1 Scattering of Light: Raman Spectroscopy Deanna O’Donnell Informal P-Chem Review June 4 th, 2009

2 A review of light Electromagnetic wave –Oscillating electric and magnetic fields Classical Interactions of light and matter –Absorption –Reflection –Refraction –Scattering Scattering –Elastic (Rayleigh scattering) –Inelastic (Raman scattering)

3 Cross section () Measure of the likelihood a molecule will absorb a photon Beer’s Law A = OD =  c l Conversion  (cm 2 )= 2303  (M -1 cm -1 ) N a  units of cm 2 Typical  values ~ cm 2 Raman  values ~ cm 2

4 History Sir. C.V. Raman discovered light scattering in 1928 Awarded Nobel Prize in physics in 1930 Experiment composed of light source (sunlight), a sample, and detector (eye) His nephew, Dr. S. Chandrasekhar, of the University of Chicago won the Nobel prize in physics in 1983 Sir. C.V. Raman

5 Raman Basics Raman spectroscopy studies the frequency change of light due to the interaction with matter The energy of a vibrational mode ( m ) depends on molecular structure and environment. Atomic mass, Bond order, Molecular substituents, Molecular geometry and Hydrogen bonding all contribute Raman signal is time weaker than incident light ( o ) Photons are not absorbed To observe Raman scattering the molecule must be polarizable

6 Selection Rules

7 More Raman Basics Raman shifts can be expressed as o ± m Stokes and Anti-stokes produce same spectrum, differing in intensity. Intensity is governed by the Maxwell-Boltzmann Distribution law. Raman shifts are measured in wavenumbers (cm 2 ) Stokes and Anti-stokes Raman Spectrum of CCl 4

8 Raman Basics Raman shifts can be expressed as o ± m Stokes and Anti-stokes produce same spectrum, differing in intensity. Intensity is governed by the Maxwell-Boltzmann Distribution law. Raman shifts are measured in wavenumbers (cm -1 ) Stokes and Anti-stokes Raman Spectrum of CCl 4 E1E1 E0E0 Stokes Scattering Anti-Stokes Scattering Rayleigh Scattering  - m  + m  virtual states

9 More Raman Basics Normal Raman  ≤10 0 Resonance Raman  ≥10 3 Energy o = 500nm o = 334nm S1S1 SoSo SoSo Spectra simplified, only totally symmetric modes enhanced – why?

10 Signal Enhancement Common method to enhance the Raman scattering is Resonance Raman Occurs when o  em Enhancement is on the order of 10 3 to 10 8  i =  ij E j  i = induced electric dipole  ij = polarizability E = electric field of the iiiiiiiiiielectromagnetic radiation I mn = I o ( o - mn ) 4  |(  ij ) mn | 2 (  ij ) mn  ( em - o ) -1

11 How do you enhance the signal? Two commonly used methods to enhance the Raman scattering are Resonance Raman Surface Enhanced Raman Resonance Raman Occurs when o  em Enhancement is on the order of 10 3 to 10 8  i =  ij E j  = induced electric dipole  ij = polarizability E = electric field of the iiiiiiiiiielectromagnetic radiation I mn = I o ( o - mn ) 4  |(  ij ) mn | 2 (  ij ) mn  ( em - o ) -1

12 Discovery Experimentally discovered by Fleischmann et al. (1974) Later explained by Van Duyne and Creighton (1977) Produces 10 5 to 10 6 enhancement Metal surfaces utilized include Ag, Au, Cu, Li, Na, K, In, Pt, Rh SERS is possible due to Electromagnetic and Chemical enhancement Other factors contribute to further enhancement NaCl, “hot spots”, concentration, orientation Surface Enhanced Raman Scattering (SERS)  i =  ij E j  = induced electric dipole  ij = polarizability E = electric field of the electromagnetic radiation

13 Good References Vibrational Spectroscopy Wilson, E.B.; Decius, J.C.; Cross, P.C.; Molecular Vibrations, ISBN: X Harris, D.C.; Bertolucci, M.D.; Symmetry and Spectroscopy, ISBN: X Raman Spectroscopy Ferraro, J.R.; Nakamoto, K.; Brown, C.W.; Introductory Raman Spectroscopy, ISBN: Radiation Chemistry Rates (use index, search engine not reliable)