Picosecond coherent anti-Stokes Raman spectroscopy of molecules in free jet expansions

By using time-resolved coherent Raman scattering, closely spaced vibrational resonances are simultaneously excited and subsequently monitored over long, nanosecond time intervals. Coherent superposition of molecular transitions leads to dramatic oscillations of the picosecond anti-Stokes scattering signals. Experimental data are presented for the ν₁ mode of CH₄ in supersonic expansions at low rotational temperatures (T_ROT ≥ 25). Our time-domain data are in good agreement with spectroscopic results. The corresponding frequency resolution of ≲3 ×10^-3 cm^-1 suggests that this method of Fourier-transform Raman spectroscopy may prove useful in high-resolution molecular spectroscopy.