Main Difference – Fluorescence vs. Luminescence
Fluorescence and luminescence both describe processes where materials emit photons without the emission being caused by heat. The main difference between fluorescence and luminescence is that luminescence describes any process where photons are emitted without heat being the cause, whereas fluorescence is, in fact, a type of luminescence where a photon is initially absorbed, which causes the atom to be in an excited singlet state. As the electron falls back to the ground state, a lower-energy photon is emitted.
What is Luminescence
Luminescence refers to radiation of light from materials, which is not caused by heat. A substance which glows when its temperature has ben raised (such as a bar of metals glowing red-hot), therefore, does not exhibit luminescence.
The light is emitted when an electron in an excited state “falls” down to the ground state. When this process occurs, a photon is emitted, carrying an amount of energy equal to the energy gap between the states. The energy that a photon carries determines its wavelength: if the wavelength is in the visible region of the electromagnetic spectrum, then “light” is seen.
Chemiluminescence is a type of luminescence where light is emitted due to a chemical reaction. During chemoluminescence, a chemical reaction produces atoms with electrons in excited states. Light is emitted as they fall to the ground state. For instance, luminol is a chemical that undergoes a chemical reaction to produce a molecule with electrons in an excited state. The iron present in haemoglobin in the blood can act as a catalyst for this reaction. Therefore, luminol is often sprayed on crime scenes to see if there had been traces of blood. If blood had been present, a bluish glow is produced that can be seen in the dark for a few seconds.
Luciferin is a chemical present in fireflies, which, when oxidised, produces a glow. Similarly, the glow in jellyfish is produced by the compound aequorin.
Electroluminescence is another type of luminescence which occurs when electrons, which are accelerated by strong electric fields, collide with a material and cause the material to ionise (as in the case of gas discharge tubes), or when electrons and holes recombine in a semiconductor material.
What is Fluorescence
Fluorescence is itself a type of luminescence called photoluminescence. Here, electrons are first excited by an external photon. The excited electron may have the same spin as it did at the ground level, or the opposite spin. When spins of all electrons in the system end up being paired, the system is said to be in a singlet state. When there is a set of electrons with unpaired spins, the system is said to be in a triplet state.
The excited electron can then go back to the ground level by emitting a photon. When an electron is in an excited triplet state, if it emits a photon to go back to the ground state, the process is referred to as phosphorescence. When an electron is in the excited singlet state, when it emits a photon to go back to the ground level, the process is referred to as fluorescence. Compared to phosphorescence, electrons spend much shorter times in their excited states in fluorescence.
The process of fluorescence takes place via several stages. First, the excited electron falls to a lower vibrational energy state, in a process named relaxation. Then, a photon is emitted as the electron falls to the ground state. After the photon emission, the electron again undergoes relaxation to fall to the lowest vibrational energy level at the ground state.
Note that during relaxation processes, the electrons lose energy but photons are not emitted. Consequently, the photons emitted during fluorescence carry less energy compared to the absorbed photon. As a result, the emission spectrum of a material undergoing fluorescence is shifted towards larger wavelengths compared to its absorption spectrum. This shift in wavelengths is called the Stokes shift.
In fluorescent lamps, ultraviolet waves are first produced by passing an electric current through a gas. The ultraviolet rays then cause fluorescence in a coating applied onto the inside of the light bulb.
Difference Between Fluorescence and Luminescence
Luminescence refers to any mechanism where photons are generated, without an input of heat.
Fluorescence refers to a specific type of luminescence where the energy to produce the photon comes from the absorption of a photon with higher energy. An excited singlet state is produced in the intermediate stages.
In luminescence processes, in general, a photon may be given off after any time. The lifetime of the electron in the excited may vary from process to process.
In fluorescence, the lifetime of the excited state is very small. Hence, photons are emitted from the atoms soon after the incident photons are absorbed.
“Luminol and Hemoglobin. Luminol glows in an alkalic solution when you add Hemoglobin and H2O2” by everyone’s idle from berlin, germany (http://www.flickr.com/photos/mgdtgd/140282001/) [CC BY-SA 2.5], via Wikimedia Commons