There are some terms that cause confusion in everyday common language. Among these terms we have the luminescence, fluorescence and phosphorescenceAre they the same terms? How are they different and what does each refer to?
We are going to see all of this in this article, so don't miss it.
What is luminescence?
The term luminescence refers primarily to the emission of light. In our environment, most objects emit light due to the energy they receive from the sun, which It is the brightest entity visible to us. Unlike the moon, which appears to emit light, it actually reflects sunlight, functioning similarly to a colossal stone mirror.
Basically, there are three main types of luminescence: fluorescence, phosphorescence and chemiluminescence. Among them, fluorescence and phosphorescence are classified as forms of photoluminescence. The distinction between photoluminescence and chemiluminescence lies in the mechanism of activation of luminescence; in photoluminescence, light acts as a trigger, while in chemiluminescence, a chemical reaction initiates the emission of light.
Both fluorescence and phosphorescence, which are forms of photoluminescence, depend on a substance's ability to absorb light and subsequently emit it at a longer wavelength, indicating a reduction in energy. However, The duration of this process differs significantly. In fluorescent reactions, the emission of light occurs instantaneously and is only observable while the light source remains active (such as ultraviolet lights).
In contrast, phosphorescent reactions allow the material to retain the absorbed energy, allowing it to emit light later, resulting in a glow that continues even after the light source has been extinguished. Thus, if the luminescence disappears immediately, it is classified as fluorescence; if it persists, it is identified as phosphorescence; and if it requires a chemical reaction to activate, it is called chemiluminescence.
For example, one could imagine a discotheque where the fabric and teeth emit a luminous glow under black light (fluorescence), the emergency exit sign radiates light (phosphorescence), and glow sticks also produce illumination (chemiluminescence).
Fluorescence
Materials that emit light instantly are called fluorescent. In these materials, the atoms absorb energy, causing them to enter an “excited” state. Upon returning to their normal state in about one hundred thousandth of a second (ranging from 10-9 to 10-6 seconds), they release this energy in the form of tiny particles of light known as photons.
Formally speaking, Fluorescence is a radiative process in which excited electrons pass from the lowest excited state (S1) to the ground state (S0). In the course of this transition, the electron dissipates part of its energy through vibrational relaxation, resulting in the emitted photon having reduced energy and, consequently, a longer wavelength.
Phosphorescence
To understand the distinctions between fluorescence and phosphorescence, it is necessary to briefly explore the concept of electron spin. Spin represents a fundamental characteristic of the electron, acting as a type of angular momentum that influences its behavior within an electromagnetic field. This property can only assume a value of ½ and can exhibit either an up or down orientation. Accordingly, the spin of an electron is denoted as +½ or -½, or alternatively represented as ↑ or ↓. Within the same orbital of an atom, electrons consistently exhibit antiparallel spin when in the singlet ground state (S0). Upon promotion to an excited state, the electron retains its spin orientation, resulting in the formation of a singlet excited state (S1), where both spin orientations remain paired in an antiparallel configuration. It is important to note that all relaxation processes associated with fluorescence are spin-neutral, ensuring that the electron spin orientation is conserved at all times.
In the case of phosphorescence, the process differs significantly. Rapid transitions (ranging from 10^-11 to 10^-6 seconds) occur between systems from the singlet excited state (S1) to an energetically more favorable triplet excited state (T1). This transition results in electron spin reversal; the resulting states are characterized by parallel spins on both electrons and are classified as metastable. In this case, relaxation occurs by phosphorescence, leading to another electron spin reversal and subsequent emission of a photon.
The transition back to the relaxed singlet state (S0) can occur after a long delay (ranging from 10^-3 to more than 100 seconds). During this relaxation process, non-radiative mechanisms consume more energy in phosphorescence relaxation compared to fluorescence, resulting in a larger energy difference between absorbed and emitted photons and consequently a larger shift in wavelength.
Excitation and emission spectra
Luminescence occurs when electrons in a substance are excited by the absorption of photons, subsequently releasing that energy in the form of radiation. In certain cases, The emitted radiation may consist of photons that have the same energy and wavelength as those absorbed.; this phenomenon is known as resonance fluorescence. More often than not, the emitted radiation has a longer wavelength, indicating a lower energy compared to the absorbed photons.
This transition to longer wavelengths is known as Stokes shift. When electrons are excited by short, invisible radiation, they ascend to higher energy states. Upon returning to their original state, they emit visible light with the same wavelength, exemplifying resonance fluorescence. However, these excited electrons can also revert to an intermediate energy level, resulting in the emission of a luminous photon carrying less energy than that of the initial excitation. This process, When induced by ultraviolet light, it usually manifests as fluorescence within the visible spectrum.In the case of phosphorescent materials, there is a delay between the excitation of electrons to high energy levels and their return to the ground state.
A specific substance does not respond to all wavelengths. However, there is usually a relationship between the excitation wavelength and the amplitude of the resulting emission. This relationship is known as the excitation spectrum. Similarly, A correlation can be observed between the amplitude and the wavelength of the emitted radiation, known as the emission spectrum.
It is important to note that the emission wavelength does not depend on the excitation wavelength, except in cases where substances possess multiple luminescence mechanisms. Consequently, minerals show different abilities to absorb UV light at specific wavelengths; some fluoresce under short-wavelength UV light, while others do so under long wavelengths, and some show indistinct fluorescence. The color of the emitted light often varies significantly with different excitation wavelengths.
The occurrence of these phenomena is not limited solely to the use of ultraviolet radiation; rather, excitation can be achieved by any radiation that has the appropriate energy. For example, X-rays are capable of inducing fluorescence in various substances, many of which also respond to different types of radiation. Magnesium tungstate, for example, displays sensitivity to almost all radiation with wavelengths shorter than 300 nm, spanning both the ultraviolet and X-ray spectra. Furthermore, certain materials can be readily excited by electrons, as exemplified by the phosphors used in television tubes.
I hope that with this information you can learn more about the differences between fluorescence, phosphorescence and luminescence.