Entering Raman's Realm

Mar 01, 2011

It is now well understood that Raman spectroscopy can provide information that is not available by other analytical means. This fact has sparked interest in the technique, yet many analysts remain uneasy because of their lack of knowledge or experience. As a result of instrument developments over the last 20 years, however, recording the spectra is not the challenge that it used to be. But the very property that makes Raman spectroscopy attractive — the wealth of information it provides — can make it appear daunting. This column provides a roadmap that will enable you, the analyst, to start recording and interpreting Raman spectra.

This column will not explain how to record Raman spectra. Instrumentation innovations have simplified that task enormously, and if you want to get started on that, your instrument manufacturer should be able to help you. What I want to spend time on is how to decide what to measure and how to extract the information from the spectra.

One of the first things to appreciate is that Raman spectroscopy is used successfully in a number of areas that do not overlap. The biggest division is between physics on the one hand and the chemical and biological sciences on the other.

To give a few examples, if you are a chemical analyst, you want to identify chemicals in applications as diverse as contaminant analysis, trace evidence analysis, reverse engineering, and structure. For more sophisticated applications, you may be interested in determining orientation, which can be done using polarization analysis of the spectra.

If you are studying metallo-organics, or unsaturated organics with electronic absorbances in the visible range, then the Raman spectra are resonance-enhanced, which means that the laser couples very strongly to the electronic transitions, and the vibrations that are coupled to these electrons are enhanced in the Raman spectra by up to six orders of magnitude. A study of this behavior can elucidate the electronic structure of these compounds.

You may also be a chemical analyst interested in inorganic materials. Raman spectra can easily differentiate polymorphs of the same chemical species, and differentiate similar entities like CaCO3 vs. MgCO3 vs. CaMgCO3.

If you are studying semiconductors or perovskites (superconductors, ferroelectrics, piezoelectrics, and so forth), the Raman spectra are used to assist in determining the physical properties of the materials, which are then used to engineer devices.

In this column installment I will deal with the first and third topics — chemical identification of organic and inorganic species. In pursuing such identification, one learns that vibrations of chemical functional groups can often, but not always, be identified. I will deal with organic materials first, and then inorganic materials.