Associative ionization in the collision of H(1s) with H(3s) and D(1s) with D(3s)

The absolute cross section for associative ionization in H(1s) + H(3s) and D(1s) + D(3s) collisions has been measured, in the energy range 0.006–3.6 eV, in a merged beam apparatus. The 3s state is populated by exciting metastable atoms, with CW laser radiation, in a static field. For both isotopes, the cross section exhibits an E⁻¹ behaviour at low energy, and a faster decrease above the ionization threshold of the 3s state. In the intermediate energy region, isotope-dependent oscillations of the cross section are observed, which we attribute to interferences between several reaction pathways.