Knowledge Bank

Evidence is presented for the existence of an indirect’’ process in the dissociative attachment of low-energy electrons to molecules. In the usual direct’’ process the incident electron first attaches itself to the molecule, and fragmentation follows. In the indirect’’ process the molecule is first fragmented by the electron which is subsequently captured by one of the neutral fragments. In setting up the scattering equations explicit consideration is given to the open channel involving the negative ion parent state. This open channel plays a special role in that it is the only channel in competition with the dissociative one which can act as a source (in addition to being a sink) of negative ion flux. The indirect’’ process is thus interpreted as a back-transfer of flux from a competing open channel into the dissociative one. Order-of-magnitude estimates for the $H2$ case at 10 eV indicate that the “indirect” process is of equal importance with the “direct” one. It is anticipated that the “indirect” process is expected to contribute to dissociative attachment in those molecules for which the negative ion potential surface crosses its parent potential surface in the vicinity of the initial states Franck-Condon region.