Name: light emission from excited molecules
Uploaded: 2024-12-05T18:32:04.000Z
Duration: 1 min 23 s
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Chemistry

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Photoluminescence: Fluorescence and Phosphorescence

When molecules absorb UV, visible, or IR EM radiation, it promotes their valence electrons to excited states.
If these electrons emit light while relaxing, it is called photoluminescence.
Photoluminescence spectra are recorded by measuring the intensity of emitted radiation in relation to either the excitation wavelength or the emission wavelength.
When the emission of light results from an electronic transition without changing the electron spin, the process is called fluorescence.
This process typically produces light with a longer wavelength than the absorbed light and is short-lived, taking place within nanoseconds.
Fluorescence is favorable for molecules exhibiting &#960;&#8594;&#960;* transitions.
In contrast, phosphorescence involves an electronic transition from an excited state to a lower energy state with a change in electron spin.
The spin alteration leads to a longer-lasting excited state and, subsequently, a delayed emission of light, ranging from seconds to minutes.
Phosphorescence occurs over a range of wavelengths at lower energies than their absorption bands and is most favorable for molecules that have n&#8594;&#960;* transitions.

photoluminescence: fluorescence and phosphorescence

Photoluminescence is a process where a molecule absorbs light energy and re-emits it in the form of light. This phenomenon occurs when a substance absorbs photons, promoting its electrons to higher energy level excited states, followed by a relaxation process in which the electrons return to their original ground state energy levels and emit light. Photoluminescence is widely observed in various materials, including semiconductors, and organic and inorganic compounds.
A pair of electrons in a singlet spin state occupy the same electronic ground state with opposite spins, while a triplet excited state occurs when an electron&#39;s spin is no longer paired with that of the ground state. There are two main types of photoluminescence based on the electron spin states involved: fluorescence and phosphorescence.
Fluorescence is a type of photoluminescence characterized by rapid decay time, typically ranging from nanoseconds to microseconds. In fluorescence, the excited and ground states have the same electron spin multiplicity, meaning that the electron spin remains unchanged during the transition. The process involves singlet-singlet transitions, where both the excited and ground states are singlet states where all electrons are paired.
Phosphorescence is another type of photoluminescence characterized by significantly longer decay times, extending from milliseconds to minutes. In phosphorescence, the excited and ground states have different electron spin multiplicities. The process involves triplet-singlet transitions, where the excited state is a triplet state (two unpaired electrons with parallel spins), and the ground state is a singlet state. These transitions are &#39;spin-forbidden,&#39; meaning the electron spin must change during the transition.
Both fluorescence and phosphorescence can be used in various applications, such as optical sensors, bioimaging, and organic light-emitting diodes. Photoluminescence spectra are recorded by measuring the intensity of emitted radiation in relation to either the excitation wavelength or the emission wavelength. Excitation spectra are obtained by monitoring emission at a fixed wavelength while varying the excitation wavelengths. Emission spectra are obtained by using a fixed wavelength to excite molecules.

Photoluminescence is a process where a molecule absorbs light energy and re-emits it in the form of light. This phenomenon occurs when a substance absorbs ...

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Analytical Chemistry

Unit 1

Introduction to Molecular Spectroscopy

KEY TERMS AND CONCEPTS

light emission from excited molecules

Photoluminescence is light emission from molecules after they absorb light energy. A substance takes in photons, which raises electrons to excited energy levels. The electrons then relax back to the ground state and release light.

This process appears in many materials, including semiconductors, organic compounds, and inorganic compounds. The main role of electron spin helps separate photoluminescence into fluorescence and phosphorescence. In a singlet spin state, two electrons share the same ground state with opposite spins. In a triplet excited state, an electron spin is no longer paired with the ground state.

Fluorescence is the faster form of photoluminescence. Its decay time is usually in the nanosecond to microsecond range. It involves singlet-singlet transitions, so the excited and ground states have the same electron spin multiplicity. The spin does not change during this transition.

Phosphorescence lasts much longer than fluorescence. Its decay time can extend from milliseconds to minutes. It involves triplet-singlet transitions, where the excited state is a triplet state and the ground state is a singlet state. These transitions are spin-forbidden, which means the electron spin must change during the process.

Scientists measure photoluminescence with spectra that compare emitted light to either excitation wavelength or emission wavelength. An excitation spectrum is collected by monitoring emission at a fixed wavelength while the excitation wavelengths change. An emission spectrum is collected by using one fixed wavelength to excite the molecules. Fluorescence and phosphorescence are both useful in optical sensors, bioimaging, and organic light-emitting diodes.

Light Emission from Excited Molecules

dual nature of electromagnetic (em) radiation

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EM Spectrum, Waves, and Photon Energy

Electromagnetic radiation has both wave and particle features. It includes electric and magnetic field components that oscillate in planes perpendicular to each other. These fields are also perpendicular to the direction the radiation moves through space. As a wave, electromagnetic radiation is described by wavelength and frequency. Wavelength is the distance between two consecutive peaks or troughs. Frequency is the number of wave cycles that pass a point in a fixed time. The electromagnetic...

interaction of em radiation with matter: spectroscopy

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EM Radiation and Matter in Spectroscopy

Electromagnetic radiation and matter interact in ways that reveal details about atoms and molecules. In spectroscopy, scientists study how EM radiation is absorbed, emitted, transmitted, reflected, or scattered by a substance. This helps identify its chemical composition and properties. EM radiation behaves as both a wave and a particle. As a wave, it has intensity and amplitude. Intensity, also called irradiance, is the number of photons that strike a given area in a certain time. Amplitude...

molecular spectroscopy: absorption and emission

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Molecular Energy Levels in Spectroscopy

Molecular spectroscopy describes how molecules absorb and emit light by moving between energy levels. These levels are called quantum states. Unlike atoms, molecules also have rotational and vibrational energy levels, so their energy structure is more complex. Each level is separated by an energy gap. The gaps between electronic, vibrational, and rotational levels are very different in size. A diatomic molecule can be used to show these three kinds of energy levels. A molecule can absorb a...

