Multidimensional least-squares resolution of excited-sate Raman spectra

Abstract

A multidimensional least-squares analysis of transient Raman data acquired during a single laser pulse is used to resolve the spectra of excited-state species from the spectra of ground-state and solvent species. The kinetics of optical excitation produce a higher order dependence on laser intensity for Raman spectra of the excited states. In the absence of saturation of the ground- or excited-state populations, the excited-state spectrum increases quadratically with laser intensity. In cases where excited states cause significant absorption of the excitation source, the scattering from a solvent band may be used as an in situ intensity reference to correct the measured laser energies for the analysis. Saturation of excited-state populations causes deviations from the sample quadratic dependence of Raman intensities on laser energy. Least-squares regression analysis with a model describing the saturation kinetics allows the spectrum of the excited state to be resolved. The method is introduced in this work and applied to the detection of the Raman spectrum of benzophenone excited triplet states. A kinetic model describing the loss of triplet states through dissociation of upper triplet states of benzophenone produced during the laser pulse explains the results observed with 316 nm excitation