Potential energy surfaces for the electronic dissociative recombination of HCNH+:: astrophysical implications on the HCN/HNC abundance ratio

The two-dimensional potential energy surfaces for the electronic dissociative recombination reaction HCN+ + e(-) --> HCNH* --> HCN + H/HNC + H have been computed using large-scale CI calculations. The lowest dissociative states of (2) Sigma(+) symmetry, resulting in the N-H or C-H bound scission yielding HCN or HNC, respectively are shown to cross the ionic HCNH+ state at its minimum geometry, indicating that the direct mechanism of electronic dissociative recombination of HCNH+ is an efficient process which should lead to an equal formation of HCN and HNC. The same conclusion can be drawn for the indirect mechanism: the crossing between the dissociative states and the lowest Rydberg state of (2) Sigma symmetry occurs at the same energy for both NH and CH dissociations. The present calculations support the astophysical hypothesis of an equal formation of HCN and HNC in the HCNH+ + e(-) dissociative recombination process. (C) 1998 Elsevier Science B.V. All rights reserved.