Raman Spectroscopy

Scattering of light by vibrating molecules

Elastic scattering

Leave the molecule in the same state, same wavelength - more abundant.

Rayleigh Scattering

Inelastic scattering

leave the molecule in a different quantum state, different wavelength - less abundant.

Raman Scattering

Rayleigh Scattering

• elastic scattering of light or other electromagnetic radiation

• Depends on the wavelength (I = 1/λ⁴)

• Isotropic

Raman Scattering

• Inelastic scatter, shifted frequency (vs=v₀±v_molecule)

• correspond either to rotational, vibrational or electronic frequencies

Scattering events

Stokes

Material absorbs energy and the emitted photon has a lower energy (higher wavelength) then incident photon.

Anti-stokes

Material loses energy and the emitted photon has a higher energy (lower wavelength) than the incident photon.

Molecular Vibrations

Intensity: Temperature Dependence

Stokes more favoured over anti stokes

The ratio of anti-Stokes to Stokes intensities will increase with temperature - more molecules in vibrationally excited state

Raman Scattering Theory

If a diatomic molecule is irradiated by this light, an electric dipole moment µ is induced.

where α is polarizability - proportionality constant

Polarizability

Tensor - ease of distortion of a collection of electric charges (how easily can electrons be moved in response to external field)

Polarizability decreases with increasing electron density, increasing bond strength and decreasing bond length.

Raman active condition

A molecular rotation or vibration must cause some change in a component of the molecular polarizability - magnitude/direction change in polarizability ellipsoid.

Polarizability Proof

Example CO₂

Example H₂O

Rule of mutual exclusion

If a molecule has a centre of symmetry, then IR active vibrations are Raman incative and vice versa.

Spectra: Features

Spectra: Effect of applied force

Polarised Raman spectroscopy

Polarised Raman spectroscopy probes information about molecular orientation and symmetry of the bond vibrations.

Resonance Raman spectroscopy

In resonance Raman the excitation wavelength is carefully chosen to overlap with (or be very close to) an electronic transition - real energy level.

High fluorescence background

Laser wavelength selection

Conventionally: 785nm

• short-wavelength: high fluorecence, high intensity

• long-wavelength: low fluorescence, weak signal

Raman Pros and Cons

Pros:

• no sample prep

• non-destructive

• no vacuum required

• short time scale

Cons:

• cant use metals or alloys

• Raman effect is very weak (Low sensitivity)

• fluorescence noise