Non-adiabatic coupling as a frictional force in (He, H, H)<sup>+</sup>dynamics and the formation of HeH<sub>2</sub><sup>+</sup>
Satyam Ravi, Soumya Mukherjee, Bijit Mukherjee, Satrajit Adhikari, N. Sathyamurthy, Michael Baer
Abstract
It is shown that the Born–Oppenheimer–Huang non-adiabatic coupling terms (NACTs) in a molecular Hamiltonian can be considered as equivalent to a friction force in classical mechanics and consequently responsible for the formation of triatomic species like HeH2+. The classical equation of motion is solved accordingly for a test case of a helium atom approaching an H2+ molecule at its potential minimum along the collinear axis and lines parallel to it and also for the C2v geometry. It is shown that the singular NACTs between the first two excited states slow down the motion of the three quasi-ions (He, H, H)+ while approaching each other and result in substantial loss of kinetic energy in the collision process. These results have profound implications for the formation of di- and triatomic species in the early universe, during the chemical synthesis epoch following the nucleosynthesis that followed the Big Bang.