Measuring Accurate Stimulated Raman Scattering Cross-sections Of Liquids

Nonlinear spectroscopy provides important information about chemical systems that is not available from linear spectra alone. However, nonlinear spectroscopies have rarely been used for quantitative measurements due to the difficulty in obtaining accurate cross-sections. To overcome this limitation, we have been developing new methods to obtain accurate two-photon absorption (2PA) and stimulated Raman scattering (SRS) cross-sections of liquids and liquid solutions using ultrafast pump-probe spectroscopy. Here, we describe recent advances in measuring absolute SRS cross-sections of common liquid solvents based on a simple setup using single-wavelength pump and broadband probe pulses derived from a tunable repetition rate Yb:KGW laser. We obtain a two-dimensional transient absorption spectrum that is integrated over the time delay between the pump and the probe to obtain the Raman response as a frequency-dependent SRS spectrum. We model the overlap between the pump and probe beams in terms of the beam diameter, crossing angle, and refractive index of the solvent in order to determine accurate values for the SRS cross-sections based on the pump-probe signal strength. Specifically, we report absolute cross-sections for the C--H stretching modes of cyclohexane, dimethyl sulfoxide, acetonitrile, methanol, benzene, and toluene using Raman excitation wavelengths of 1030 nm (anti-Stokes), 515 nm (Stokes and anti-Stokes), and 343 nm (Stokes). The measured cross-sections follow the expected $\sim\nu^4$ frequency dependence. Our measurements also allow for direct comparison of the absolute SRS cross-sections for Stokes and anti-Stokes scattering from a single (515 nm) Raman excitation laser, which may be important for quantitative measurements using coherent nonlinear spectroscopies. For example, the SRS cross-sections that we measure for these solvents provide valuable internal reference standards for the determination of accurate 2PA cross-sections of solutes