Key Differences 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.
Physics of Fluorescence and Phosphorescence
One of the foundations of quantum mechanics is the Pauli Exclusion Principle.
No two fermions in an atom or molecule may have the same set of quantum numbers, according to this theory. Fermions are particles with a half-integer intrinsic angular momentum or "spin" of H = \[\frac{h}{2 \pi}\], where h is Planck's constant. Electrons are “spin 1/2” fermions since they have an angular momentum of H/2. The Pauli Exclusion Principle states that two electrons in the same atomic or molecular orbital must have opposite spins, which are referred to as "up" and "down" for our purposes.
A molecule of singlet state is one in which all of the electrons are matched in up and down pairs.
A triplet state is one in which two electrons in different orbitals have the same up-down or down-down direction.
Understanding the fluorescent and phosphorescent difference necessitates an understanding of electron spin and the singlet and triplet conditions. According to the Pauli Exclusion Theorem, no two electrons in an atom should have the same four quantum numbers (n, l, ml, ms), and only two electrons can share each orbital, with opposite spin states. Spin pairing refers to the opposing spin states. Because of this spin coupling, some molecules are diamagnetic and lack a magnetic field. The static electric field does not trap or repel electrons in diamagnetic molecules. Since unpaired electrons have magnetic moments that are drawn to the magnetic field, free radicals are paramagnetic.
When a molecule is exposed to a magnetic field, all of the electron spins in the molecular electric state are paired, and the electronic energy levels do not split.
When an unpaired electron is exposed to a magnetic field, it will take two different orientations, each of which imparts different energy to the device. When one electron is excited to a higher energy level, it may form a singlet or a triplet. The electron is promoted in the same spin direction as it was in the ground state in an excited singlet state. The promoted electron in a triplet excited state has the same spin direction as the other unpaired electron. The difference of spins of the excited singlet, ground singlet, and excited triplet is shown in the given figure,
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Jablonski Diagram Fluorescence and Phosphorescence
A molecule's energy states are shown in a Jablonski diagram.
(The man's name is pronounced Jaboski correctly.) The accented N in the Russian word“HeT,” or Nyet in English, is similar to the English “Y,” the L with a line through it is similar to the “W” in water, and the L with a line through it is similar to the “W” in water. For English spelling, Jaboski is spelt YabWoNYski, with the emphasis on the second syllable. He is one of those poor ones who, like Khrushchev and Gengis Kahn, will have their names mispronounced by English speakers for the rest of their lives.)
The energy levels are grouped vertically, and the spin states are grouped horizontally in those diagrams. The thick lines labelled S0 and S1 are electrical levels, and the thinner lines are vibrational levels; the black lines reflect energy levels. Electrons in up or down angular momentum states are represented by the gold dots with arrows.
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Difference Between Fluorescence and Phosphorescence
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.
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.
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.
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.
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.
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.
Factors Affecting Fluorescence and Phosphorescence
The common factors affecting fluorescence and phosphorescence
1. Nature of Molecule
Having conjugated double bonds.
2. Temperature/Viscosity
The viscosity of the medium varies as the temperature changes, affecting the number of collisions between fluorophore molecules and solvent molecules.
The viscosity of the medium varies as the temperature changes, affecting the number of collisions between fluorophore molecules and solvent molecules.
A drop in fluorescence is almost always followed by an increase in temperature.
Fluorescence intensity rises as viscosity rises.
The probability of deactivation by internal transfer and vibrational relaxation increases as the number of collisions between molecules increases.
The reaction must be held at a constant temperature of +/- 0.1°C.
Applications of Fluorescence and Phosphorescence Spectroscopy
Phosphorescent
Shine-in-the-dark dolls, stickers, paint, wristwatch and clock dials are all examples of phosphorescent items that glow after being filled with bright light, such as a regular reading or space light. The glow of clock dial or toys or in bulb after switching off the light in the room. Hence the glow remains for some minutes or even hours in a dark room. Phosphorescent materials in sigh board illuminate during the night.
Fluorescence
Gemstones fluoresce, including gypsum, talc, jellyfish, chlorophyll extract, vitamins etc are the common examples of fluorescent. Mineralogy, gemology, medicine, chemical sensors (fluorescence spectroscopy), fluorescent labelling, dyes, biological detectors, cosmic-ray tracking, vacuum fluorescent screens, and cathode-ray tubes are only a few of the uses for fluorescence.
Fluorescent coatings are used in energy-saving fluorescent lamps and LED lamps to transform short-wavelength UV or blue light into longer-wavelength yellow light, simulating the warm light of inefficient incandescent lamps.
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FAQs on Fluorescence and Phosphorescence in Physics
1. What are fluorescence and phosphorescence in simple terms?
Both are processes where a substance absorbs light or other electromagnetic radiation and then emits it.
- Fluorescence happens almost instantly; the substance glows only while the radiation source is on. Think of a fluorescent tube light.
- Phosphorescence involves a significant delay; the substance can continue to glow for seconds, minutes, or even hours after the radiation source is turned off. Think of glow-in-the-dark stars.
2. What is the key difference in how fluorescence and phosphorescence work at the atomic level?
The main difference lies in the behavior of electrons. In fluorescence, an electron jumps to a higher energy state and quickly falls back, releasing light instantly. In phosphorescence, the electron gets temporarily trapped in a 'forbidden' energy state (a triplet state). It takes much longer for it to escape this state and fall back down, causing the delayed glow.
3. Can you give some real-world examples of fluorescence and phosphorescence?
Certainly! You can see these phenomena in many places:
- Examples of Fluorescence: Highlighter pens, fluorescent lights, tonic water under a black light, and security features on banknotes.
- Examples of Phosphorescence: Glow-in-the-dark toys and stickers, luminous watch dials, and some safety signs that glow after being exposed to light.
4. Why does the glow from phosphorescence last so much longer than fluorescence?
The long-lasting glow of phosphorescence is due to a slow, quantum-mechanically 'forbidden' transition. The excited electron gets stuck in a triplet state, a kind of holding area from which it cannot easily return to its ground state. Because the path back is inefficient, the electrons trickle back one by one over a longer period, releasing light slowly and creating a persistent glow.
5. How does a Jablonski diagram help us understand these concepts?
A Jablonski diagram is like a map that shows the different energy levels an electron can occupy. It clearly illustrates the pathways for both phenomena. It shows the initial absorption of energy, the rapid drop back to the ground state in fluorescence, and the special detour to a triplet state (called intersystem crossing) that causes the delay seen in phosphorescence.
6. Is the light emitted in fluorescence the same color as the light absorbed?
No, usually it is not. The emitted light almost always has a longer wavelength (and thus lower energy) than the absorbed light. This is known as Stokes Shift. It happens because some of the absorbed energy is lost as non-radiative vibrations or heat before the light is re-emitted, leaving less energy for the emitted photon.
7. What kinds of factors can affect or stop fluorescence?
Several factors can influence fluorescence. The process can be reduced or stopped by something called quenching. This can be caused by:
- High concentrations of the fluorescent substance itself.
- The presence of other substances that can take the energy away (like oxygen).
- Changes in temperature or the pH of the solution.
