Spectral intensity is then interpreted in terms of the temperature of different regions of the flame. CARS signal is coherent and emitted in one direction and can be observed without monochromator. It is on Anti-Stokes side and thus avoids fluorescence. All modes that are Raman active and some inactive Raman and IR modes are active in CARS. 2 P − S = 2 P − ( P − M ) = P + M
With high intensity pulse at frequency , scattered radiation contains frequencies 2 (hyper-Rayleigh scattering) and (2 ± M ) (Stokes and anti-Stokes hyper-Raman scattering), where Μ is normal vibration of molecule, caused by two incident photons (2 ) of the laser. Different symmetry selection rules apply, and hyper- Raman effect contains all IR active modes. Hyper-Raman scattering is produced by very high intensity pulses. Two photons of the exciting radiation produce the Raman spectrum. A non-linear effect in which the vibrational modes interact with the second harmonic of the excitation beam . This requires very high power, but allows the observation of vibrational modes that are normally "silent".
Two laser beams , P (pump beam) and S (Stokes beam) impinge on gaseous sample, interact when P - S = M for Raman active mode. The Stokes beam is amplified and the pump beam is attenuated. The vibrationally excited molecules lose excitation to translational energy, changing the pressure in the cell, which can be detected with a microphone. In this case there is no Rayleigh line, and low energy rotational lines can be studied. Photoacoustic Raman Spectroscopy (PARS) For each Stokes photon created by the Raman process, one molecule is transferred from the lower state to the upper state of the transition. Collisional relaxation of these excited molecule produces changes that is detected by a microphone.
Problems 1. Consider the vibrational mode that corresponds to the uniform expansion of the benzene ring. a) Is it Raman active or infrared active? b) Why? 2. A linear molecule has the following fundamental vibrational transitions: 370 cm -1 , 520 cm -1 , 880 cm -1 and 1550 cm -1 (all Raman active) The wavelength of the excitation radiation is 500 nm. a) Calculate the wavenumbers of scattered radiation in Stokes range. b) How many atoms has this molecule? 3. Calculate the temperature of the sample (CCl 4 ) from Raman spectrum, knowing the excitation wavelength and I S /I AS ratio for 460 cm -1 band (I S /I A = 6; l ex = 488 nm) h = 6,63∙10 -34 J∙s, c= 3∙10 8 m/s, k B =1,38∙10 -23 J/K
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- Fall '19
- Raman Spectroscopy, Raman scattering, Sir Chandrasekhara Venkata Raman