For a Vibrational Raman, will the S or O branch be at the higher wavelength?
For a Rotational Raman, will the S or O branch be at the higher wavelength?
To determine whether the S or O branch will be at the higher wavelength for vibrational Raman and rotational Raman spectroscopy, we need to understand the energy levels involved in these spectroscopic techniques.
In both cases, Raman spectroscopy involves the interaction of light with the sample, leading to the scattering of photons. The scattered photons can have either a higher or lower energy compared to the incident photons, depending on the energy changes occurring in the sample.
1. Vibrational Raman spectroscopy: In this technique, the energy levels involved are related to the vibrational states of the molecules. When a molecule absorbs a photon, it can undergo a vibrational transition where its energy increases or decreases.
a) Stokes Raman scattering: If the molecule transitions to a higher vibrational energy level, it absorbs energy from the incident light, resulting in photons scattered at a lower energy (higher wavelength) compared to the incident photons. This is called the Stokes branch.
b) Anti-Stokes Raman scattering: If the molecule transitions to a lower vibrational energy level, it emits energy as it scatters light, resulting in photons scattered at a higher energy (lower wavelength) compared to the incident photons. This is called the Anti-Stokes branch.
Therefore, for vibrational Raman spectroscopy, the Stokes branch will be at a higher wavelength (lower energy) while the Anti-Stokes branch will be at a lower wavelength (higher energy).
2. Rotational Raman spectroscopy: In this technique, the energy levels involved are related to the rotational states of the molecules. When a molecule absorbs a photon, it can undergo a rotational transition where its angular momentum changes.
In rotational Raman scattering, the energy changes are relatively small compared to vibrational transitions. As a result, the shifts in wavelength between the Stokes and Anti-Stokes branches are usually smaller and less easily distinguishable than in vibrational Raman scattering. Therefore, it is not common to differentiate the S and O branches in rotational Raman spectroscopy.
In summary,
- For vibrational Raman spectroscopy, the Stokes branch is at a higher wavelength (lower energy) and the Anti-Stokes branch is at a lower wavelength (higher energy).
- For rotational Raman spectroscopy, it is not generally distinguishable between S and O branches.