Vibrational coupling effects in the energy redistribution of alkylbenzenes

Ultrafast time- and frequency-resolved coherent anti-Stokes Raman Scattering spectroscopy (CARS) was used to detect vibrational energy redistribution in liquid alkylbenzenes at ambient-temperature. The C–H stretching vibrational modes (vibrational frequencies near ∼3000 cm⁻¹) on substituents were selectively excited, the substituents were CH₃ (toluene), (CH₃)₂CH (isopropylbenzene) and (CH₃)₃C (t-butylbenzene), vibrational energy flow from the substitution modes to the phenyl modes was observed in the electronic ground state of alkylbenzenes. According to the selective excitation CARS experimental results, beats among these relevant modes were observed and vibrational coupling information was extracted via Fourier analysis. Vibrational coupling and substitution structure that effect energy redistribution processes were discussed.