The negatively-charged ion probably forms in steps. First, a molecule of C2H attaches to a molecule of C6H2, producing a molecule of C8H2 and a hydrogen atom. Radiation (squiggly line) breaks one hydrogen atom from the C8H2, leaving C8H and a hydrogen atom. Finally, an electron attaches itself to the C8H molecule, freeing a burst of radiation (overall glow seen around the molecule) and leaving the negatively-charged ion C8H-.
Bill Saxton, NRAO/AUI/NSF