REACTIONS OF SIZE-SELECTED POSITIVELY CHARGED NICKEL CLUSTERS WITH CARBON-MONOXIDE IN MOLECULAR-BEAMS

Reactions of small thermalized positively charged nickel clusters with carbon monoxide were studied in a molecular beam experiment. The nick el clusters were produced in a high intensity cluster ion source and t hermalized in a large helium-filled quadrupole ion guide. The clusters were size selected by a quadrupole mass spectrometer. The mass-and ch arge-selected nickel clusters then passed through a Linear quadrupole drift tube filled with a mixture of helium buffer gas and carbon monox ide. The reaction products were then analyzed by a quadrupole mass-spe ctrometer, Using this technique, saturation limits for Ni-n(+) cluster s with n=4-31 were measured and the competitive reaction channels were identified. Under certain experimental conditions carbide formation w as observed in the case of the nickel tetramer, pentamer, and hexamer. The structure of the nickel carbonyl clusters is discussed within the framework of the polyhedral skeletal electron pair theory. The cluste r growth may be explained by a pentagonal sequence of structures for n =4-7, capping of the pentagonal bipyramid to buildup an icosahedron at Ni-13(+),and further capping of this icosahedron to form a double ico sahedron at Ni-19(+). (C) 1997 American Institute of Physics.