lecture_13_Raman Spectroscopy and Application

lecture_13_Raman Spectroscopy and Application - Lecture #13...

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Lecture #13 Raman Spectroscopy and Application Reading: Chapter 18, page 481 –497 ; Problems: 18-1, 3, 5. • Basics of Raman spectroscopy; • Instrumentation of Raman spectroscopy; • Qualitative and quantitative applications; • Resonance and Surface Enhanced Raman Spectroscopy.
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Some remarks about Raman spectroscopy C.V. Raman , an Indian physicist, discovered in 1928; received Nobel Prize in 1930. • Another spectroscopic technique which probes the vibrational structure of molecules ( what is the other one ?). • Can be applied to gaseous, liquid, and solid samples. • Almost all modern Raman instruments use laser sources for excitation. Complementary to IR spectroscopy. It used to be somewhat specialized, but nowadays is becoming more commonly used instrumentation with the development of new detectors for improved sensitivity.
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The Nobel Prize in Physics 1930 "for his work on the scattering of light and for the discovery of the effect named after him" b. 1888 d. 1970 Calcutta University Calcutta, India India Sir Chandrasekhara Venkata Raman Raman Spectroscopy
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What is Raman scattering? Excited electronic state Ground electronic state 0 1 2 3 0 1 2 3 Virtual states Excitation Rayleigh Scattering Raman Scattering Anti-Stokes, E=h ν + E Stokes, E=h - E = 0 E
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Raman Spectrum A complete Raman spectrum consists of: • a Rayleigh scattering peak (high intensity, same wavelength as excitation, E = 0) • a series of Stokes-shifted peaks (lower frequencies, E=h ν - E, longer wavelength) • a series of anti-Stokes shifted peaks (higher frequencies, E=h + E , shorter wavelength) • spectrum independent of excitation wavelength (488, 632.8, or 1064 nm) • only the Stokes peaks are generally reported for Raman spectra, since Stokes peaks have higher intensity than anti-Stokes peaks (why ?). --- (see last slide) the fraction of the molecules in the first vibrationally excited state is lower than those populated at the vibrationally ground state. Increasing temperature generally increases the population at vibrationally excited states, and thus increases the intensity of anti- Stokes peaks. Figure 18-2. Spectrum of CCl 4 , using an Ar + laser at 488 nm.
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Raman and IR spectra are complementary Spectra of PETN explosive. Univ. of Leeds • Energy shifts (frequency difference) observed in Raman spectra should be identical to the energies of its IR absorption bands, while the peak intensity may be different. • Some peaks occur in one spectrum are absent
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This note was uploaded on 05/15/2010 for the course CHEM 434 taught by Professor None during the Spring '07 term at University of Michigan-Dearborn.

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lecture_13_Raman Spectroscopy and Application - Lecture #13...

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