1. Absorption and Energy Levels: When a molecule absorbs light, an electron is excited from its ground electronic state to a higher energy level. This is the absorption process.
2. Vibrational Relaxation: The excited molecule is initially in a high vibrational state within the excited electronic state. This is very short-lived and the molecule quickly loses energy through collisions with surrounding molecules, relaxing to the lowest vibrational level of the excited state. This process is called vibrational relaxation.
3. Emission and Energy Loss: The excited molecule then emits a photon and returns to the ground electronic state. This emitted photon has a lower energy than the absorbed photon because some of the absorbed energy was lost during vibrational relaxation. This is the emission process.
Since energy is inversely proportional to wavelength, the emitted photon has a longer wavelength than the absorbed photon, leading to the Stokes shift.
Key points to remember:
* Stokes shift is the difference in wavelength between the absorbed and emitted light.
* Vibrational relaxation is the key reason for the Stokes shift, as it causes a loss of energy between absorption and emission.
* This difference in wavelength is important in fluorescence applications because it helps distinguish the emitted light from the excitation light.
Here's an analogy: Imagine a ball rolling up a hill. The ball gains potential energy as it goes up. Then, it rolls down the hill, losing some of its energy due to friction. The ball's final potential energy is lower than its initial potential energy. Similarly, the excited molecule loses some energy during vibrational relaxation, resulting in a lower energy emitted photon.
