587 08 wk08a nonlinear spect

587 08 wk08a nonlinear spect - Nonlinear Spectroscopy...

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1 Nonlinear Spectroscopy • Nonlinear spectroscopies are coherent spectroscopies • Polarization of the molecule, ability to induce a dipole described as P = α E+ β E 1 E 2 + γ E 1 E 2 E 3 +… •2 nd order optical spectroscopies – Second Harmonic Generation Spectroscopy ω SHG = 2 vis – Sum Frequency Generation Spectroscopy SFG = vis + IR – Difference Frequency Generation Spectroscopy DFG = vis - IR •3 rd order optical spectroscopies – Coherent Anti Stokes Raman Spectroscopy (CARS) • CARS is one type of 3 rd order spectroscopy, most common Advantages and Disadvantages of using nonlinear techniques Pros Nonlinear processes are highly sensitive Nonlinear signal with number density CARS: Electric field 3 α number of molecules SHG, SFG, DFG: E-field 2 α number of molecules Note that IR and Raman are linear: E-field α number of molecules The detected photons are directionally emitted Unlike nonlinear Raman where the molecules scatter in all directions
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587 08 wk08a nonlinear spect - Nonlinear Spectroscopy...

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