FTMS STUDIES OF SPUTTERED METAL CLUSTER IONS .4. SIZE-SELECTIVE EFFECTS IN THE CHEMISTRY OF FEN+ WITH NH3 AND PDN+ WITH D2 OR C2H4

Fe(n)+ and Pd(n)+ clusters up to n = 19 and n = 25, respectively, are produced in an external ion source by sputtering of the respective metal foils with Xe+ primary ions at 20 keV. They are transferred to the ICR cell of a home-built Fourier transform mass spectrometer, where they are thermalized to nearly room temperature and stored for several tens of seconds. During this time, their reactions with a gas leaked in at low level are studied. Thus in the presence of ammonia, most Fe(n)+ clusters react by simply adsorbing intact NH3 molecules. Only Fe4+ ions show dehydrogenation/adsorption to Fe4(NH)m+ intermediates (m = 1, 2) that in a complex scheme go on adsorbing complete NH3 units. To clarify the reaction scheme, one has to isolate each species in the ion cell, which often requires the ejection of ions very close in mass. This led to the development of a special isolation technique that avoids the use of isotopically pure metal samples. Pd(n)+ cluster ions (n = 2 ...9) dehydrogenate C2H4 in general to yield Pd(n)(C2H2)+, yet Pd6+ appear totally unreactive. Towards D2, Pd7+ ions seem inert, whereas Pd8+ adsorb up to two molecules.