Effects of Surface Crossing in Chemical Reactions: The H3+ System

Approximate potential-energy surfaces for the two lowest singlet states of H3+ are calculated using the diatomics-in-molecules approach. The nonadiabatic terms which couple these surfaces can be directly computed in this approximation. From the magnitudes of these coupling terms it is apparent that, for excitation energies below about 10 eV, nonadiabatic transitions must be confined almost entirely to a region localized at the avoided crossing of the two surfaces. This fact suggests the following simplified picture of the dynamics of the H++H2 reaction: As the H+ and H2 approach, they remain on the lower potential surface (there is no initial electron jump). They continue to follow adiabatically the lower surface in the close-collision region, so the probability of a nonadiabatic transition does not appear to be related to the lifetime of the collision complex. It is while the products are receding that electronic transitions become important. Consequences of this model on the threshold for formation of H2+ and on the partition of vibrational energy in the products are discussed, and a comparison with recent experiments of Krenos and Wolfgang is included.