Nonadiabatic Effects in the High-Energy Scattering of Normal Helium Atoms

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Abstract
There is a large discrepancy (∼9 eV) between the calculated adiabatic electronic energy of the ground state (X 1Σg+) of the system He2 and the effective scattering potential deduced from experiments with high‐energy (2 keV) atomic beams, at small interatomic distances (R=0.53 Å). It is shown that at least a significant part (25%) of this discrepancy arises from nonadiabatic effects of high relative angular velocity of the atoms in the collisions typical of the experiment. A perturbation‐theory treatment is included to give an estimate of the over‐all magnitude of the effect; but a variational calculation is employed with the nonadiabatic Hamiltonian to produce the minimum estimates of nonadiabatic energy shifts to which we refer above. The theory correctly predicts the observed approximate agreement between adiabatic energies and the experimental potentials at larger distances (R=1.06 Å). A method of extending the variational calculation to include continuum contributions is described; it employs a pseudo‐Hamiltonian to simplify the calculation. Arguments connected with this formulation are advanced to suggest that when continuum contributions are taken into account it may be possible to account for all or most of the observed discrepancy.