7.4 Reading Raman Spectra

Learning Objectives

Read Raman spectra using Raman shift units.

Prior Knowledge and Skills

3.7 Electromagnetic Energy: Units Conversion

Key Terms

  • Wavenumber
  • Raman Shift
  • Intensity

Guided Inquiry

Watch this video which provides a great visual demonstration of the Raman effect, and shows how Raman scattered light is plotted on a graph:

NanophotonGlobal. (January 6, 2017). Visual guide to Raman spectroscopy | Nanophoton.

7.4.1. In this example, Raman scattering from liquid water is shown.  What is the wavelength of the laser in nm, and what is the wavelength of Raman scattered light for the major peak of water (as stated in the video?)

7.4.2. What is plotted on the y-axis?

7.4.3. If the laser was changed to 364 nm, at what wavelength would the major peak of water appear?

In practice, Raman spectra are plotted as Raman shift.  Raman shift is the difference between the peak energies and the excitation laser energy.  This allows comparison of a spectrum to other spectra even when different laser excitation energies are used.  Conventional Raman spectra are plotted in wavenumber (cm-1) units, not in wavelength.

The following links show examples of Raman spectra of different materials.  Please look at these examples to compare and contrast the plots.

InPhotonics. InPhotote Spectrometer Data. (ret. 3/17/2019) http://www.inphotonics.com/INPdata.htm

National University of Ireland, Galway. Small Molecule/Forensics Research. (ret. 3/17/2019) http://www.nuigalway.ie/nanoscale/researchprojects/forensicsraman/

Sylvia-Monique Thomas. Infrared and Raman spectroscopy. (ret. 3/17/2019) https://serc.carleton.edu/NAGTWorkshops/mineralogy/mineral_physics/raman_ir.html

7.4.4. In the olivine spectrum, what is the energy difference between the tallest peak and the laser energy, in wavenumbers?

7.4.5. By looking at this data, can you tell which laser energy/wavelength was used?

7.4.6. What is/are the variable/s plotted on the y-axis (look at several sources)?  We are essentially plotting the counts from the detector in the y-axis, which is why people report this in different ways.  Peak intensity in reflectance spectra can vary due to experimental conditions, count times, orientation of the sample, surface characteristics, etc.

Summary

Raman spectra are plotted in units of intensity (which can be arbitrary) versus Raman shift in wavenumbers (cm-1).  The Raman shift is used so that data can be easily compared even when different laser wavelengths are used.

References

InPhotonics. InPhotote Spectrometer Data. (ret. 3/17/2019) http://www.inphotonics.com/INPdata.htm

NanophotonGlobal. (January 6, 2017). Visual guide to Raman spectroscopy | Nanophoton. https://youtu.be/G1jXR5PhUc0

National University of Ireland, Galway. Small Molecule/Forensics Research. (ret. 3/17/2019) http://www.nuigalway.ie/nanoscale/researchprojects/forensicsraman/

Sylvia-Monique Thomas. Infrared and Raman spectroscopy. (ret. 3/17/2019) https://serc.carleton.edu/NAGTWorkshops/mineralogy/mineral_physics/raman_ir.html