Energy spectra of electrons detached in collisions of Cl − and Br − with atomic hydrogen and deuterium have been measured for laboratory frame ion energies between 0.2 and 8.0 eV. Their shapes agree very well with the predictions of nonlocal resonance theory. Both types of structure predicted by the theory are observed. They are the 'v steps', at ro-vibrational thresholds, and the 'S steps', which are a consequence of interchannel coupling,which raises the cross section when a higher

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... channel closes. They exhibit the behaviour predicted by theory both when the collision energy is varied and upon isotope substitution. The 'v steps' move to higher electron energies with higher collision energy and when hydrogen is substituted by deuterium, reflecting the higher maximum energy available to the electron. The positions of the S steps do not depend on collision energy, and are essentially equal to differences of vibrational energies of the product molecules HCl, DCl, HBr and DBr. The relative cross sections for formation of low vibrational levels (i.e., emission of fast electrons) are smaller in the deuterated compounds, reflecting the slower motion of D compared to H and consequently preferred detachment at high internuclear separations. (Some figures in this article are in colour only in the electronic version) Introduction Associative electron detachment is an important plasma process. Here we are concerned with the collisions of the halogen anions Cl − and Br − with atomic hydrogen, where H designates both the 1 H and 2 H isotopes, X is the halogen, E rel is the centre-of-mass (CMS) collision energy, v and J the vibrational and rotational quanta of the product molecule and ε the energy of the detached electron. † Dedicated to Professor Hartmut Hotop, on the occasion of his 60th birthday.