CHEM3440Lec11F06 - Raman Spectroscopy Another spectroscopic...

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CHEM*3440 Chemical Instrumentation Topic 11 Raman Spectroscopy Raman Spectroscopy Another spectroscopic technique which probes the rovibrational structure of molecules. C.V. Raman discovered in 1928; received Nobel Prize in 1931. Can probe gases, liquids, and solids. Must use a laser source for excitation. Resurgence in recent years due to the development of new detectors with improved sensitivity. Shift back away from FT-Raman to dispersive Raman with multichannel detector systems. How It Works - Virtual States Excited Electronic State Ground Electronic State Virtual Electronic State …it only exists while the electrical field of the photon is present. Excitation Rayleigh Scattering Raman Scattering Stokes Anti-Stokes The Spectrum A complete Raman spectrum consists of: a Rayleigh scattered peak (high intensity, same wavelength as excitation) a series of Stokes-shifted peaks (low intensity, longer wavelength) a series of anti-Stokes shifted peaks (still lower intensity, shorter wavelength) spectrum independent of excitation wavelength (488, 632.8, or 1064 nm)
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Compare IR and Raman Spectra Spectra of PETN explosive. From D.N. Batchelder, Univ. of Leeds Some Raman Advantages Here are some reasons why someone would prefer to use Raman Spectroscopy. Non-destructive to samples (minimal sample prep) Higher temperature studies possible (don ! t care about IR radiation) Easily examine low wavenumber region: 100 cm -1 readily achieved. Better microscopy; using visible light so can focus more tightly. Easy sample prep: water is an excellent solvent for Raman. Can probe sample through transparent containers (glass or plastic bag). Origin of Raman Effect The oscillating electric field of the excitation light. The induced dipole moment from this oscillating field. The molecular polarizability changes with bond length. The bond length oscillates at vibrational frequency.
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