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ultraviolet and visible (uv&ndash;vis) spectroscopy: overview

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UV-Vis Spectroscopy for Light Absorption

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uv&ndash;vis spectroscopy: molecular electronic transitions

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UV-Vis Spectroscopy and Molecular Absorption

UV-Vis spectroscopy shows how molecules absorb ultraviolet and visible light. It is used to study electronic structure by tracking how electrons move between molecular orbitals. The absorbed light must match the energy gap between the ground state and an excited state. In this process, an electron moves from the highest occupied molecular orbital, or HOMO, to the lowest unoccupied molecular orbital, or LUMO. This jump raises the electron to a higher energy level. The energy difference between...

uv&ndash;vis spectrometers

UV-Vis Spectrophotometer Parts and Samples

A UV-visible spectrophotometer measures how much UV and visible light a sample absorbs. It uses a light source, a wavelength selector, a sample holder, and a detector to build the spectrum. This makes it useful for checking liquid samples in the lab. The light source depends on the region being measured. A deuterium lamp provides UV radiation, and a tungsten lamp provides visible radiation. A monochromator or prism acts as a diffraction grating and splits the incoming light into different...

uv&ndash;vis spectrum

How Molecular Structure Shapes UV-Vis Bands

UV-Vis spectroscopy shows how a molecule absorbs light in the ultraviolet and visible ranges. When light passes through a substance, some of it is absorbed and the rest is reflected or transmitted. Molecules that absorb between 180 and 400 nm give a UV spectrum, while absorption in the 400 to 780 nm range gives a visible spectrum. A UV-Vis spectrum is a plot of absorbance versus wavelength. The y-axis shows molar absorptivity, ε, or log ε, and the x-axis shows wavelength. Absorbance describes...

uv&ndash;vis spectroscopy: beer&ndash;lambert law

Using Absorbance to Find Concentration

The Beer-Lambert law links absorbance, concentration, and path length in a solution. It is based on ideas from Johann Heinrich Lambert and August Beer. Their work helps explain how light changes as it passes through a medium. Lambert's law says that when light passes through a medium, the loss in intensity is directly proportional to the original intensity and the path length. Beer’s law adds that the transmittance of a solution stays constant if the product of concentration and path length...

Photoluminescence is light emission from molecules after they absorb light energy. A substance takes in photons, which raises electrons to excited energy levels. The electrons then relax back to the ground state and release light. This process appears in many materials, including semiconductors, organic compounds, and inorganic compounds. The main role of electron spin helps separate photoluminescence into fluorescence and phosphorescence. In a singlet spin state, two electrons share the same...

variables affecting phosphorescence and fluorescence

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Factors That Change Light Emission in Molecules

Fluorescence and phosphorescence depend on several factors that change how molecules emit light. These effects matter in analytical chemistry, biological imaging, and materials science because they help scientists detect molecular properties and see cellular structures. Molecular structure is one of the main influences. Aromatic compounds with low-energy pi-bonding to pi-non-bonding transitions often give strong fluorescence. Aliphatic and alicyclic carbonyl compounds, as well as highly...

deactivation processes: jablonski diagram

Jablonski Diagram: Light Loss Pathways

Jablonski diagrams show how excited molecules lose energy after absorbing light. They are used to track both light-emitting and non-light-emitting steps in luminescence. The diagram gives a simple picture of the energy states a molecule can move through. The lowest line in a Jablonski diagram is the ground-state energy of the molecule. This state is usually a singlet state, and it represents the condition of most molecules in solution at room temperature. The lines above it show vibrational...

photoluminescence: applications

Fluorescent and Phosphorescent Analysis

Photoluminescence is useful in chemical analysis because it is sensitive and selective. It can support both direct and indirect measurements of an analyte, which is the substance being tested. When an analyte has a strong quantum yield, it can be measured directly by fluorescence or phosphorescence. Indirect analysis is used when the analyte is not naturally fluorescent or phosphorescent, or when its quantum yield is poor. One approach is to react the analyte with a reagent to make a...

fluorescence and phosphorescence: instrumentation

fluorescence-and-phosphorescence-instrumentation

Measuring Fluorescence and Phosphorescence

Fluorometers and spectrofluorometers are instruments used to measure molecular fluorescence. They do this by choosing specific excitation and emission wavelengths. The two instruments use different light sources and wavelength-selection methods. A fluorometer uses absorption interference filters to select the excitation and emission wavelengths. Its excitation source is usually a low-pressure mercury vapor lamp. This lamp gives intense lines across the ultraviolet and visible regions.

key terms and concepts

Learn how excited molecules absorb photons and emit light through fluorescence and phosphorescence, plus how excitation and emission spectra are measured.

Light Emission from Excited Molecules | JoVE K12

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South Carolina College and Career Ready Science Standards

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Waves and Their Applications in Technologies for Information Transfer

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Physics

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Communicate scientific and technical information about why the atomic-level, subatomic-level, and/or molecular level structure is important in the functioning of designed materials.  Clarification Statement: Emphasis is on the attractive and repulsive forces that determine the functioning of the material. Examples could include why electrically conductive materials are often made of metal, fireworks and neon signs are made of certain elements, flexible but durable materials are made up of long chained molecules, and pharmaceuticals are designed to interact with specific receptors.

Light Emission from Excited Molecules

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