In anti-Stokes shift, the excitation and emission wavelengths are reversed compared to the usual Stokes shift observed in fluorescence or phosphorescence processes. This phenomenon occurs due to the involvement of higher energy vibrational states in the excited state of a molecule.
In typical fluorescence or phosphorescence processes, the molecule absorbs a photon of shorter wavelength (higher energy) and undergoes a transition to a higher energy electronic state. From this excited state, the molecule then relaxes back to its ground state by emitting a photon of longer wavelength (lower energy). This is known as the Stokes shift.
In contrast, in anti-Stokes shift, the molecule absorbs a photon of longer wavelength (lower energy) and undergoes a transition to a higher energy vibrational state within the same electronic state. From this higher vibrational state, the molecule relaxes to a lower vibrational state and emits a photon of shorter wavelength (higher energy). The emitted photon has higher energy than the absorbed photon, resulting in an anti-Stokes shift.
The reason for this reversal is related to the energy difference between the vibrational levels within the electronic states. The vibrational energy levels in a molecule are quantized, and the spacing between these levels increases with higher vibrational quantum numbers. When a molecule absorbs a photon, it gains energy primarily in electronic transitions but can also populate higher vibrational levels within the excited electronic state.
In anti-Stokes shift, the absorption of a lower energy photon (longer wavelength) promotes the molecule to a higher vibrational level within the excited state. The relaxation from this higher vibrational level to a lower one leads to the emission of a higher energy photon (shorter wavelength). The emitted photon has a shorter wavelength because it corresponds to the energy difference between the lower vibrational level and the ground state.
Overall, the anti-Stokes shift occurs when the absorption and emission involve different vibrational levels within the same electronic state, resulting in the emission of a higher energy photon (shorter wavelength) compared to the absorbed photon.