Femtosecond pulse excitation of vibrational wave packets in chloroform: The effect of gold nanoparticles

Excitation of pure chloroform or a gold colloid in chloroform (average nanoparticle diameter of 2.5 nm) with 740-nm femtosecond pulses of 23-fs duration leads to oscillations in the differential absorption signal ΔA(λ, t) recorded using white-light continuum. The main oscillation modes for both systems are close to the chloroform Raman resonance frequencies of 260, 367, and 668 cm⁻¹. However, marked narrowing and splitting of bands in the Fourier-transform spectra of oscillations are observed in the system of gold colloid in chloroform. The intensity of oscillations linearly increases with the pump pulse intensity in the cases of pure chloroform and the system of gold colloid in chloroform. Apparently, the principal mechanism of excitation of coherent packets in chloroform is femtosecond impulsive stimulated Raman scattering (ISRS). Dissolution of gold nanoparticles leads to a four- to sixfold enhancement of the Raman resonance signal of chloroform in the gold colloid as compared with pure chloroform. The increase in the intensity of Raman resonances is presumably due to amplification in the near field of gold nanoparticles.