14 Raman Scattering

Raman scattering is an inelastic scattering mechanism that was first observed by C.V. Raman in 1928, earning him the Nobel Prize in Physics in 1930.

Virtual State and Vibrational Transitions

When a non-resonant photon is incident on a molecule, it distorts the molecule’s electron density, effectively moving a valence electron to a higher-energy virtual electronic state. This virtual state is not formally allowed and exists only for the time that the photon interacts with the molecule (~10^–15 s), after which the molecule returns to its electronic ground state. For elastic (i.e. Rayleigh) scattering, the molecule also returns to its original vibrational level in the ground electronic state. For Raman scattering, the molecule returns to a vibrational level in the ground electronic state that is different than its original vibrational level:

• If the new vibrational level is higher in energy than the original one, then the result is Stokes Raman scattering that has a longer wavelength than the incident light (i.e. less energy).

• If the new vibrational level is lower in energy than the original one, then the result is anti-Stokes Raman scattering that has a shorter wavelength than the incident light (i.e. more energy). For this process to occur, a molecule must initially be in an excited vibrational state.

Although the absolute wavelength of Raman scattering depends on the incident wavelength, the shift in wavelength is independent of the incident wavelength. As this shift represents a vibrational transition, it gives chemical information similar and complementary to IR absorption.

Raman-Active Vibrations

The selection rules for IR absorption and Raman are scattering a different: whereas IR absorption requires a change in dipole moment with a vibration, Raman scattering requires a change in polarizability with a vibration. Polarizability is how easily the electron density (or bonds) of a molecule can be deformed. Although some vibrations are active for both IR absorption and Raman scattering, other vibrations are active for one but not for the other.

Enhanced Raman Scattering

Raman scattering is a very inefficient process, with an estimated 0.000001–0.001% of the incident light being scattered by the mechanism. (Rayleigh scattering is orders of magnitude more efficient.) Notably, anti-Stokes Raman scattering is less intense than Stokes Raman scattering because the number of molecules that are initially in an excited vibrational state is very small (as per the Boltzmann distribution).

Methods are known for increasing the efficiency of Raman scattering. For example, higher frequencies (i.e. shorter wavelengths) of light are Raman scattered more efficiently. In resonance Raman, the incident light is chosen to be close to the wavelength for an allowed electronic transition, resulting in an orders-of-magnitude increase in Raman scattering efficiency. The reason is that the large change in bond length near the allowed electronic transition results in a large change in polarizability.

Another enhancement method is surface enhanced Raman spectroscopy (SERS), where a sample is applied to a roughened or otherwise nanostructured metal surface. Chemical interactions between a molecule and the metal surface can increase its polarizability, resulting in enhancements up to 10³-fold. In addition, the interaction of incident light with metal nanostructures can create localized electric field enhancements that increase Raman scattering between 10³– and 10¹⁰-fold.

Connections

• Like IR absorption (Ch. 12), Raman scattering is a type of vibrational spectroscopy that provides information about bonding.
• Similar to light absorption (Ch. 6, Ch. 12), there are selection rules for Raman scattering. Although the requirement for a change in polarizability is special to Raman, the strong preference for transitions between adjacent vibrational levels is shared with IR absorption.
• For the same reason that hot transitions are weak in IR absorption (i.e. the Boltzmann distribution, Ch. 12), anti-Stokes Raman scattering is much weaker than Stokes Raman scattering.
• Analogous to IR spectra (Ch. 12), Raman spectra have a fingerprint region and a group frequency region.

Post-Reading Questions

1. A sample is illuminated with a 532 nm laser and the spectrum of scattered light is measured. Peaks are observed at 508 nm, 532 nm, and 556 nm. Identify each peak as Rayleigh scatter, Stokes Raman, and anti-Stokes Raman.
2. Fill in the blanks: The anti-Stokes Raman peaks are _____ intense than the Stokes Raman peaks; the Rayleigh scatter peak is _____ intense than the Raman peaks.
3. A sample is illuminated with a 532 nm laser and a Raman peak is observed with a shift of 800 cm^–1. What will be the value of the Raman shift if the sample is illuminated with a new 785 nm laser? How will the intensity of the Raman peak change with illumination at 785 nm?

Topic Learning Objectives

The chapter is a primer for the following learning objectives, which will be covered in lecture and/or with additional assigned reading:

• Draw energy level diagrams that illustrate the mechanism of Raman Scattering.
• Predict whether or not a vibrational mode is Raman active for a simple small molecule or functional group.
• Be aware of strategies for mitigating the low efficiency of Raman Scattering.