Double Resonance Excitation of Fluorescence by Stimulated Raman Scattering

Fluorescence spectroscopy is usually limited in its specificity by broad absorption and fluorescence profile. Recent papers have suggested that double resonance methods of fluorescence excitation could provide higher specificity. A vibrational excitation is first created by either direct infrared absorption or stimulated Raman scattering and is followed by excitation to an excited electronic level which fluoresces. Since the positions of vibrational levels are much less strongly affected by environmental changes than electronic levels, the vibrational features are sharp and characteristic of the molecule. It was shown that the rapid vibrational relaxation of molecules in condensed phases can be overcome by efficient pumping with stimulated Raman or infrared excitation. In this paper, we report the results of an experiment that tests the feasibility of double resonance excitation of fluorescence by stimulated Raman scattering. It is found that the method is limited by a strong, interfering two-photon process.