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From quantum mechanics to auto-mechanics

Frontiers in Spectroscopy. Ohio State University, March 2004. Nonlinear Spectroscopy:. From quantum mechanics to auto-mechanics. Phase Matching: Conservation of momentum. Physics: Grating formation + Readout. Calculation of the Signal intensity: ~ Field effects + Atom effects.

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From quantum mechanics to auto-mechanics

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  1. Frontiers in Spectroscopy. Ohio State University, March 2004 Nonlinear Spectroscopy: From quantum mechanics to auto-mechanics Paul Ewart

  2. Phase Matching: Conservation of momentum Physics: Grating formation + Readout Calculation of the Signal intensity: ~ Field effects + Atom effects

  3. The nonlinear wave equation

  4. Phase matching geometries

  5. Forward Folded Boxcars Geometry

  6. We need to find the Polarization induced by the incoming 3 fields

  7. The polarization induced by 2 pump and 1 probe photon is given by r21

  8. Susceptibility for saturated absorption

  9. Note: Signal proportional To phase conjugate of probe } Solve coupled equations

  10. Note reduced sensitivity to collisions of saturated signal

  11. Calculable on PC Full spectrum in <1 minute

  12. DFWM Spectra of C2: Saturation and spectral modelling Unsaturated Saturated Strongly saturated Experiment BLE Theory Abrams & Lind

  13. DFWM Lineshapes: power broadening with pump and probe saturation

  14. Laser Induced Thermal Gratings

  15. Laser Induced Thermal Gratings • Energy per pulse • Einstein B-coefficient • Grating “wavelength” • Laser pulse shape in time • Laser pulse profile in space • Einstein A-coefficient • Quenching rate • Diffusion rate

  16. Thermal Grating evolution (Bulk gas dynamics)

  17. Density Perturbation in LITGS • Acoustic gratings interfering… • Speed of sound Temperature (2) Temperature grating… Decay by diffusion Pressure

  18. Laser Induced Thermal Grating Scattering: simulation

  19. LITGS from OH in high pressure methane/air flame

  20. Analysis of LIGS signals The LIGS signal is a convolution of several functions: Excitation by pump laser pulse l (t) Quenching rates q(t) Evolution of the stationary and acoustic gratings r(s) t is the normalized time s is the Laplace transform variable density perturbation. r(s) Fourier transformed to R(w) l (t) Fourier transformed to L(w) q(t)Fourier transformed to Q(w) LIGS signal Z(w) = product of the Fourier transforms: Z(w) = R(w) . Q(w) . L(w) … Rapid fitting of simulated LIGS signals to data using T, P as fitting variables with calculated gas dynamic parameters.

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