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. To better understand how luminescence works in different substances, you can consult the impact of astronomical phenomena on luminescence.
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.
Examples of this can be found in natural phenomena and technological applications, such as in fluorescent and phosphorescent materialsFor example, one could imagine a disco where the fabric and teeth glow under black light (fluorescence), an emergency exit sign radiates light (phosphorescence), and glow sticks also produce light (chemiluminescence). This is also true for marking objects and in scientific studies, where the difference in luminescence duration is crucial.
Fluorescence
Materials that emit light instantly are called fluorescent. In these materials, atoms absorb energy, causing them to enter an "excited" state. When they return to their normal state in approximately one hundred-thousandth of a second (ranging from 10^-9 to 10^-6 seconds), they release this energy in the form of tiny light particles known as photons.
Formally speaking, Fluorescence is a radiative process in which excited electrons They pass from the lowest excited state (S1) to the ground state (S0). During this transition, the electron dissipates some of its energy through vibrational relaxation, resulting in the emitted photon having a reduced energy and, consequently, a longer wavelength.
For practical applications and examples of fluorescence, you can refer to "Applications and uses of fluorescence in science and technology".
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. 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, as it involves transitions between states with different spin orientations.. 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 an 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 the subsequent emission of a photon.
The transition back to the relaxed singlet state (S0) can occur after a long delay (varying from 10^-3 to over 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. It is interesting to note how the difference in the atomic structure of the materials causes these variations in luminescence phenomena.
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.
An interesting aspect to note is that the intensity and color of the emitted light depend on the substance and the excitation wavelength, which is fundamental in the design of fluorescent and phosphorescent materials. The relationship between excitation and emission wavelengths, known as excitation and emission spectra, is key to understanding how and when these phenomena occur.
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, shows sensitivity to almost all radiation with wavelengths shorter than 300 nm, encompassing both the ultraviolet and X-ray spectra. Furthermore, certain materials can be easily excited by electrons, as exemplified by the phosphors used in television tubes.
And how do these phenomena relate to other natural events?
Understanding the differences between luminescence, fluorescence and phosphorescence also helps to understand natural phenomena such as cirrus cloud layers and other atmospheric phenomenaThis knowledge enriches the interpretation of light spectra and the interaction of light with different materials in our environment, as well as opening the door to new scientific and technological applications. Discovering how these phenomena occur and what conditions favor each of them could be key to advances in areas such as mineralogy, astronomy, and biomedicine.
