Theoretical studies and model calculations of femtosecond stimulated Raman spectroscopy

• Main Author: Qiu, Xueqiong
• Other Authors: Lee Soo Ying
• Format: Theses and Dissertations
• Language: English
• Published: 2012
• Subjects: DRNTU::Science::Physics::Optics and light
• Online Access: https://hdl.handle.net/10356/48905
• Institution: Nanyang Technological University

The theoretical descriptions of Femtosecond stimulated Raman spectroscopy (FSRS) and its features were studied using three- state model and displaced harmonic oscillator model. Two molecules were studied using displaced harmonic oscillator model: Rhodamine 6G (R6G) and crystal violet (CV) and compared to experiment. In the quantum-mechanical description of FSRS for a three-state model, With the help of Feynman diagram, a completed series of expressions of third-order polarizations were obtained to represent all the Stokes, Rayleigh, and anti-Stokes lines. It's showed that eight terms contribute to the FSRS, and we grouped them in four groups, which correspond to four nonlinear processes - stimulated Raman scattering (SRS(I) and SRS(II)) and inverse Raman scattering (IRS(I) and IRS(II)). The SRS(I) contributes to the Stokes band, IRS(I) contributes to anti-Stokes band of FSRS spectra, while the SRS(II) and IRS(II) form the baseline of FSRS spectra. In order to get analytical expressions for the FSRS spectra, a displaced harmonic oscillator model was proposed. Then, the analytical expressions of third-order polarizations were obtained in the form of triple integrals over four-time correlation functions. The analytical expressions for the FSRS were successfully applied to R6G. The comparison of calculated results with experimental results provided good test of our theory and helped to understand all the features of FSRS. The effects of various of parameters on FSRS line shapes were studied: the Raman pump pulse temporal width, the vibrational dephasing time of ground state, the lifetime of excited state, and the inhomogeneous damping constant for energy difference between ground and excited states. The IRS spectra of CV was calculated using displaced harmonic oscillator model. The change in phase of the line shapes with Raman pump wavelength was reproduced. And it could be explained by the expression of the third-order susceptibility for IRS. In the limit of monochromatic Raman pump and probe pulses, we obtained the third-order susceptibility and recovered the well-known expression for the third-order susceptibility of IRS.