CHEMICAL REACTION OF SILICON CLUSTER IONS WITH ETHYLENE BY USING FT-ICR MASS SPECTROMETER Masamichi Kohno1 and Shigeo Maruyama1,2 1. Engineering Research Institute, The University of Tokyo 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan 2. Department of Mechanical Engineering, The University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Mass-selected silicon cluster ions were levitated in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer and monitored during chemisorption reaction with ethylene (C2H4). Through measurements of "time-dependency" of this reaction process, drastic change of the reactivity depending on the number of absorbed ethylene was observed, in addition to the strong dependency to silicon cluster size.