief. The relative number of molecules that fluoresce is small because fluorescence requires structural features that slow the rate of the nonradiative relaxation and enhance the rate of fluorescence. In most molecules, these features are not there; therefore, they undergo nonradiative relaxation, and fluorescence does not occur. Molecular fluorescence bands are made up of a large number of closely spaced lines; therefore, usually it is hard to resolve.
What is Phosphorescence?
When molecules absorb light and go to the excited state they have two options. They can either release energy and come back to the ground state immediately or undergo other non-radiative processes. If the excited molecule undergoes a non radiative process, it emits some energy and come to a triplet state where the energy is somewhat lesser than the energy of the exited state, but it is higher than the ground state energy. Molecules can stay a bit longer in this less energy triplet state. This state is known as the metastable state. Then metastable state (triplet state) can slowly decay by emitting photons, and come back to the ground state (singlet state). When this happens it is known as phosphorescence.
What is the difference between Fluorescence and Phosphorescence?
• When light is supplied to a sample of molecules, we immediately see the fluorescence. Fluorescence stops as soon as we take away the light source. But phosphorescence tends to stay little longer even after the irradiating light source is removed.
• Fluorescence takes place when excited energy is released, and the molecule comes back to the ground state from the singlet-excited stage. Phosphorescence takes place when a molecule is coming back to the ground state form the triplet excited state (metastable state).
• The energy released in the fluorescence process is higher than that in the phosphorescence.
• In fluorescence, the absorbed amount of energy is released back but, in phosphorescence, released energy is lower than what is absorbed.