What is the Difference Between Stokes and Anti-Stokes Lines

The key difference between stokes and anti-stokes lines is that stokes lines have a longer wavelength than the wavelength of exciting radiation that is responsible for the fluorescence or Raman effect, whereas Anti-stokes lines occur in fluorescence or Raman spectra when atoms or molecules are already in an excited state.

Stokes lines represent radiation of particular wavelengths present in the line spectra that is associated with fluorescence and the Raman effect. Anti-stokes lines represent the radiation of particular wavelengths present in fluorescence and in Raman spectra when the atoms or molecules of the material exist in an excited state.

CONTENT

1. Overview and Key Difference
2. What are Stokes Lines
3. What are Anti-Stokes Lines
4. Stokes vs Anti-Stokes Lines in Tabular Form
5. Summary – Stokes vs Anti-Stokes Lines

What are Stokes Lines?

Stokes lines represent radiation of particular wavelengths present in the line spectra associated with fluorescence (emission of light from a substance that has absorbed energy previously) and the Raman effect (change in the wavelength of light that happens when a light beam is deflected by molecules). This was named after the 19^th-century British physicist Sir George Gabriel Stokes. These stokes lines are typically longer wavelengths than the wavelength of the exciting radiation responsible for fluorescence or the Raman effect.

Stokes lines can be described as scattered photons that are reduced in energy relative to the incident photons that can interact with the molecule. Moreover, the reduction of energy of the scattered photons is usually proportional to the energy of vibrational levels of the molecule.

What are Anti-Stokes Lines?

Anti-stokes lines represent the radiation of particular wavelengths present in fluorescence and in Raman spectra when the atoms or molecules of the material exist in an excited state. Therefore, it is the opposite of stokes lines. Here, the radiated line energy gives the sum of the pre-excitation energy and the energy absorbed from the exciting radiation. Therefore, anti-stoke lines typically have a shorter wavelength compared to the light that produces them. Moreover, the difference between the frequency of the emitted light and absorbed light can be named the Stokes shift.

Stokes vs Anti-Stokes Lines in Tabular Form

Figure 01: Raman Effect

We can describe anti-stokes lines as the scattered photons that have increased energy compared to the incident photons that come to interact with the molecule. Usually, increasing the energy of scattered photons is proportional to the energy of the vibrational levels of the molecule.

What is the Difference Between Stokes and Anti-Stokes Lines?

The terms stokes lines and anti-stokes lines are important in spectroscopic detections. The key difference between stokes and anti-stokes lines is that stokes lines have a longer wavelength than the wavelength of exciting radiation that is responsible for the fluorescence or Raman effect, whereas Anti-stokes lines occur in fluorescence or Raman spectra when atoms or molecules are already in an excited state. While stokes lines are not in the excited state, anti-stokes lines are already in the excited state.

The below infographic presents the differences between stokes and anti-stokes lines in tabular form in tabular form for side by side comparison.

Summary – Stokes vs Anti-Stokes Lines

Stokes lines and anti-stokes lines are described in physical and analytical chemistry. The key difference between stokes and anti-stokes lines is that stokes lines have a longer wavelength than the wavelength of exciting radiation that is responsible for the fluorescence or Raman effect, whereas Anti-stokes lines occur in fluorescence or Raman spectra when atoms or molecules are already in an excited state.

Reference:

1. “Stokes Lines.” Encyclopædia Britannica, Encyclopædia Britannica, Inc.
2. Thomas Wenzel and Charles A. Dana, et al. “Molecular and Atomic Spectroscopy (Wenzel).” Chemistry LibreTexts, 9 May 2021.