Ab initio potential energy surfaces for excited electronic states of the molecular ion HCN+

Potential energy surfaces for the linear (X) over tilde (2)Pi, 2 (2)Pi, (A) over tilde (2)Sigma(+), (B) over tilde (2)Sigma(+), 3 (2)Sigma(+), 1 (4)Pi and 1(4)Sigma(+) states and for the bent B(2)Sigma(+)/ 3(2)A' state of HCN+ have been calculated by a multi-reference configuration interaction (MRCI) method. The persistence of vibrational structure in the photoelectron spectrum of the (B) over tilde state above the experimental dissociation asymptote is interpreted in terms of a local maximum on the potential energy surface with respect to stretching the CH bond. The global minimum for this state has a bent geometry with a bond angle of 129degrees. This is discussed in terms of an avoided intersection with the A' component of the 2 (2)Pi state for bent geometries. The 3 (2)Sigma(+) surface is quasi-bound and it is suggested that a satellite state at 22.5 eV in the photoelectron spectrum of HCN is the 3(2)Sigma(+) state. The predissociation of the (B) over tilde (2)Sigma(+) state is discussed and it is suggested that this may occur via non-adiabatic transitions to the 1 (4)Pi/1 (4)A' surface.