Raman Spectroscopy

Describe Rayleigh light scattering

A collision between a photon and a molecule can result in scattering which is elastic 

• Being elastic means it has no change in energy of the photon or the molecule 

• Weak

Provide a diagram detailing rayleigh light scattering

Describe Raman light scattering

A collision between a photon and a molecule can result in scattering which is inelastic 

• Energy is exchanged between the photon and the molecule

• Very weak

Draw diagrams for the two types of raman light scattering naming each

draw a graph of intensity against wavenumber

Give equations for stokes and anti-stokes

What is scattering intensity proportional too

Give the gross selection rule for Raman scattering

Raman scattering must have a change in polarizability on : 

• Rotational Raman 

• Vibrational Raman 

What is polarizability

Polarizability is the ease with which the electrons within a molecule can be moved by an applied electric field. 

draw the polarizability ellipsoid and describe what it entails

what is an anisotropic ellipsoid

Anisotropic ellipsoid : different in different directions 

• Changes as the molecule rotates 

• Rotational Raman transition is allowed 

What is an isotropic ellipsoid

Isotropic ellipsoid : same in all directions 

• Applies only to spherical rotors 

• Doesn’t change when rotating 

Describe rotational raman spectroscopy

• Molecules change their rotational energy by inelastic light scattering 

• Rotational energy levels are the same as described prior and depend only on the molecule 

Describe specific selection rule for rotational raman spectroscopy

• Specific selection rule 

  • Delta J = 0, +- 2 (in diatomic molecules 

    • Different from absorption and emission 

 

Describe and draw the rotational raman spectroscopy graph

• Range of J levels populated and as such spectrum has several lines 

  • Spacing between the line are different from what they were in microwaves 

• On the stokes lines molecule gain energy on the Anti-stokes they lose energy. 

Describe Vibrational Raman Spectroscopy

• A molecule changes its vibrational energy by inelastic light scattering 

• Vibrational energy levels 

Describe Specific selection rule for Vibrational Raman Spectroscopy

Specific selection rule 

• Delta v = +- 1 (Harmonic oscillator) 

  • Same as for absorption and emission 

Describe Vibrational Raman Spectrum

• Spectrum 

  • Most molecules in v = 0 

  • One stokes band in spectrum 

  • One anti-stokes band (v = 1-> 0) generally not observed 

 

• generally reported and plotted as a change in wavenumber 

•  Δ v ~ = v ~ exc – v ~ stokes (Sometimes called the Raman shift) 

• showing only the Stokes region (energy gained by molecule) to resemble an IR spectrum 

What is the mutual exclusion rule

• Molecule has a centre of inversion, then it is Raman active and IR inactive.

• Vice versa

What happens if a molecule has no center of inversion

If a molecule does not have a center of inversion then some or all modes may be both Raman and IR active 

What are band positions determined by for raman vibrations

Bands Positions are determined by the molecule via its energy levels 

What are band strengths determined by for raman vibrations

Band strengths are determined by the spectroscopic technique via its selection rules