Resolution of transient excited-state Raman scattering from ground-state and solvent bands is a challenging spectroscopic measurement since excited-state spectral features are often of low intensity, overlapping
the dominant ground-state and solvent bands. The Raman spectra of these intermediates can be resolved, however, by acquiring time-resolved data and using multidimensional data analysis methods. In the absence
of a physical model describing the kinetic behavior of a reaction, resolution of the pure-component spectra from these data can be accomplished using self-modeling curve resolution, a factor analysis technique
that relies on the correlation in the data along a changing composition dimension to resolve the component spectra. A two-laser UV pump-probe resonance-enhanced Raman instrument was utilized to monitor
the kinetics of amine quenching of excited-triplet states of benzophenone. The formation and decay of transient intermediates were monitored over time, from 15 ns to 100 μs. Factor analysis of the time-resolved
spectral data identified three significant components in the data. The time-resolved intensities at each Raman wavenumber shift were projected onto the three significant eigenvectors, and least-squares
criteria were developed to find the common plane in the space of the eigenvectors that includes the observed data. Within that plane, the three pure-component spectra were resolved using geometric criteria
of convex hull analysis. The resolved spectra were found to arise from benzophenone excited-triplet states, diphenylketyl radicals, and the solvent and ground-state benzophenone.
Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112-0850
Publication date: April 1, 2003
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