THRESHOLD EXCITATION IN H-2 AND D2 BY ELECTRON-IMPACT AND PREDISSOCIATION OF TRIPLET-STATES MEASURED BY ELECTRON-PHOTON COINCIDENCES

An electron scattering apparatus is described which allows the simultaneous detection of scattered electrons (energy resolution Delta E=20 meV) and emitted photons (visible, near UV, VUV). Optical and energy-loss excitation functions for various processes in H2 and D2 are measured covering the energy-loss range 11-16 eV. Strong and very sharp threshold peaks are observed in the excitation functions of many triplet states for which an interpretation based on the final-state-interaction model is proposed. Coincidence measurements between threshold electrons and VUV photons (1120-1300 AA) are performed giving direct evidence for predissociation into H(1s)+H(2p) of the triplet states above the dissociation limit of 14.7 eV. Using selection rules and known potential curves most of the predissociating triplet states are identified.