Complex formation in proton-hydrogen collisions. II. Isotope effects

Abstract

Classical trajectory calculations have been performed on the DIM potential energy surface of H +H 2 for collision energies between 20 meV and 2 eV. Complex formation cross sections have been determined for many combinations of projectile and target masses, showing that capture behind the centrifugal barrier is a necessary but by no means sufficient condition for the formation of a long-lived complex. Its probability depends on energy and masses in a way which suggests that the initial energy loss of the projectile on its approach to, and first encounter with the target plays a crucial role in the “trapping” process. H/D isotope effects exceed 20% at energies above 1 20 of the potential well depth, and reach more than 200% at higher energies. At collision energies below 1 100 of the well depth the isotope effect disappears, and the complex formation cross section becomes equal to the capture cross section if the target has no (classical) internal excitation. If, on the contrary, the target is internally excited this equality is invalidated, and an isotope effect of a few percent due to different zero point energies remains. Microreversibility arguments show that these effects should have a perceptible influence on the results of a “dynamically biased” phase space theory.