L10: Introduction to Raman Spectroscopy

Raman Spectroscopy discovery and main idea

In 1928 C. V. Raman discovered that the wavelength of a small fraction of the radiation scattered by molecules is different from the incident of radiation, and is usually shifted to lower frequencies

signals are usually very weak; about % of the incident laser radiation → must be separated from raleigh scattering

How are weak Raman signals overcome

• FT (Fourier Transform) techniques → improve signal processing and resolution.

• Powerful lasers → generate more scattered photons.

• Sensitive visible–NIR detectors

How are Raman Spectra acquired

Laser excitation: Raman spectra are acquired by irradiating the sample with a strong visible laser, mostly Rayleigh scattering but a small amount of Raman scattering

Scattered radiation measured at 90 degrees: detector is typically placed at 90 degree angle to the incoming laser beam

Excitation eavelength is chosen carefully to avoid fluorescence produced by laser light

Raman Diagrams

Resonant absorption

molecule absorbs energy matching an electronic transition and emits radiation corresponding to the energy difference between electronic levels, producing fluorescence or phosphorescence

Rayleigh scattering

molecule is excited to a virtual state and returns to its original state, emitting a photon with the same energy as the incident light.

Stokes scattering

The molecule reaches a virtual state and loses energy, emitting a lower- energy photon that appears as a Stokes line on the red side of the incident beam.

Anti-Stokes scattering

molecule gains energy from vibrational states and emits a higher- energy photon that appears as an anti-Stokes line on the blue side of the incident beam

Advantages of Raman Spectroscopy

• Raman is a scattering effect and does not have to result in vibrational transitions

• Raman requires a change in polarizability of the bond; often, Raman-activebands are IR-inactive, and vice versa

• Raman can also operate in water and through conventional optical materials, unlike IR spectroscopy, which must use crystalline halides (NaCl and KBr)

Components of Raman Spectrometers

• Excitation source

• Sample light and collection optics

• Wavelength selector

• Detector

• Signal processor

• Display

Notch (interference) filters

Remove Rayleigh scattering by blocking wavelengths within ±80–120 cm⁻¹ of the laser line.

However, this could prevent detection of low-frequency Raman modes within that range

Holographic gratings

Commonly used in Raman spectrometers because they have fewer defects and produce much less stray light than ruled gratings

Multiple dispersion stages

Double- and triple-stage spectrometers further reduce stray light and allow detection of low-frequency Raman signals (~3–5 cm⁻¹) without notch filters

Raman Sampling in Gases

Raman Sampling in Liquids

Raman Spectroscopy in Solids

Excitation Sources in Raman Spectroscopy

• Lasers are commonly used; Raman intensity increases with excitation frequency, but high-energy sources may cause photodecomposition.

• Lower-energy excitation reduces fluorescence but decreases Raman intensity

• FT techniques improve signal-to-noise ratio (SNR)

analyte concentration

Raman intensity is proportional to

Fiber Optic Multi Channel Raman Spectrometers

FT Raman Spectrometer

Handheld and Standoff Raman Spectrometer