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Resonance Raman Spectroscopy

  1. SAMRAT PRITHVIRAJ CHAUHAN GOVERNMENT COLLEGE AJMER Resonance Raman Spectroscopy Submitted by Rohini Narwal M.Sc. Chemistry Semester 2nd 2020-2021 Department Of Chemistry
  2. T ABLE OF CONTENT 1 • Introduction 2 • Raman Effect 3 • Resonance Raman Effect 4 • Theory 5 • Instrumentation 6 • RRS of Pentacene 7 • Merits and Demerits of RRS 8 • Applications of RRS 9 • Conclusion 10 • References
  3. INTRODUCTION  Raman Spectroscopy is based on scattering of radiation which is phenomenon discovered in 1928 by physicist Sir C.V.Raman and he won the Noble Prize in 1930 for his work.  The field of Raman spectroscopy was greatly enhanced by the advent of laser technology during 1960s.  Resonance Raman spectroscopy also helped to advance the field: 1) The Resonance Raman Spectroscopy is a particular application of the general Raman spectroscopy where the incident laser radiation has a frequency that matches the energy of an electronic transition in the sample. 2) This technique is more selective compared to non resonance Raman spectroscopy. 3) It works by exciting the analyte with incident radiation corresponding to the electronic absorption bands. This
  4. RAMAN EFFECT  Resonance Effect : When a beam of light is passed through a transparent substance, a small amount of the radiation energy is scattered, the scattering persisting even if all dust particles or other extraneous matter are rigorously exclude from the substance.  If monochromatic radiation, of narrow frequency band, is used the scattered energy will almost entirely of radiation of the incident frequency but in addition, certain discrete frequencies above and below that of the incident beam will be scattered, it is called Raman Scattering.
  5. RESONANCE RAMAN EFFECT  The phenomenon in which Raman line intensities are greatly enhanced by excitation with wavelengths that closely approach that of an electronic absorption peak of an analyte is known as Resonance Raman Effect.  Pre-resonance is the condition where the laser excitation is around 100 wavenumbers below the electronic transition.  And the spectrum so obtained is called Resonance Raman Spectrum.
  6. THEORY  The theory of resonance Raman effect is rather complex.  Kramers Heisenberg Dirac (KHD) equation Normal modes with large displacement in excited state are most intense  The Raman polarizability is given by the term (α ρσ)GF where ρ and σ are the polarizations of incident and Raman scattered light and the terms G and F correspond to the ground and final vibronic states of the molecule.  The excited state is indicated by E.  The polarization dependent dipole operator is given by either rρ or rσ.  The summation symbol indicates that the Raman polarizability is given by the sum of all of the vibronic states of the molecule.  ν GE, νEF, and νL are the frequencies of the ground to excited state transition, excited state to final state transition, and laser.
  7. INSTRUMENTATION  The main components of a resonance raman spectroscopy system are 1) Light source 2) Optical components such as lenses and mirrors to focus the light onto a sample and collect the scattered light 3) A spectrometer 4) A detector  The light source is typically a VIS,NIR laser emitting monochromatic light. Types of lasers are gas lasers e.g. Ar⁺(488 and 514.5nm), diode-pumped solid state lasers or tunable lasers are most suitable for RRS.  Notch filters are used to filter the Rayleigh line intensity before the scattered light is entering the spectrometer and the detector (CCD camera).
  8. Schematic diagram of Resonance Raman Spectrometer
  9. RESONANCE RAMAN SPECTROSCOPY OF PENTACENE Pentacene is a polycyclic aromatic hydrocarbon which is consisting of five linearly-fused benzene rings. Pentacene is a compound of great interest in the world of molecular electronics and is solids at room temperature. It absorb light in the visible region of the spectrum make it good candidates for resonance Raman Pentacene appears almost black. This highly conjugated compound is an organic
  10.  A resonance Raman spectrum of pentacene acquired using 633 nm excitation.  The highest energy bands in the fingerprint region will be due to aromatic ring vibrational modes at approximately 1600 cm-1.  The most prominent bands in the spectrum appear at 1158,1176,1371,1532 and 1597cm-1 . The first four of these bands have been assigned to Ag symmetry species and the 1597 cm-1 band to the B3g symmetry species.  An expand view of the portion 1600 to 3000 cm-1 of the Raman spectrum consisting of overtones and combination modes. The peak at 3194 cm-1 can be attributed to aromatic C-H stretching.
