Chapter 5Raman Spectrometry Florida International University：第5rama法佛羅里達(dá)國際大學(xué)

1、advanced analytical chemistry chm 6157 y. caiflorida international universityupdated on 9/18/2008chapter 5raman spectrometrychapter 5 raman spectrometry1.fundamental aspects 1.1an example of raman effectgreen laserblue laserpentanereview through an orange glass filter which does not transits green o

2、r 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 scatteringadvanced analytical chemistry chm 6157 y. caiflorida international universityupdated on 9/18/2008chapter 5 raman spect

3、rometry1.2 background and brief historya beam of monochromatic light incident on a sampletransmittedabsorbedscatteredv most of the scattered light has the same wavelength as the incident light. v a small fraction of the scattered light is shifted in wavelength by the molecular vibrations and rotatio

4、ns 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

5、for qualitative analysisthe intensity of a raman spectrum is proportional to concentration for quantitative analysisadvanced analytical chemistry chm 6157 y. caiflorida international universityupdated on 9/26/2006chapter 5 raman spectrometry this scattering phenomena was discovered by the indian sci

6、entist c.v. raman 1928 c.v. raman and k.s. krishnan, a new type of secondary radiation, nature, 1928, 121, 5011888-1970advanced analytical chemistry chm 6157 y. caiflorida international universityupdated on 9/28/2006chapter 5 raman spectrometry 1.3 raman effecta photon raises the energy of the molec

7、ule 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 assum

8、e any of an infinite number of values, virtual statesthe molecule immediately relaxes back to the original electronic state by emitting a photon. advanced analytical chemistry chm 6157 y. caiflorida international universityupdated on 9/26/2006chapter 5 raman spectrometry rayleigh scattering:the mole

9、cule returns to the vibrational energy levels from which it started. no energy is transferred to the molecule. raman scattering: stokes raman scatteringe = h - e anti-stokes raman scatteringe = h - erayleigh, john william strutt1842-1919advanced analytical chemistry chm 6157 y. caiflorida internatio

10、nal universityupdated on 9/26/2006chapter 5 raman spectrometry light intensitiesat thermal equilibrium, the fraction of the molecules in one vibrational energy level relative to another is given by the bolzmann distribution equation:kteggnnexp2121advanced analytical chemistry chm 6157 y. caiflorida

11、international universityupdated on 9/28/2006chapter 5 raman spectrometry 1.4 what 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 universityupdated on 9/28/2

12、006chapter 5 raman spectrometry 1.5 raman scattering and infrared absorptioninfrared 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 prom

13、ote 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

14、a bond varies as a function of distance of the internuclear separation (r).advanced analytical chemistry chm 6157 y. caiflorida international universityupdated on 9/28/2006chapter 5 raman spectrometry ir measures the frequency of the absorbed light raman measures the frequency shift between incident

15、 and emitted light raman shift does not changes with incident light!advanced analytical chemistry chm 6157 y. caiflorida international universityupdated on 9/28/2006chapter 5 raman spectrometry 2.instrumentation2.1 sources high intensity of light sources are needed.laser sources are normally employe

16、d.type sourcewavelength, nmargon ion488 or 514.5krypton ion530.9 or 647.1helium/neon632.8diode laser 782 or 830 (near-infrared)nd/yag1064 (near-infrared)table some common laser sources for raman spectroscopyadvanced analytical chemistry chm 6157 y. caiflorida international universityupdated on 9/28/

17、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 e