Rare atoms, molecules and radicals in the coma of P/Halley

We have searched for rare molecules and radicals in the coma of P/Halley us ing the ion data obtained by IMS-Giotto. Whereas our established methods we re used in the ionosphere, a new model was developed for the interpretation of the ion data in the outer coma. Ne/H2O < 1.5 x 10(-3) was determined in the coma of the comet. Upper limits for the production of Na were derived from the very low abundance of Na+. Methyl cyanide and (probably) ethyl cya nide were identified with abundances of CH3CN/H2O = (1.4 +/- .6) x 10(-3) a nd C2H5CN/H2O = (2.8 +/- 1.6) x 10(-4). These results and upper limits for other N-bearing species confirm that nitrogen is depleted in the Halley mat erial. C4H was identified and a point source strength of C4H/H2O = (2.3 +/- .8) x 10(-3) was derived. Our upper limit for C3H is lower than the abundan ce of C4H. This is in agreement with the enhanced abundances of CnH species with even numbers of C-atoms found in interstellar molecular clouds, sugge sting that the C4H in Halley was synthesized under molecular cloud conditio ns. Thus, C4H and other organics with unpaired electrons may turn out to be indicators for a molecular cloud origin of cometary constituents.