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Abstract:
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 500 pA of a mass-selected Ni-20(+)
cluster ion beam from 9 mm down to a spot size of 2 mm in diameter. The
transmittance is 70%+/- 5% at a rf voltage of 420 V-pp 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 and simulations to be a very useful
technique for increasing molecule or cluster densities on a substrate
and thus reducing deposition time. (c) 2006 American Institute of
Physics.
