Scattering of Light: Raman Spectroscopy

1. Scattering of Light: Raman Spectroscopy Deanna O’Donnell Informal P-Chem Review June 4th, 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 (s) • Measure of the likelihood a molecule will absorb a photon • Beer’s Law A = OD = e c l • Conversion s(cm2)= 2303 e(M-1cm-1) Na • s units of cm2 • Typical s values ~10-15 cm2 • Raman s values ~10-30 cm2

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 (nm) depends on molecular structure and environment. • Atomic mass, Bond order, Molecular substituents, Molecular geometry and Hydrogen bonding all contribute • Raman signal is 10-6 time weaker than incident light (no) • 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 no ± nm 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 (cm2) Stokes and Anti-stokes Raman Spectrum of CCl4

8. virtual states n0- nm n0 n0+ nm E1 E0 Stokes Scattering Rayleigh Scattering Anti-Stokes Scattering Raman Basics • Raman shifts can be expressed as no ± nm • 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 CCl4

9. More Raman Basics Spectra simplified, only totally symmetric modes enhanced – why? S1 So So Energy no = 334nm no = 500nm Resonance Raman e≥103 Normal Raman e≤100

10. Signal Enhancement • Common method to enhance the Raman scattering is • Resonance Raman • Resonance Raman • Occurs when no nem • Enhancement is on the order of 103 to 108 mi = aij Ej Imn = Io(no-nmn)4S|(aij)mn|2 mi = induced electric dipole aij = polarizability E = electric field of the iiiiiiiiiielectromagnetic radiation (aij)mn  (nem-no)-1 http://www.personal.dundee.ac.uk/~tjdines/Raman/RR3.HTM

11. Two commonly used methods to enhance the Raman scattering are Resonance Raman Surface Enhanced Raman Resonance Raman Occurs when no nem Enhancement is on the order of 103 to 108 How do you enhance the signal? mi = aij Ej Imn = Io(no-nmn)4S|(aij)mn|2 m = induced electric dipole aij = polarizability E = electric field of the iiiiiiiiiielectromagnetic radiation (aij)mn  (nem-no)-1

12. Surface Enhanced Raman Scattering (SERS) • Discovery • Experimentally discovered by Fleischmann et al. (1974) • Later explained by Van Duyne and Creighton (1977) • Produces 105 to 106 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 m = induced electric dipole aij = polarizability E = electric field of the electromagnetic radiation mi = aij Ej

13. Good References Vibrational Spectroscopy Wilson, E.B.; Decius, J.C.; Cross, P.C.; Molecular Vibrations, ISBN:0-486-63941-X Harris, D.C.; Bertolucci, M.D.; Symmetry and Spectroscopy, ISBN: 0-486-66144-X Raman Spectroscopy Ferraro, J.R.; Nakamoto, K.; Brown, C.W.; Introductory Raman Spectroscopy, ISBN: 978-0-12-254105-6 Radiation Chemistry Rates (use index, search engine not reliable)