Theoretical investigation of the direct observation of anharmonic coupling in CDCl3 in the time domain with femtosecond stimulated Raman scattering

The femtosecond stimulated Raman scattering (FSRS) difference spectra of CDCl3 from the ground potential energy surface, with and without off-resonance impulsive stimulated Raman pumping, taken at various delay times from the impulsive pump by Kukura [Phys. Rev. Lett. 96, 238303 (2006)], showed sidebands of the C-Cl bends with periodic changes in phase as well as having an inversion symmetry to the high and low frequency sides of the Stokes C-D stretch at 2255 cm(-1). The semiclassical coupled-wave approach and a one-dimensional (1D) oscillator model for the C-D stretch whose frequency is modulated in time by the C-Cl bends could account for some features of the experimental results. Here, a quantum mechanical investigation is made of the FSRS difference spectra with a modulated 1D oscillator and three-dimensional (3D) harmonic and anharmonic potentials. It is shown that (i) the sidebands are allowed, with or without anharmonic coupling between the C-D stretch and the C-Cl bends, (ii) in the 3D harmonic model the sidebands have mirror symmetry about the Stokes C-D stretch, (iii) in the 3D anharmonic model with appropriate coupling terms between the C-D stretch and the C-Cl bends, the experimental results are well accounted for in both the phase changes in the FSRS difference spectra with time delay as well as the inversion symmetry of the sidebands for the C-Cl bends about the Stokes C-D stretch, (iv) there is a correspondence between the phase changes in the FSRS difference spectra and the wavepacket motion induced by the impulsive pump pulse on the ground potential energy surface as a function of the delay time, and (v) changing the polarization of the impulsive pump pulse, say, directly affects the coordinate dependence of the linear transition dipole moment in the asymmetric mode and hence its phase in the FSRS difference spectra, but not so for the symmetric modes where the displacement of the excited state surface governs the FSRS difference spectra more than the transition dipole moment. This work illustrates the potential of off-resonance FSRS in studying polyatomic molecular dynamics.