Theoretical study of the photoelectron spectrum of allene

Multi-mode dynamical E⊗B Jahn-Teller coupling effects in the X ²E and A ²E states of the allene cation are systematically investigated. Ab initio electronic potential energies of the X ²E, A ²E and B ²B₂ states of C₃H⁺₄ have been obtained as a function of normal coordinates of A₁, B₁ and B₂ symmetry. The potential-energy functions reveal the Jahn-Teller activity (B₁, B₂ symmetry) and Condon activity (A₁ symmetry) of the normal modes. It is shown that the recently published (Yang et al. Chem. Phys. Letters 171 (1990) 9) high-resolution photoelectron spectrum of the X ²E band of allene can quantitatively be interpreted in terms of Jahn-Teller coupling involving the mode ν₄ (torsion) and the antisymmetric CC stretching mode ν₆. The vibronic fine structure of the A ²E band reported by Yang et al. is shown to arise primarily from the excitation of the Jahn-Teller active antisymmetric HCH bending mode ν₇ and the totally symmetric HCH bending mode ν₂.