The emission wavelength can be longer than the excitation wavelength in certain situations due to phenomena such as Stokes shift and fluorescence.
Stokes shift occurs when the emitted light has a longer wavelength (lower energy) than the absorbed (excitation) light. This shift happens because during the absorption process, some energy is lost or dissipated in various ways, such as heat or vibrational relaxation. As a result, the emitted light has less energy and a longer wavelength.
Fluorescence is another phenomenon where the emitted light has a longer wavelength than the absorbed light. In fluorescence, a substance absorbs photons of higher energy (shorter wavelength) and then re-emits them at a longer wavelength. This occurs due to the relaxation of excited electrons in the substance, which release excess energy as photons of lower energy (longer wavelength).
Both Stokes shift and fluorescence are common in various fields such as spectroscopy and luminescent materials. It's important to note that not all emission wavelengths are longer than excitation wavelengths, as it depends on the specific properties and mechanisms of the system under consideration.