The underlying principle of Raman spectroscopy is based on the interaction between light and matter, specifically molecules' inelastic scattering of photons. When a monochromatic beam of light, typically from a laser source, interacts with a sample, most scattered light has the same frequency as the incident light. This is known as Rayleigh scattering.

However, a small fraction of the scattered light exhibits a frequency shift due to the exchange of energy between the incident photons and the vibrational energy levels of the molecules in the sample. This phenomenon is called Raman Scattering.

To obtain Raman spectra, the sample is irradiated with a monochromatic beam of laser light. The scattered radiation is collected at a suitable angle (usually 180°) using a spectrometer equipped with a sensitive detector. The measurement of the frequency-shifted scattered light as a function of wavelength or wavenumber provides the sample's Raman spectrum.

Resonance Raman scattering occurs when the excitation wavelength is close to an electronic transition of the molecule. This enhances the Raman scattering intensity, making it easier to detect even weak signals. However, choosing an appropriate excitation wavelength is crucial to avoid fluorescence interference, which can overwhelm the Raman signal. In many cases, near-infrared or visible lasers are used to minimize fluorescence.

When a molecule absorbs a photon, it is temporarily elevated to a virtual energy level. This virtual state is short-lived, and the molecule quickly returns to a vibrational energy level by emitting a photon. The difference in energy between the incident and emitted photons corresponds to the molecule's vibrational energy levels.

There are two types of Raman scattering: Stokes and anti-Stokes. In Stokes scattering, the emitted photon has lower energy (longer wavelength) than the incident photon, while in anti-Stokes scattering, the emitted photon has higher energy (shorter wavelength).

The frequency shifts of the inelastically scattered radiation correspond to the vibrational frequency of the molecule. By analyzing these frequency shifts, valuable information about the molecular structure and chemical composition of the sample can be obtained.

Raman spectra are typically presented as plots of intensity versus wavenumber (reciprocal of wavelength). The peaks in the spectrum represent specific vibrational modes of the molecules in the sample, providing a unique "fingerprint" for identification.

Consider a sample of carbon tetrachloride. Both Stokes and anti-Stokes scattering will occur when irradiated with a monochromatic laser. The intensity of the Stokes signal will be stronger than the anti-Stokes signal, as the population of molecules in the ground vibrational state is higher than those in the excited state at room temperature.

Vibrational modes can be identified by analyzing the Raman spectrum of carbon tetrachloride. The frequency shifts associated with these vibrational modes provide valuable information about the molecule's chemical composition and bonding.