  11. DIFFERENCES IN THE RAMAN SPECTRA AS A FUNCTION OF EXCITATION WAVELENGTH
  12.  A profile of Raman spectra of pentacene acquired using 405, 473, 532, 633, and 785 nm excitation.  All of the excitation wavelengths fall within the absorption spectrum of pentacene and therefore yield resonantly enhanced Raman spectra .  Although absorption occurs for all of the other laser excitation wavelengths, they do not all couple to the same electronic transition.  Using a longer excitation wavelength at 532 nm, we see definite changes in the relative intensities of the bands.  At 633 nm excitation, the spectrum is similar to that obtained with 532 nm excitation, but we observe a small recovery of the strengths of the 1408 and 1456 cm-1 bands.  Finally, at 785 nm excitation we observe a very different Raman spectrum with respect to relative intensities because at this wavelength we are now out of resonance.
  13. MERITS OF RRS  It improves the sensitivity, which allows detection of the sample at micromolar concentration, whereas FTIR and conventional Raman require millimolar concentration.  The state of the sample required for RRS can be solid, fiber, gel, or solution.  The time scale is on the order of 10⁻15–10⁻14 s, which is too short, and is on the order of molecular vibrations.  No additional probes are required for analysis. DEMERITS OF RRS  The disadvantage of this technique is that the laser light used for excitation can damage the sample, which can be solved by either agitation of the sample or using flow methods.  Fluorescence is a problem for Resonance Raman techniques, particularly when using sources in the visible range which can swamp the Raman signal.
  14. APPLICATIONS OF RESONANCE RAMAN SPECTROSCOPY  RR Spectroscopy provides structural information. GEOMETRY •Active –site structure •Isotope shifts •Characteristic frequencies •Symmetry •Relative changes across a series ELECTRONIC STRUCTURE •Bond lengths •Excited state structure •Nature of electronic transition •Excitation profiles Reactivity •Potential energy surface(s) •Reaction coordinate •Spectral/frequency changes as a function of time •Rapid freeze quenching •Continuous flow/mixing
  15.  RRS is a powerful technique for monitoring the structure and dynamics of proteins and peptides in solution.  The ultraviolet resonance Raman spectra are employed for the conformation analysis of proteins and amyloid fibres.  RRS is employed in biomedical applications including single blood cell detection by the trapping method.  The antioxidative capacity of the skin in vivo has been studied using RRS. RRS to identify carotenoids in lower epidermis and dermis in ca skin
  16. CONCLUSION  Vibrational overtones and combination modes can appear in resonance raman spectra whereas they are frequently absent from non-resonant raman spectra.  A tunable laser is preferred for resonance Raman and can be an advantage. That is because only one laser is necessary to do analyses of multiples samples in which each one requires a different excitation wavelength. This allows the user to switch out samples without having to switch out the lasers as well.  If the excitation wavelength is in resonance with an electronic transition of the sample, then enhancement of the signal strength of some Raman bands can occur. Consequently, the resonance Raman spectrum can appear quite different from the normal Raman spectrum because of the sometimes very significant differences in relative intensities.
  17. REFERENCES  Molecular Structure and Spectroscopy by G.ARULDHAS  Fundamental of molecular spectroscopy by COLIN N.BANWELL AND ELAINE M.McCASH  Chemistry LibreTexts™ <chem.libretexts.org>  Spectroscopy solution for materials analysis <spectroscopyonline.com>
  18. For Your Time and Attention Thank You
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