Definition of fluorescence and phosphorescence spectroscopy

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Photonics Dictionary

fluorescence and phosphorescence spectroscopy: Fluorescence and phosphorescence spectroscopy are techniques used to study the emission of light (photons) by molecules following their excitation by external energy sources. While both processes involve the emission of light, they differ in the nature of the excited states involved and the timescales over which emission occurs.

Fluorescence spectroscopy:

Principle: Fluorescence occurs when molecules absorb photons of higher energy (usually from ultraviolet or visible light) and are temporarily promoted to excited electronic states. These excited states are typically short-lived, with lifetimes ranging from nanoseconds to microseconds. Upon returning to the ground state, the molecules emit photons of lower energy (longer wavelength), often in the visible or near-infrared range.

Instrumentation: Fluorescence spectroscopy typically involves illuminating the sample with light of a specific wavelength (the excitation wavelength) and measuring the emitted fluorescence using a detector. The resulting fluorescence spectrum provides information about the structure, concentration, and environment of the fluorescent molecules.

Applications: Fluorescence spectroscopy is widely used in biochemistry, molecular biology, environmental science, materials science, and analytical chemistry. It is used for studying biomolecular interactions, detecting and quantifying analytes in biological and environmental samples, and characterizing fluorescent probes and markers.

Phosphorescence spectroscopy:

Principle: Phosphorescence is a type of delayed fluorescence that occurs when molecules undergo intersystem crossing from an excited singlet state to a lower-energy triplet state. Unlike fluorescence, phosphorescence involves a change in the spin multiplicity of the electronic states, resulting in longer-lived excited states with lifetimes ranging from microseconds to milliseconds or even longer. Phosphorescent molecules emit light as they return to the ground state from the triplet state.

Instrumentation: Phosphorescence spectroscopy involves similar instrumentation to fluorescence spectroscopy, but with additional considerations for detecting longer-lived emissions and minimizing background noise. It often requires specialized equipment capable of measuring emissions over extended time periods.

Applications: Phosphorescence spectroscopy is used in various fields, including materials science, organic electronics, photophysics, and analytical chemistry. It is particularly valuable for studying the properties of phosphorescent materials, including organic light-emitting diodes (OLEDs), phosphorescent dyes, and transition metal complexes.

Key differences:

Lifetime of excited states: Fluorescent states have relatively short lifetimes (nanoseconds to microseconds), while phosphorescent states have longer lifetimes (microseconds to milliseconds or longer).

Spin multiplicity: Fluorescence involves transitions between singlet electronic states, while phosphorescence involves transitions between singlet and triplet electronic states, leading to differences in selection rules and transition probabilities.

Energy transfer processes: Fluorescence typically involves rapid relaxation processes, such as internal conversion and vibrational relaxation, whereas phosphorescence may involve slower processes such as intersystem crossing and spin-forbidden transitions.

In summary, fluorescence and phosphorescence spectroscopy are valuable tools for studying the emission of light by molecules and materials. They provide insights into electronic structure, molecular dynamics, and environmental interactions and find applications in a wide range of scientific and technological fields.