Quantitative Spectral Analysis of Coherent Anti-Stokes Raman Scattering Signals: C-H Stretching Modes of the Methyl Group.

Coherent anti-Stokes Raman scattering (CARS) vibrational spectroscopy has been extensively developed into a powerful analytical technique to study various molecules. Quantitative interpretation of CARS spectra can help to improve CARS for chemical analysis and extend its analytical applications. In this work, we quantitatively analyzed CARS signals originating from the methyl groups in poly(dimethylsiloxane) (PDMS), with the help of the bond additivity method. Experimentally, a home-built CARS spectrometer modified from a commercial sum frequency generation spectrometer was used to collect CARS spectra from a PDMS film. Theoretically, we successfully reproduced the peak intensity ratio of C-H symmetric and asymmetric stretching modes of the PDMS methyl group in different polarization combinations based on bond additivity method and Raman depolarization ratio. This research shows that bond additivity theory can help to obtain the third-order nonlinear susceptibility tensor properties probed by different polarization combinations used in CARS spectroscopy. The method developed in this work could also be applied to CARS vibrational stretching analysis of other functional groups, providing quantitative understanding of CARS spectrum for applications in spectroscopy.