Selective excitation CARS spectroscopy: An efficient method for analysis of vibrational coupling and intramolecular vibrational energy transfer

Abstract

Selective excitation of the high-frequency CH stretching vibrational modes around 3000 cm−1 and the low-frequency vibrational modes around 1500 cm−1 of toluene in the narrow femtosecond time- and frequency-resolved CARS spectrum induces the transfer of the intramolecular vibrational energy from the vibrational modes in the selectively-excited region to the vibrational modes outside the excitation region. The vibrational energy transfer from high-frequency parent modes (vibrational modes are selectively excited by pump and Stokes laser pulses) to low-frequency daughter modes (vibrational modes are indirectly excited by energy transfer) via vibrational coupling and the dynamic information of vibrational modes involved in coupling are identified by fast Fourier Transformation (FFT) analysis. The lifetime of the vibrational modes is described, and the energy transfer pathway is confirmed. When the majority of toluene vibrational modes are collectively excited in the multiplex femtosecond time- and frequency-resolved CARS, the vibrational coupling information can be traced from the time-domain CARS spectrum and its frequency-domain FFT power spectrum. Vibrational energy transfer cannot be detected in the multiplex CARS spectrum of toluene. According to experimental results from the selective excitation CARS spectrum and multiplex CARS spectrum, the selective excitation CARS spectroscopy is an efficient method to track intramolecular vibrational energy transfer from parent modes to daughter modes and identify the energy transfer pathway.