Sensitization of photoeffects by hyper-Rayleigh scattering (HRS)

We have observed fluorescence from Coumarin334 (C334) in a variety of solvents using a fundamental wavelength (840 nm) twice that of radiation actinic for excitation of this fluorescence, and we attribute this fluorescence to second harmonic, hyper-Rayleigh light scattering (HRS). Fluorescence intensity is compared to that obtained by direct excitation of fluorescence under the same conditions to provide an estimate of the efficiency of the HRS process; Φ=ca. 10−6, strongly solvent dependent. Photochemical actinometry indicates a fundamental optical power of 1 mW cm−2 under our conditions. By mapping the action spectrum for second harmonic excitation of fluorescence, we exclude the possibility suggested by the solvent dependence of Φ, namely that the principal pathway for excitation of C334 involves second harmonic scattering by solvent, followed by absorption of actinic light by the solute. Instead, we find that the action spectrum does not correlate with the absorption spectrum of C334. Therefore, we hypothesize that second harmonic scattering is coupled to an electronic transition in the C334 chromophore, which probably involves a higher excited state than that responsible for emission.