Abstract

The characterization of the ground and excited electronic states of the allyl radical is important to the understanding of free radical chemistry. Resonance Raman spectroscopy can provide detailed information on the allyl radical excited state dynamics through the intensities of the ground state normal modes. Previous resonance Raman studies have examined the promotion of the valence a₂ electron to the valence b₁ orbital, the ²A₂ → ²B₁ transition. The intensities of the Raman spectra at 224 nm indicate initial excited state dynamics consistent with a disrotary photoisomerization of the allyl radical to form the cyclopropyl radical. Resonance Raman spectroscopy has been utilized to examine the nature of the weakly allowed transitions between 235 nm and 250 nm. Rydberg states have been predicted to lie in this energy range. Analysis of the Raman spectra revealed enhancement in the fundamental, overtones, and combinations of the non-totally symmetric modes v₉ and v₁₂. Intensity in the fundamentals of these non-totally symmetric modes is evidence for B-term Raman scattering and hence vibronic coupling. The sharp resonances in the enhancement of the Raman spectra simplify the assignment of the overtone and combination tones ground state vibrational frequencies. Schemes for the observed vibronic coupling and ground state anharmonic couplings will be presented.

© 1992 Optical Society of America