Using light to describe the ancient world

Apatite crystal irradiated by a laser for Raman Spectroscopy

The first technique I want to introduce is one of the latest spectroscopic technologies used to study fossils. Raman Spectroscopy involves shining (irradiating) a sample with a very gentle laser and measuring the light that is scattered away. I discussed scattering below, but here we need to be a bit more technical. Laser light is a stream of photons with a single, specific wavelength (energy). The light from your green laser pointer probably has a wavelength of 532 nm. When a photon from a laser encounters a sample it may be reflected, absorbed or scattered (see The Light Fantastic). On very rare occasions, the light will undergo a special type of scattering called inelastic scattering: the photon will cause molecular-level movement in the sample. There are costs to everything, however, and the movement might drain a small amount of energy from the interacting photon. Occasionally, the movement will bolster the photon, and it will actually be more energetic after the interaction. The photons are then scattered away after the energy changes. The goal of Raman spectroscopy is to collect these inelastically scattered photons, because the small changes in energy are directly related to the type of molecu

lar-level movement they helped sponsor. Molecular movement (vibrations) are highly specific, and only certain types of molecule can perform certain vibrations. What is more, the exact structural arrangement of molecules controls the amount of energy taken from, or donated to, inelastically scattered photons. The net result is that Raman Spectroscopy can be used to explore the chemistry and physical structure of a substance. The best part, Raman Spectroscopy is completely non-destructive.


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