The Fundamentals of Raman Spectroscopy

Raman spectroscopy is an analytical technique based on Raman scattering. Here, we describe the basics of Raman scattering and how it is used as an spectroscopy and imaging tool.

What is Raman Scattering?

When photons interacts with matter, specifically molecules, there are a few events that can take place. One of them is the inelastic scattering of the photon , or Raman scattering, that happens when the incoming photon changes direction and energy (or equivalently, color) after the interaction. How much the photon's energy changes corresponds to the energy of a vibrational level in the molecule. Not all vibrational levels in a molecule are "Raman active", there are quantum rules that help determine which ones are or aren't active, but a mapping of those that are results in a spectrum like the one shown in Figure 1. A spectrum like that is achieved by illuminating the sample with a single color light source, in other words a laser, and collecting the photons after they have interacted with the molecules in the sample, and thus changed energy. Each peak in the spectrum is generated by photons which energy changed by the same amount, and together all the peaks in the spectrum create a unique fingerprint for a molecule.

Figure 1. Raman spectrum of polystyrene acquired with a 785 nm excitation laser. Relative wavenumbers are a measure of the change in the photons energy with respect to their initial value.

Raman scattering as an spectroscopy and imaging tool

Since each molecule's Raman spectrum is unique, it becomes a very powerful tool to do chemical identification. Not only that, but Raman scattering is proportional to the concentration of molecules in the volume probed, so it also is a quantitative tool. In many instances a spectrum is all is needed to characterized a sample, for example when the sample is solution, or homogeneous as the polystyrene in Figure 1. In other cases, when the sample is not uniform or more information about th different components distribution is required, Raman spectra collected on different spots on the sample can be used to reconstruct a Raman map or image. Figure 2 shows how Raman scattering can be used as an imaging tool.

Raman Scattering in the context of light-matter interactions

When photons interacts with matter, specifically molecules, there are a few things that can happen to them. Table 1. below shows a comparison in terms of cross-sections (or likelihood) of some of the phenomena that can take place durign the photon-molecule interaction. The most likely event is that the photon will get absorbed in a process called electronic absorption, and the energy of the photon turns into heat.

The next event in likelihood is fluorescence. That is when the photon gets absorbed and later re-emitted with a shorter wavelength. The change in wavelength (or color) is due to the loss of some of the original photon's energy in the form of heat to the sample.

Least likely are the scattering events, where the incoming photons change direction and in some instances also wavelength. In the event the photon scatters elastically, that is, only change in direction, it is called Rayleigh scattering and it is slightly more likelly than the inelastic scattering events, or when the photons also change wavelength. The inelastic scattering is called Raman scattering, and the amount of energy that the photon losses or gains during the interaction corresponds to vibrational levels of the molecules in the sample.

Table 1. The different phenomena that can take place during the light-matter interaction, their likelihood (cross-section), and photons fate.
Process Cross-section Photon fate
Electronic absorption 10-20 m2 Photon is absorbed and its energy turn to heat in the sample.
Fluorescence 10-20 m2 Photon is absorbed and later re-emitted with longer wavelength (or less energy).
Vibrational absorption 10-23 m2 Photon is absorbed and its energy turn to heat in the sample.
Rayleigh scattering 10-32 m2 Photon is scattered without wavelength (or energy) change.
Raman scattering 10-33 m2 Photon is scattered and the energy changes. The amount of change corresponds to a vibrational energy level in the molecule.