Conical octopole ion guide: Design, focusing, and its application to the deposition of low energetic clusters

A design of a radio-frequency (rf) octopole ion guide with truncated conical rods arranged in a conical geometry is presented. The performance is tested in a cluster deposition apparatus used for the soft-landing of size-selected clusters on well-characterized substrates used as a model system in heterogeneous catalysis in ultrahigh vacuum. This device allows us to focus 500pA of a mass-selected Ni20+ cluster ion beam from 9mm down to a spot size of 2mm in diameter. The transmittance is 70%±5% at a rf voltage of 420Vpp applied over an amateur radio transceiver with an interposed homemade amplifier-transformer circuit. An increase of the cluster density by a factor of 15 has been achieved. Three ion trajectories are simulated by using SIMION6, which are relevant for this focusing device: transmitted, reflected, and absorbed. The observed effects in the simulations can be successfully explained by the adiabatic approximation. The focusing behavior of the conical octopole lens is demonstrated by experiment a...

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