Resonance Raman depolization ratios for cyclopentadiene demonstrate the presence of two overlapping electronic transitions with perpendicular polarizations in the low energy absorption band: the 1B₂ and 2A₁ states

The absorption spectrum of 1,3-cyclopentadiene in the 260-220 nm region exhibits some vibrational structure superimposed on a broad continuum. The depolarization ratios (p) of resonantly enhanced Raman transitions of the three strongest totally symmetric modes of vibration all deviate significantly from the value of 1 3 corresponding to a single electronic transition. This directly and unambiguously demonstrates the presence of two overlapping electronic transitions with perpendicular transition moments. The variation of p with excitation wavelength is consistent with association of the weak structured absorption with the transition to the 2A₁ state, analogous to the 2A_g state of centrosymmetric polyenes, and the much stronger underlying continuous absorption with the transition to the 1B₂ state, analogous to the corresponding 1B_u state of butadiene.