The òE/B̃²B₂photoelectron bands of allene beyond the linear coupling scheme: an ab initio dynamical study including all fifteen vibrational modes†

Abstract

In an earlier publication [J. Chem. Phys. 1999, 111, 10452] we theoretically investigated the photoelectron spectrum of allene (C₃H₄⁺) pertinent to the òE/B̃²B₂ interacting electronic manifold of its radical cation (C₃H₄⁺). Employing a linear vibronic coupling scheme it was demonstrated that in addition to the E⊗B Jahn−Teller activity within the 2e electronic manifold, there is a strong (E⊗B) + E pseudo-Jahn−Teller interaction with the B̃²B₂ electronic state, which causes the diffuse structures observed at high energies. Here, the same photoelectron spectrum is reinvestigated including all fifteen vibrational degrees of freedom of the system and a higher order coupling scheme. The coupling parameters of the Hamiltonian are calculated by ab initio methods. The photoelectron band is calculated by the wave packet propagation method within the multiconfiguration time-dependent Hartree (MCTDH) scheme and compared with the experimental results of Baltzer et al. [Chem. Phys. 1995, 196, 551]. The progressions at low energies are identified unambiguously by calculating the "stick" vibronic spectrum within the òE electronic manifold, considering five relevant vibrational modes, and the effect of the higher order couplings is clearly demonstrated. The calculations show that it is necessary to re-assign the progressions in the low-energy region of the spectrum to the vibrational mode v₃, which is of C=C stretching and H—C—H bending character and the combination of symmetric (v₂) and antisymmetric (v₇) H—C—H bending vibrational modes. In addition, we report on the time-dependent nuclear dynamics by snapshots of the time-evolved wave packet and by the diabatic electronic populations.