Raman Spectroscopy Through Rose-Colored Glasses

Our visible laboratory at the Natural History Museum in Los Angeles contains quite a bit of equipment. One of the workhorses in the MinSci lab is the Raman microscope. What is a Raman microscope you ask? Check out this animation we made in collaboration with Bailee Desrocher to visually explain how Raman spectroscopy works. There is a lot of fancy math and chemistry that is skipped, but the basics are there.

The Raman effect is a pretty simple phenomenon: light interacts with a substance, most of that light scatters without loss in energy, but some light does transfer some energy to the molecules and scatters back at a slightly different color. This apparent change in color is an extremely weak phenomenon, and so it goes unnoticed in our daily lives most of the time (unless you look at the ocean, it’s blue because of the Raman effect). Plus, sunlight contains a large spectrum of colors, so the new colors generated get washed out. As usual, some experimental troubleshooting had to be done in order to observe to observe this weak color change effect of the scattered light.

In his first 1928 experiments, Sir Chandrasekhara Venkata Raman used sunlight and some filters. A good overview of his life, by Barry Master published in Optics and Photonics News, but here is a summary. First, he needed a high-intensity light source. Raman used a narrow beam of sunlight using a telescope. In order to understand how light could generate new colors, you have to narrow the sunlight spectrum down to a single color. So Raman used a violet filter, which allowed only violet light to pass. Placing a substance in front of the intense violet light beam still only showed violet light being scattered back. The problem is that today we know that only about 1 in 10,000,000 photons has a direct interaction with the molecules in the substance. Most of the light bounces off the substance without really transferring any energy, which is why Raman still observed violet light. So the next step is to somehow filter out the violet light to allow other colors to be observed. As soon as Raman put a green filter in front of the scattered light — Eureka! — he saw green light. This meant that green light was being generated by the interaction of violet light with the substance. He immediately knew that this was an important discovery, so he wrote a paper to the journal Nature, and two years later he won the Nobel Prize for this work. Later on, he showed that all the samples (solids, liquids, and gases) that he analyzed had a different color spectrum of the scattered light. This has now become known as the Raman Effect.

Crude hand drawing of how C.V. Raman set up his experiment. His original groundbreaking experiments used liquids, but crystals are much nicer to look at in schematic. Sunlight is filtered, to only allow the violet part of the spectrum to pass. That violet light interacts with a material, and then light scatters off the material in all directions. Next, the violet scattered without loss in energy (elastically scattered), and so it needs to be filtered to observe what new colors have been generated. Raman used a green filter for this.

It essentially proved the quantum nature of light, and it also showed that materials can be identified by observing their Raman spectrum. Basically, the Raman spectrum is a fingerprint of every material that scatters light like this. For mineralogy, the Raman spectrum of a mineral can then be compared to a database of Raman spectra to find the best fits, just like fingerprint databases. Such databases, like RRUFF, do exist and are free to use. There are even efforts to try and calculate Raman spectra (such as the WURM project) based on what we know of the quantum nature of light and how it interacts with materials. That is super important, because these calculations give us a better understanding of the fundamentals of how the Raman Effect works.

The technique is not without its limitations. For example, Raman can be used to identify how molecules are configured (like if you have SiO₄ or SiO₆), and how they may be linked to other molecules in the crystal, but it doesn’t tell you the precise location of the atoms in the crystal, or what the atomic coordinates are for each of the atoms. The latter is the scientific realm of crystallography and X-ray diffraction, but that will have to wait for another post. Also, Raman spectroscopy can suffer from fluorescence, which can severally limit the usable data that can be observed (different laser frequencies can be used to limit fluorescence). Additionally, not all materials are Raman active, as there are some materials that simply do not have a Raman effect, such as metals and some minerals (e.g. halite).

Despite those limitations, Raman is quite useful, and the following are a few examples from my own work. Raman spectroscopy can be used to collect data very rapidly, and so it can be used to monitor chemical processes in minerals in near real-time. A recent study, that I published showed how the microporous mineral zorite is able to absorb rare earth elements out of water. Doing the experiment in real-time adds that 4D aspect of data acquisition, that helps understand the chemical dynamics. Raman can also be used to identify biosignatures in minerals, and it can also be used to make mineralogical maps showing the distribution of minerals within a specimen. There are many such useful applications.

So there you have it. The Raman spectroscopy technique is quite powerful, but does have limitations. As you read other articles in my Medium space, you’ll have a better understanding of why scientists use Raman spectroscopy to characterize the chemical nature of materials.

DIY Raman: There are a few good resources online on how to build your own Raman spectrometer. I have not attempted to make these myself, but I hope you try it and let me know how it goes.

Aaron Celestian is the Mineralogy Curator at the Natural History Museum of Los Angeles. He researches how minerals interact with their environments and with living things, and how those minerals can be used to solve problems like climate change, pollution, and disease.

Keeping science accessible. Researching how minerals can be used to solve problems like climate change, pollution, and disease. @ NHMLA, USC, NASA-JPL