Monitoring the Formation and Decay of Transient Photosensitized Intermediates Using Pump-Probe UV Resonance Raman Spectroscopy. II: Kinetic Modeling and Multidimensional Least-Squares Analysis

Abstract

Analysis of transient excited-state Raman spectra is a challenging spectroscopic measurement since transient spectral features are often overlapped with dominant ground-state and solvent bands. In the previous manuscript, resolution of component Raman spectra from the time-resolved amine quenching of excited-triplet benzophenone was accomplished using self-modeling curve resolution, a model-free factor analysis technique that relies on correlation in the data along a changing composition dimension. The results are consistent with the production of diphenylketyl radicals by H-atom abstraction from the amine and subsequent free-radical decay by recombination reactions. A kinetic model for this chemistry is developed in the present work, based on the observed Raman scattering data and the structures of product species confirmed by mass spectral analysis. The model is applied to the analysis of the time-dependent Raman scattering data using multidimensional least-squares methods, and it yielded well-resolved spectra of benzophenone excited-triplet states, diphenyl ketyl radical, and the solvent and ground-state precursors. The best-fit kinetic parameters agree well with the time-dependent triplet-state and ketyl-radical concentration profiles.