The phenomenon of molecules absorbing light with a shorter wavelength than the incident light is known as "Stokes shift." The Stokes shift occurs in certain molecules due to various factors, including electronic transitions, energy relaxation processes, and molecular vibrations.
When light interacts with molecules, it can induce electronic transitions within the molecules' energy levels. In some cases, these transitions involve absorption of photons with higher energy (shorter wavelength) than the incident light. The absorbed energy excites the molecule to a higher energy state.
However, the molecule doesn't remain in this excited state indefinitely. It undergoes relaxation processes to return to its ground state. During this relaxation, the excess energy can be dissipated through several pathways, including emission of light. This emitted light typically has lower energy (longer wavelength) than the absorbed light, resulting in a Stokes shift.
Several mechanisms contribute to the Stokes shift. One prominent mechanism is the relaxation through molecular vibrations. After absorbing energy, the molecule vibrates, and during relaxation, it loses energy through vibrational relaxation processes. This energy loss leads to a shift towards longer wavelengths.
Additionally, in some cases, the excited molecule can undergo internal conversion, where the excess energy is converted into heat rather than emitted as light. This also contributes to the observed Stokes shift.
It's important to note that not all molecules exhibit a Stokes shift. The extent of the shift depends on the specific molecular structure, electronic properties, and energy relaxation pathways of the molecule in question. Different molecules may exhibit varying degrees of Stokes shift or even exhibit anti-Stokes shift, where the emitted light has higher energy than the incident light.