Light interacts with matter in various ways, and one fascinating phenomenon is fluorescence, where a substance absorbs light at one color and then re-emits it as a different color. This re-emitted light is typically of a longer wavelength. The difference in wavelength between the light a molecule absorbs and the light it subsequently emits is a fundamental concept known as the Stokes shift. This spectral shift is a universal law in fluorescence, named after Irish physicist George Gabriel Stokes who first observed this principle.
What is Stokes Shift?
Since longer wavelengths correspond to lower energy, the emitted light carries less energy than the absorbed light. This difference in energy between the absorbed and emitted photons is precisely what the Stokes shift quantifies.
When a molecule absorbs a photon, it gains energy and transitions to an excited state. This excited state is temporary, lasting for a very short period, typically around 10 nanoseconds (10^-8 seconds).
The molecule then releases some of this absorbed energy before emitting a new photon. Due to this energy loss, the emitted photon always has lower energy and a longer wavelength than the absorbed photon.
The Molecular Explanation
The underlying reasons for the Stokes shift involve several rapid molecular processes that occur after a molecule absorbs light.
When a molecule absorbs a photon, its electrons jump from a lower energy state, called the ground state (S0), to a higher electronic energy state (S1 or even higher states). This absorption process happens extremely quickly, within femtoseconds (10^-15 seconds), meaning the atomic nuclei and surrounding solvent molecules remain essentially fixed in their positions during this initial energy uptake.
Once in the excited electronic state, the molecule quickly undergoes a process called vibrational relaxation. The excited electronic state contains multiple vibrational energy levels, and the molecule rapidly loses energy as it falls from higher vibrational levels to the lowest vibrational level within that excited electronic state. This energy is typically dissipated as heat to the surrounding environment, rather than being re-emitted as light.
Following vibrational relaxation, surrounding solvent molecules reorient themselves around the excited molecule. Excited molecules often have a different charge distribution than their ground state. Solvent molecules adjust to stabilize this new distribution, further lowering the excited state’s energy.
After these relaxation processes, which collectively take place on the nanosecond timescale, the molecule emits a photon and returns to its ground electronic state. Because energy was lost during vibrational relaxation and solvent reorganization, the emitted photon has less energy than the absorbed photon. This energy difference directly accounts for the shift to a longer wavelength in the emitted fluorescent light, defining the Stokes shift.
Real-World Applications
In fluorescence microscopy, the Stokes shift is fundamental because it allows scientists to distinguish the weak emitted fluorescent signal from the much stronger excitation light. By using optical filters that block the shorter excitation wavelengths and only allow the longer emission wavelengths to pass through, researchers can clearly visualize fluorescently labeled structures within cells and tissues, reducing background noise and enhancing image contrast.
Biosensors also rely on the Stokes shift. A change in the environment or target molecule binding can cause a fluorescent probe to emit light at a different wavelength or intensity. This spectral change provides a clear signal for diagnostics or monitoring cellular processes.
Medical diagnostics benefit from Stokes shift in techniques like flow cytometry and immunoassays. In flow cytometry, different cell types can be identified by labeling them with multiple fluorescent dyes, each with a distinct Stokes shift. This allows simultaneous detection of various markers, invaluable for complex biological analyses. The separation of excitation and emission spectra also increases the signal-to-noise ratio, enabling detection of low-abundance substances.
The Stokes shift also plays a role in optoelectronic devices, such as organic light-emitting diodes (OLEDs) and solar concentrators. In OLEDs, understanding the energy conversion and emission properties, which are influenced by the Stokes shift, helps in designing efficient and brightly emitting materials. For solar concentrators, materials with large Stokes shifts can absorb sunlight over a broad spectrum and re-emit it at a longer wavelength, which can then be efficiently collected by photovoltaic cells.
Variations and Related Concepts
While the typical Stokes shift involves emitted light having lower energy than absorbed light, a less common phenomenon known as the Anti-Stokes shift also exists. In this case, the emitted light possesses higher energy and thus a shorter wavelength than the absorbed light. This occurs when the molecule gains additional energy from its surroundings, often from thermal vibrations, before emitting a photon.
One notable application of the Anti-Stokes shift is in Raman spectroscopy, a technique used to analyze the vibrational modes of molecules. In Raman scattering, a small fraction of scattered light can be shifted to higher energies (Anti-Stokes lines) if the molecules are already in an excited vibrational state due to thermal energy. The ratio of Anti-Stokes to Stokes lines in Raman spectra can provide information about the temperature of the sample.
Another related concept is photon upconversion, which is an Anti-Stokes process where two or more lower-energy photons are absorbed, and then a single higher-energy photon is emitted. This process is particularly useful in bio-applications because it allows for the use of longer-wavelength excitation light, such as near-infrared light, which can penetrate deeper into biological tissues. This reduces background interference from tissue autofluorescence and enables deeper imaging for medical diagnostics and bioimaging.