Difference Between Fluorescence and Phosphorescence

Fluorescence and phosphorescence both are types of molecular luminescence. A photon is absorbed by a sample solution atom, which excites a species. The emission spectrum can be used to do both qualitative and quantitative research. Since both fluorescence phosphorescence are excited by the emission of a photon, they are often referred to as photoluminescence.

Fluorescence varies from phosphorescence in that the electron spin does not change during the electronic energy transfer that causes fluorescence, resulting in short-lived electrons 10⁻⁵ s in the excited state.

The key difference between fluorescence and phosphorescence is that Fluorescence stops as soon as we take away the light source whereas phosphorescence tends to stay little longer even after the irradiating light source is removed.

When a molecule or atom absorbs energy, it can undergo various changes. Fluorescence and phosphorescence are two such processes. In addition to the above key difference, there are some other differences between the two terms such as the energy released in the fluorescence process is higher than that in the phosphorescence.

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. We call this relaxation process as fluorescence. Fluorescence takes place much more rapidly. Generally, it completes in about 10-5 seconds or less time from the time of excitation.

When gaseous atoms undergo fluorescence, atomic fluorescence takes place when 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, we call the resulting emission as resonance fluorescence.

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.

We call this state 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 we call it phosphorescence.

Difference Between Fluorescence and Phosphorescence

1. The main difference between fluorescent and phosphorescent is, fluorescence is the absorption of energy by atoms or molecules followed by immediate emission of light or electromagnetic radiation. On the other hand, phosphorescence is the absorption of energy by atoms or molecules followed by delayed emission of electromagnetic radiation.
2. Fluorescent emission of radiation or light suddenly stops on the removal of the soucre of excitation. On the other hand, phosphorescence emission of radiation remains for some time even after the removal of the source of excitation.
3. If we distinguish between fluorescence and phosphorescence, the time period or interval between the absorption and emission is the primary factor. It is very short for fluorescence and comparatively long for phosphorescence.
4. Another differentiate between fluorescence and phosphorescence is the emitted photon (light) has lower energy than the absorbed photon and emission occurs at a longer wavelength than the incident light in fluorescence and the longer wavelength than fluorescence in phosphorescence.
5. In fluorescent materials, gives an “an immediate flash or afterglow” on excitation. And the phosphorescent materials appear to “glow in the dark” because of the slow emission of light over time.
6. Fluorescence and phosphorescence difference is the excited atom has a comparatively short lifetime before its transition to a low energy state in fluorescence, and for phosphorescence, the excited atom has a comparatively long time before its transition to a low energy state.

Fluorescence vs. Phosphorescence: Comparison Chart