Theory Overview
The Raman Effect
Raman spectroscopy is a non-destructive chemical analysis technique that provides detailed information about molecular structure, phase, and even crystal properties. It works by shining monochromatic light (a laser) on a sample and detecting the tiny fraction of light that scatters back with shifted color (wavelength). These color shifts form a unique “fingerprint” of the material because each Raman peak corresponds to a specific molecular vibration or bond. In practical terms, a Raman spectrum lets you identify substances (from polymers to pharmaceuticals) by their molecular makeup, without cutting, staining, or altering the sample.
For a DIY project, Raman’s value is in giving you rich chemical information with fairly simple hardware – essentially a laser, some filters, and a spectrometer. It’s feasible to build your own Raman setup, especially now that inexpensive lasers and high-sensitivity detectors (like CCDs) are available. Understanding fundamentally how and why Raman works, helps you prioritize what to aim for: a setup that maximizes the weak Raman signal and minimizes background noise (from things like stray light or fluorescence). Keep in mind that Raman signals are inherently weak, so design choices will focus on squeezing out as much signal-to-noise as possible.
Rayleigh and Raman Scattering
When light interacts with molecules, most photons scatter elastically – they bounce off without losing energy. This is called Rayleigh scattering, and the scattered light has the same color (wavelength) as the incoming light. In a Raman experiment, this Rayleigh-scattered light is essentially a strong background with no analytical value (it doesn’t tell you anything about molecular vibrations). However, a tiny fraction of the photons (on the order of 1 in 107 or 108) interact inelastically with the molecules. These rare events are Raman scattering: the photon exchanges energy with molecular vibrational modes, resulting in a shifted wavelength of the scattered photon.
In simple terms, imagine throwing a ball (photon) at a resting target (molecule). In most cases, the ball bounces back with the same energy it had (Rayleigh scatter). But once in a while, the ball gives a tiny bit of its energy to excite a vibration in the target – so it comes back a little “slower,” i.e. with less energy (this would be a Raman scattered photon). Conversely, if the molecule was already vibrating, it might transfer energy to the photon, which then scatters with more energy than it came in with. These energy exchanges correspond to the Raman effect.
Crucially, the energy lost or gained by the photon equals the energy of a molecular vibration. Because molecules have specific allowed vibrational energies, the amount of energy change (and thus the wavelength shift) is specific to particular bonds or groups of atoms. This is why the collection of Raman shifts forms a fingerprint for the molecule.
Stokes vs. Anti-Stokes
When a Raman scattering event occurs, the outgoing photon can be either lower in energy or higher in energy compared to the incoming laser photon. These two cases are known as Stokes and anti-Stokes scattering, respectively, and they show up as symmetric sides of the spectrum around the laser line (which is typically filtered out in the measurement). Here’s what’s happening at the molecular level for each case:
Stokes
The incoming photon excites a molecule from the ground state to a higher virtual state. The molecule then relaxes down to a vibrationally excited state (above the ground state). In this process, the molecule has taken some energy from the photon – so the scattered photon comes out with less energy than it had originally. Less energy means a longer wavelength (a red shift). These red-shifted photons are the Stokes lines. For example, if you excite with a green laser, a Stokes-shifted Raman photon might emerge as orange or red-colored light.
Anti-Stokes
Here we start with a molecule that is already in an excited vibrational state (e.g. the sample has some thermal energy). The photon interaction can de-excite the molecule back down to the ground state, stealing that vibrational energy and giving it to the scattered photon. The result is the scattered photon comes out with more energy than it had initially (because it gained the molecule’s vibrational energy). More energy means a shorter wavelength (a blue shift) – this is an anti-Stokes Raman photon.
From a spectrometer perspective, Stokes lines appear at wavelengths longer than the laser (to the “right” side of the laser line), and anti-Stokes lines at shorter wavelengths (to the “left” side of the laser line). They are roughly symmetric in how far they are shifted, but not symmetric in intensity. Stokes scattering is much stronger than anti-Stokes for most samples at room temperature. This is because at ambient temperatures, most molecules are in the ground vibrational state initially (very few are thermally excited to the first vibrational level). There are simply more molecules available to undergo Stokes processes. The anti-Stokes process requires molecules to be in an excited vibrational state to begin with, which is statistically less likely except at high temperatures. In a typical DIY context, you can just remember that anti-Stokes lines will be weak and often buried in noise.
For this reason, most DIY Raman setups (and even many commercial ones) focus on detecting the Stokes spectrum only. It’s easier to measure because the signals are stronger. Often the instrumentation is designed to block the laser (Rayleigh line) and everything shorter than the laser wavelength, effectively passing only the Stokes-shifted light. If you ever do need to observe anti-Stokes lines, you’d need a different filter setup and perhaps a more sensitive detector since the signals are more faint.
Because Raman scattering is so weak, any competing light can overshadow it. This includes: the intense Rayleigh-scattered laser light, fluorescence from the sample or impurities (which can be 10⁴–10⁶ times stronger) and even faint ambient light leaking into the setup.Even a small light leak or stray reflection can present itself as a false Raman peak or bury genuine ones in noise. In practice, this means robust filtering, dark enclosures, and signal processing to best make out the real Raman features.
Raman-Shift & Wavenumber
Using a spectrometer, we measure the wavelengths of the scattered light and convert them into the Raman shift (cm⁻¹), which indicates the vibrational energy difference. The wavelength of incidence – the excitation source (our laser) – is known at roughly around 532nm. This also highlights the importance of employing a laser with the narrowest possible wavelength (or using a bandpass filter to compensate cheap modules).
Δν (cm−1) = ( 1/λLaser − 1/λRaman ) × 107
- λLaser and λRaman are in nanometers (nm)
- The factor 107 converts from nm⁻¹ to cm⁻¹
- Wavenumber is the reciprocal of wavelength (1/λ), and it’s directly proportional to energy — higher wavenumber means higher energy
This notation provides a laser-independent measure of the vibrational energy: a Raman peak at 1000 cm⁻¹ always represents the same vibrational mode, regardless of whether a green (532 nm) or red (785 nm) laser is used. This makes Raman spectra comparable across instruments using different excitation sources (wavelength).