Determination of 3-D molecular orientation by polarization broadband CARS spectroscopy
Young J. Lee
Theoretical description is presented about a new analysis method to determine 3-diminsional (3-D) molecular orientation by concurrently analyzing polarization profiles of multiple Raman modes measured by broadband coherent anti-Stokes Raman scattering (CARS) spectroscopy. Conventional analysis of polarization Raman spectroscopy is very limited in providing 3-D orientational information due to intrinsic nature of signal generation by 2-D rotating light polarization. Here we propose to use dimensionless quantities, such as intensity ratios and phase difference, observed from multiple polarization profiles simultaneously measured by a single polarization scanning of BCARS at a single location. We describe how 3-D molecular orientation can be determined from the mathematical relations between the dimensionless observables and orientational angles. The 3-D orientation measurement has been demonstrated for two simplified cases, both in the absence and in the presence of orientational broadening by using a model distribution function. Even in the presence of broadening, this analysis method can measure not only the mean 3-D orientation angles but also the degree of orientational broadening as defined in the model function.