Experiment #6
Raman Spectroscopy of CCl3H, CCl3D, and CCl4 Introduction
{ The Raman phenomenon is a light scattering process. Although the inelastic scattering of light was predicted by Smekal in 1923, it was not until 1928 that it was observed in practice. { The Raman effect was named after one of its discoverers, the Indian scientist Sir C. V. Raman who observed the effect by means of sunlight (1928, together with K. S. Krishnan and independently by Grigory Landsberg and Leonid Mandelstam). Raman won the Nobel Prize in Physics in 1930 for this discovery accomplished using sunlight, a narrow band photographic filter to create monochromatic light and a "crossed" filter to block this monochromatic light. He found that light of changed frequency passed through the "crossed" filter. Raman Spectrometer Classical Theory
{ Rayleigh scattering z The induced oscillations of electric charge due to the applied field emit light mostly at the same frequency as the applied field { Raman scattering z About 1 out of 108 photons is scattered inelastically (i.e., energy changed) z Increase in energy: anti-Stokes shift z Decrease in energy: Stokes shift Polarization
{ Induced movement of charge in response to an electric field (light)
µin = αE { The frequency and intensity of the scattered radiation is determined by the frequency and magnitude of the oscillating dipole moment induced by the incident light. Vibrational frequency of a Polarizability Raman active mode in a molecule
α = α0 + α1cos(ωvt)
E = E cos(ω t) frequency 0 i of incident light
µin = [α0 + α1cos(ωvt)]E0cos(ωit)
µin = α0E0cos(ωit) +½α1E0[cos(ωi−ωv)t + cos(ωi+ωv)t] Raleigh Stokes Anti-Stokes scattering scattering scattering Raman Transitions Raman Active
{ In order for a particular mode of vibration to be Raman active its polarizability has to change (must be non-zero) with respect to its
normal mode coordinate qi. ∂α α1 = qi ∂qi Quantum Mechanics
{ Where ψ1 is the first excited vibrational state of a given mode, ψ0 is the ground state and αij a polarizability element in the molecular coordinate system. If this integral is non-zero a Raman peak is observed. ψ α ψ dq ∫ 1 ij 0
{ Whether or not the integral is zero is determined by symmetry z Even, symmetric functions (e.g., cos(x)) are finite
z odd, asymmetric functions (e.g., sin(x)) are zero Molecular Insights
{ For totally symmetric vibrations the induced molecular dipole is always in the same direction as the applied field thus light scattered due to a symmetric vibrations is polarized parallel to the applied field. { For asymmetric vibrations the polarizability is anisotropic hence the induced dipoles are no longer parallel to the applied field and the scattered light is said to depolarized with respect to the incident light.
{ Depolarization ratio pd = pa / pe z The ratio of the two intensities corresponding to the light collected with the perpendicular pe and parallel pa orientation of the polarizing filter. Depolarization Relative Intensity
Anti-Stokes transitions Boltzmann’s distribution
I AS Nv1= Nv1= hv ∝ =−exp INS v0= NkTv0= Stokes transitions
4 i I ASiνν0 +−hv i i = exp ISiνν0 − kT
Laser frequency Raman Spectra of (10,5)CNTs Stokes to anti-Stokes intensity ratio in Raman spectra of the soft mode in KH2PO4 near the phase transition temperature Journal of Non-Crystalline Solids, Volume 354, Issues 2-9, 15 January 2008, Pages 112-116 Raman Spectra (Stokes Shift)
CHCl3 CDCl3
The Chemical Educator, Vol. 7, No. 1, Published on Web 02/01/2002