Raman spectra in the electronic excited state: CARS spectra of lasing rhodamine 6G

By choosing ω₂ (4880 Å) preresonant with the S₁S₀ transition of rhodamine 6G, and ω₁ (4537 Å) resonant with the S₂S₁ absorption, the resonant CARS spectra for lasing rhodamine 6G in methanol, ethanol and acetonitrile have been observed in the 1500–1650 cm^?1 range. Strong positive bands, attributed to CARS from the S₁ state, replace the negative ground state preresonant bands as the S₀ state saturates at high laser beam power densities. A water solution behaves quite differently, possibly as a result of strong association of water with rhodamine in the excited state.