Raman Spectroscopy

Raman Effect & Raman Scattering

Raman spectroscopy is a nondestructive analysis technique that focuses on the vibrations of molecules and their energy shifts caused by scattered laser light.

Atoms form molecules because molecules are more stable and have lower energy. Energy is stored in bonds between the molecules. When a photon hits a molecule, light can be scattered by the molecule and usually has the same energy or wavelength as the initial incident beam. This is called Rayleigh scattering, defined by the elastic scattering process in which there is no net energy loss. Occasionally, however, the light scattered by the molecule has a different energy than the initial incident beam. This is called Raman Scattering, defined by the inelastic scattering process in which there is a net energy loss.

The lost energy in Raman scattering is due to an interaction between the scattered light and the bonds of the molecule. Bonds can vibrate, and these vibrations have certain well-defined energy states, without possibility of existing at any energy level in between these states. When bonds absorb the energy lost in Stokes Raman scattering, they transition from a lower energy state to a higher energy state. The resulting Raman-scattered light has a lower energy and a longer wavelength than the incident laser beam. In Anti-Stokes Raman scattering, bonds relax from a higher energy state to a lower energy state. The resulting Raman-scattered light has a higher energy and shorter wavelength than the incident laser beam. Statistically, Stokes Raman scattering has a higher probability of occurring, and Raman spectroscopy usually measures Stokes Raman scattering.

Raman Shift

A Raman shift is the difference in energy between the scattered beam and the initial beam. The scattered beam’s energy must always be defined in reference to the incident beam. The magnitude of a Raman shift is affected by the structure of the molecule. Molecules with heavier atoms and weaker bonds have lower vibrational frequency and therefore a smaller Raman shift. Molecules with lighter atoms and stronger bonds have higher vibrational frequency and therefore a larger Raman shift. One molecule may have several different Raman shifts, because there are many ways in which a molecule can vibrate, and each vibration can lead to its own Raman shift.

Applications of Raman Spectroscopy

Raman spectroscopy has applications in all fields, including materials characterization, metallurgy, medicine, biology, and more. Specifically, carbon nanostructures can be well-analyzed due to carbon being highly active and suitable for Raman spectroscopy. Raman spectroscopy can also be used to identify and confirm tumor diagnoses, pathogens, drugs, and biomolecules. Furthermore, Raman instruments have been sent to space for analysis of space materials.

Advantages & Limitations of Raman Spectroscopy

Raman spectroscopy is a nondestructive technique that is also fast and reliable. Because Raman and IR have different selection rules (Raman detects change in polarizability, while IR detects change in dipole momentum), some Raman-active vibrational modes are not IR-active. Thus, Raman and IR are complementary spectroscopy techniques that can provide full analysis of a molecule when used together. Furthermore, water can interfere with IR spectroscopy but not Raman, allowing for the analysis of aqueous solutions using Raman spectroscopy.

However, because the probability of Raman scattering is so low, difficulties arise in the analysis of low-concentration samples. More complicated types of Raman spectroscopy must be used to enhance the signal for low-concentration samples. Fluorescence or overheating of samples due to the power of the incident beam can also interfere with Raman spectra. Because water is polar and not linear, the O-H bonds are IR-active rather than Raman-active.

Anatomy of a Raman Spectrometer

There are five main components of a Raman spectrometer: incident laser, filters, optics, grating, and detector.

  1. The laser provides the source of photons—which all have the same energy—to excite the vibrational modes in the sample.
  2. Optics focus and redirect the beam to the sample.
  3. Filters help separate the Raman signal from any Rayleigh scatter and improve the signal-to-noise ratio.
  4. The grating acts as a prism and separates light by wavelength, differentiating frequencies of Raman scattered light.
  5. The detector reads and displays the Raman signal.
Schematic of a Raman spectrometer. Starting at the incident laser, the beam hits optics to focus on the sample, then goes through a beam splitter and filter. In the spectrometer, the grating separates Raman scattered light and is detected by the CCD detector.

Reading Raman Spectra

Each molecule has many vibrational modes, and each mode may give a distinct Raman shift. In this way, every molecule has a unique spectroscopic fingerprint, and reading a Raman spectrum’s peak placement (energy or wavenumber of the shift in the scattered beam) and peak magnitude (intensity of scattered beam) can help identify and quantify the presence of a certain material.

Each compound has a different Raman spectrum. The vertical axis indicates Raman intensity (AU in this example, but given in photon counts), while the horizontal axis indicates the wavenumber of the Raman shift in (cm-1). Comparing an experimental spectrum with a database with known spectra allows for the identification of materials.

Types of Raman Spectrometers

Surface-Enhanced Raman Scattering (SERS)

Placement of the analyte in close proximity to or adsorbed onto a rough metal surface enhances the Raman signal, making SERS a suitable option for the analysis of low-concentration materials. Striking the metal with the incident laser beam produces localized surface plasmon, or a group of oscillating electrons as a result of a laser beam. This intensifies the electromagnetic field in the area. Using a rough metal surface further enhances the effect due to an increased number of adsorption regions.

Surface-Enhanced Hyper Raman Scattering (SEHRS)

As the non-linear analog to SERS, SEHRS uses two-photon excitation to interact with the analyte. Two photons are delivered in succession such that they reach the sample almost simultaneously, resulting in a quadratic relationship between the incident beam and the signal intensity.

Tip-Enhanced Raman Scattering (TERS)

TERS utilizes a similar concept to SERS using metal to enhance Raman signal. However, a metal tip or nanoparticle is used instead of a metal surface in SERS. This generates the surface plasmon and concentrates the effects of the electromagnetic field on a specific point in the sample directly under the tip.

Stimulated Raman Scattering (SRS)

SRS takes advantage of coherence, which happens when two waves with the same frequency and amplitude are in phase with each other. This causes vibrational modes to oscillate, polarizing atoms and producing a stronger signal. Because SRS uses two different incident beams, two types of changes can be tracked and detected. Simulated Raman loss (SRL) detects the loss of photons in the pump beam, while Simulated Raman gain (SRG) detects the gain of photons in the Stokes beam. Furthermore, SRS is time-resolved, meaning that signal can be collected at different points in time.

Coherent Anti-Stokes Raman Scattering (CARS)

Like SRS, CARS takes advantage of coherence to generate a stronger signal. When the frequency difference between the pump beam and the Stokes beam matches the frequency of the Raman active mode, a coherent beam excites an excited-state bond, and a third probe beam is used to probe the bond. As the bond relaxes back to ground state, it releases a photon with a higher frequency than the incident beam in Anti-Stokes fashion. The CARS setup allows only one bond to be probed at a time.

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