Excited state geometry of uracil from the resonant Raman overtone spectrum using a Kramers–Kronig technique

Abstract

The Kramers–Kronig transform technique of Blazej and Peticolas is extended to permit the calculation of overtone and combination bands in a resonant Raman spectrum relative to the fundamentals. The method is applied to the ultraviolet (257 nm) resonant Raman spectrum of uracil which exhibits five fundamental bands and numerous overtone and combination bands. The method permits the determination of Δej the shift of the excited state potential energy minimum along the jth normal mode for each of the five resonant Raman active modes. The Δej values obtained are in reasonable agreement with those obtained earlier from the absorption spectrum and excitation profiles of the fundamentals. However, no internal standard for the experimental determination of Raman intensities is required nor is it necessary to assume any value for Γ the vibronic linewidth. The shape of the absorption band and relative intensities of the harmonics to the fundamental are sufficient to obtain the value of each Δej.