Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/18/2008Chapter 5Raman Spectrometry Chapter 5 Raman Spectrometry 1.Fundamental aspects 1.1An example of Raman Effect Green laser Blue laser Pentane Review through an orange glass filter which does not transits green or blue light (see a much dimmer red line) Review through no filter Review through an orange glass filter which does not transits green or blue light (see orange color) Raman Scattering

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/18/2008Chapter 5 Raman Spectrometry 1.2Background and brief history  A beam of monochromatic light incident on a sample transmitted absorbed Scattered  Most of the scattered light has the same wavelength as the incident light.  A small fraction of the scattered light is shifted in wavelength by the molecular vibrations and rotations of the molecular in the sample. The shifts in wavelength depend upon the chemical structure of the molecular responsible for the scattering. The spectrum of this wavelength-shifted light is called a Raman spectrum.  Many sharp bands that are characteristics of the specific molecules in the sample – for qualitative analysis  The intensity of a Raman spectrum is proportional to concentration – for quantitative analysis

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/26/2006Chapter 5 Raman Spectrometry  This scattering phenomena was discovered by the Indian scientist C.V. Raman 1928 C.V. Raman and K.S. Krishnan, A new type of secondary radiation, Nature, 1928, 121,

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 5 Raman Spectrometry 1.3Raman effect  A photon raises the energy of the molecule from the ground state to a “virtual state’.  The increase in energy is equal to the energy of photon h.  The process is not quantized, just a distortion of the electron distribution of a covalent bond.  Depending upon the frequency of the radiation from the source, the energy of the molecule can assume any of an infinite number of values, virtual states  The molecule immediately relaxes back to the original electronic state by emitting a photon.

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/26/2006Chapter 5 Raman Spectrometry Rayleigh scattering: The molecule returns to the vibrational energy levels from which it started. No energy is transferred to the molecule. Raman Scattering: Stokes Raman Scattering E = h - ΔE Anti-Stokes Raman Scattering E = h - ΔE Rayleigh, John William Strutt

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/26/2006Chapter 5 Raman Spectrometry Light Intensities At thermal equilibrium, the fraction of the molecules in one vibrational energy level relative to another is given by the Bolzmann distribution equation:

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 5 Raman Spectrometry 1.4What is light scattering The re-emission of light from the light- induced oscillation in the electron cloud is called scattering.

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 5 Raman Spectrometry 1.5Raman scattering and infrared absorption  Infrared absorption requires that a vibrational mode of the molecule have a change in dipole moment or charge distribution associated with it. Only then can radiation of the same frequency interact with the molecule and promote it an excited vibrational state.  Scattering involves a momentary distortion of the electrons distributed around a bond in a molecule, followed by reemission of the radiation as the bond returns to its normal state. It is important to note that Raman scattering requires that the polarizability of a bond varies as a function of distance of the internuclear separation (r).

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 5 Raman Spectrometry  IR measures the frequency of the absorbed light  Raman measures the frequency shift between incident and emitted light  Raman shift does not changes with incident light!

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 5 Raman Spectrometry 2.Instrumentation 2.1Sources High intensity of light sources are needed. Laser sources are normally employed. Type sourceWavelength, nm Argon ion488 or Krypton ion530.9 or Helium/neon632.8 Diode laser782 or 830 (near-infrared) Nd/YAG1064 (near-infrared) Table Some Common Laser Sources for Raman Spectroscopy

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 5 Raman Spectrometry Two advantages with near-infrared radiation sources Can operated at much higher power without causing photodecomposition of the sample. Reduce fluorescence interference because they are not energetic enough to populate a significant number of fluorescence producing excited electronic energy states in most molecules. One must choose the one that is not absorbed by the sample and solvent (except with resonance Raman measurement).

Advanced Analytical Chemistry – CHM 6157® Y. CAIFlorida International University Updated on 9/28/2006Chapter 5 Raman Spectrometry