Fluorescence

Key features of fluorescence

• Fluorescence takes place when a substance absorbs light of a specific energy level, gets excited to a higher energy state, and then quickly emits light of a lower energy (longer wavelength) as it returns to its ground state. This emission typically happens within a very short time frame, ranging from nanoseconds to milliseconds. Fluorescence involves:
  • Light absorption: The substance absorbs light of a specific wavelength, exciting an electron within the molecule to a higher energy level.
  • Excited state: The excited electron wants to return to its ground state.
  • Energy emission: Instead of dropping back down, the excited molecule releases some absorbed energy as light of a lower energy (longer wavelength). This difference reflects the “lost” energy used for excitation.
• Rapid process: This emission happens very quickly, from nanoseconds to milliseconds.

Examples of fluorescence

• Fluorescent dyes: Used in highlighters, clothing, and biological experiments. These dyes absorb ultraviolet light and emit visible light, making them appear bright.
• Minerals: Some minerals fluoresce under ultraviolet light, used in identification and dating techniques.
• Chlorophyll: The green pigment in plants fluoresces under certain wavelengths, contributing to photosynthesis.

Distinguishing fluorescence from bioluminescence

• Fluorescence differs from bioluminescence as it doesn’t require complex biological reactions. It’s a purely physical process triggered by light absorption.
• While some bioluminescent systems might exhibit weak fluorescence, the primary light emission mechanism involves enzymatic reactions and doesn’t follow the principles of fluorescence.

The sub-atomic process

The subatomic process involved in fluorescence can be broken down into several key steps:

• Light Absorption: The process starts with a molecule (the fluorophore) absorbing a photon of light with a specific energy level.
  • This energy excites an electron within the molecule, promoting it from its ground state to a higher energy level (often a singlet excited state).
  • The energy of the absorbed photon and the specific electron transition determine the wavelength of the absorbed light.
• Internal Relaxation: In some cases, the excited electron might undergo non-radiative transitions within the molecule. This involves losing some energy through processes like vibrations or collisions with other molecules, without emitting light.
  • This internal relaxation typically happens within picoseconds (trillionths of a second) and doesn’t directly contribute to the observed fluorescence.
  • Radiative Emission: Eventually, the excited electron returns to its ground state, releasing energy in the form of a photon.
  • This emitted photon usually has a lower energy (longer wavelength) than the absorbed photon due to the internal energy losses mentioned above.
  • The specific energy difference between the absorbed and emitted light determines the colour of the emitted fluorescence.
• Excited State Lifetime: The time it takes for the excited electron to emit a photon and return to its ground state is known as the excited state lifetime. This typically ranges from nanoseconds to nanoseconds in fluorescent molecules.
  • A shorter lifetime indicates a faster emission rate, influencing the intensity and overall efficiency of the fluorescence process.
• Additional details: The specific energy levels involved, electron transitions, and excited state lifetimes depend on the structure and characteristics of the fluorescent molecule.
  • Fluorescence is sensitive to factors like temperature and the surrounding environment, which can affect the internal relaxation processes and emission properties.
  • While the basic principles remain the same, there are different types of fluorescence and specialized fluorophores used in various applications.