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 potent ial energy functions for all electronic components of the (X) over til de (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 be en 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 c is-bent (A) over tilde' (2)A(2)((2) Pi(g)) shake-up state, which is no t 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) Sig ma(g)(+) and (A) over tilde (2) Pi(g) electronic states are vibronical ly coupled in the conical intersection regions lying close to the equi librium structure of the (A) over tilde (2) Pi(g) ((2) Sigma(g)(+)) St ate by the upsilon(3), upsilon(4) and upsilon(5) modes, which explains why all experimentally observed vibrational bending modes have lifeti mes in the range of only 150 to 2 ps.