On the mechanism of the energy redistribution in the A 2Ag(2Σg+) state of the HCCH+ ion

Using ab initio calculations large parts of the six-dimensional potential energy functions for all electronic components of the X 2Πu A 2Σg+ and A′ 2Πg states of the acetylene radical cation have been mapped. It has been found that the lowest doubles state of HCCH+ is not the trans-bent A 2Ag(2Σg+) state, as expected, but the cis-bent A′ 2A2(2Πg) shake-up state, which is not Franck-Condon active in the ionization process because of its long RCC distance. The A state has only one trans-bent (2Ag) minimum, the Ā′ state has three minima (two cis-bent (2A2 and 2B2) and one trans-bent (2Bg)) on the adiabatic potential energy functions. The three X 2Πu, A 2Σg+ and A′ 2Πg electronic state are vibronically coupled in the conical intersection regions lying close to the equilibrium structure of the A 2Ag(2Σg+) state by the ν3, ν4 and ν5 modes, which explains why all experimentally observed vibrational bending modes have lifetimes in the range of only 150 to 2 ps.