Resonance Secondary Radiation and Coherent Raman Scattering Spectroscopies Driven by Pulsed and Incoherent Light: Theoretical and Experimental Studies
This thesis investigates how two nonlinear spectroscopies, resonance secondary radiation (RSR), and coherent Raman scattering, depend on the temporal and coherence characteristics of the incident radiation field. Both theoretical and experimental work are presented for each of these spectroscopies. RSR refers to the overall process of light-induced excitation leading to spontaneous emission. A simple homogeneously -broadened three-level system coupled to a material bath as well as to the external radiation field proves to be a simple theoretical model in which to discuss RSR. Using the density matrix formalism, one finds that dephasing processes introduced into the applied light, by making it incoherent or short-pulsed, can cause a "light-induced" spectral redistribution to occur in the RSR spectrum. This effect is similar to the more familiar "material-bath-induced" spectral redistribution. Experimental RSR work studies the resonant emission from low temperature mixed naphthalene crystals driven by incoherent light. Empirically, these RSR spectra are found to behave as Raman-like spectra would, but, theoretical considerations show that when inhomogeneous broadening effects are included these spectra are more accurately characterized as fluorescence-like. Four-wave mixing spectroscopy has the ability to use incoherent light in order to achieve ultrafast time resolution. This thesis examines an experimental arrangement of coherent Raman spectroscopy (CRS) that uses incoherent light together with sharply-frequency-resolved detection to obtain dephasing measurements in transparent molecular liquids. One finds that when the CRS wave is sharply-frequency -resolved, the CRS signal reveals a new class of Rabi-detuning oscillations which are damped by the dephasing of the Raman -active mode. This frequency-resolved mode of detection is compared to the more conventional "white" mode of detection. In addition to the appearance of Rabi oscillations, CRS experiments that use sharp frequency resolution are often capable of achieving much better signal to noise contrast than would be achieved with "white" detection. A density matrix treatment of this experiment accounts for such observations.
- Pub Date:
- PULSED LIGHT;
- Chemistry: Physical; Physics: Molecular; Physics: Optics