Fluorescence
Introduction
Fluorescence is a type of luminescence that occurs when a substance absorbs photons and re-emits them after a brief interval. The phenomenon is named after the mineral fluorite, which exhibits this property. Fluorescence is a form of photoluminescence, a broader category of light emission processes in materials.
Physical Principles
The process of fluorescence occurs in several steps. Firstly, the fluorescent molecule, or fluorophore, absorbs a photon of light, which excites an electron to a higher energy state. This is known as the excitation process. The energy difference between the ground state and the excited state corresponds to the energy of the absorbed photon.
Following excitation, the electron undergoes a non-radiative relaxation, known as internal conversion, where the molecule loses energy through vibrational relaxation. This process typically occurs in picoseconds or less.
The electron then returns to the ground state by emitting a photon, a process known as fluorescence emission. The emitted photon has less energy, and therefore a longer wavelength, than the absorbed photon due to the energy lost during internal conversion. This shift in energy, known as the Stokes shift, is a key characteristic of fluorescence.
Properties of Fluorescence
Fluorescence has several key properties that distinguish it from other forms of luminescence. These include:
- Lifetime: The time between absorption and emission is known as the fluorescence lifetime. This can range from picoseconds to nanoseconds, making fluorescence a relatively fast process.
- Spectral properties: The absorption and emission spectra of a fluorophore are typically mirror images of each other, due to the similar vibrational levels involved in both processes.
- Quantum yield: The quantum yield of fluorescence is the ratio of the number of photons emitted to the number of photons absorbed. It is a measure of the efficiency of the fluorescence process.
- Dependence on environment: The fluorescence properties of a molecule can be influenced by its environment, including factors such as temperature, pH, and the presence of other molecules. This makes fluorescence a powerful tool for studying molecular interactions and dynamics.
Applications of Fluorescence
Fluorescence has a wide range of applications in various fields, including:
- Fluorescence microscopy: This technique uses fluorescence to image samples, providing higher contrast and specificity than traditional light microscopy.
- Fluorescence spectroscopy: This analytical method uses fluorescence to determine the properties of molecules, including their concentration, environment, and interactions with other molecules.
- Fluorescent labeling: Fluorescent dyes can be attached to molecules of interest, allowing them to be tracked and studied in biological systems.
- Fluorescent lamps: These energy-efficient light sources work by exciting a gas, which then emits UV light. The UV light strikes a fluorescent coating on the inside of the lamp, causing it to glow.