摘要：

Ultraviolet resonance Raman (UVRR) spectra of solutions of phenol, tyrosine, and tryptophan at pH 7 and 12 show Raman bands that are assigned to the photoionization products phenoxy, tyrosyl, and tryptophanyl radicals, respectively. These photoinduced transients are only detected at high laser power densities (high average power, tightly focused, pulsed-laser excitation) when exciting within the π → π* transitions of the precursor compounds. Each of the radicals show UVRR spectra distinct from their precursors. A pump-probe experiment shows that the phenolate transient (phenoxy radical) produced by the incident laser beam at pH 12 has a lifetime significantly longer than 10 ns. The dependence of the Raman scattering intensity upon incident laser-power density is consistent with a simple model in which the free radical is produced by a monophotonic photoionization process. Saturation effects are also evident at high power densities. Photochemistry, absorption band optical saturation, and nonlinear optical processes are proven and potential impediments for measurements of Raman spectra and excitation profiles with pulsed laser excitation. On the other hand, the facile production and detection of free radical species with UVRR suggest that this technique will be useful in the investigation of radical structure and chemistry.

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