FTMS studies of sputtered metal cluster ions (IV): size-selective effects in the chemistry of Fe ⁺ _n with NH₃ and Pd ⁺ _n with D₂ or C₂H₄

Abstract

Fe ⁺ _n and Pd ⁺ _n clusters up ton=19 andn=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 Fe ⁺₄ 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 Pd ⁺₆ appear totally unreactive. Towards D₂, Pd ⁺₇ ions seem inert, whereas Pd ⁺₈ adsorb up to two molecules.