ON THE MECHANISM OF THE ENERGY REDISTRIBUTION IN THE (A)OVER-TILDE(2)A(G)((2)SIGMA(+)(G)) 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) over tilde (2) Pi(u), (A) over tilde' (2) Sigma(g)(+) and (A) over tilde' 2 Pi(g) states of the acetylene radical cation have been mapped. It has been found that the lowest doublet state of HCCH+ is not the trans-bent (A) over tilde (2)A(g) ((2) Sigma(g)(+)) state, as expected, but the cis-bent (A) over tilde' (2)A(2)((2) Pi(g)) shake-up state, which is not Franck-Condon active in the ionization process because of its long R(CC) distance. The A state has only one trans-bent ((2)A(g)) minimum, the (A) over tilde' state has three minima (two cis-bent ((2)A(2) and B-2(2)) and one trans-bent (B-2(g))) on the adiabatic potential energy functions. The three (X) over tilde (2) Pi(g), (A) over tilde (2) Sigma(g)(+) and (A) over tilde (2) Pi(g) electronic states are vibronically coupled in the conical intersection regions lying close to the equilibrium structure of the (A) over tilde (2) Pi(g) ((2) Sigma(g)(+)) State by the upsilon(3), upsilon(4) and upsilon(5) modes, which explains why all experimentally observed vibrational bending modes have lifetimes in the range of only 150 to 2 ps.