Core–shell effects in the ionization of doped helium nanodroplets

Benjamin Shepperson,Andrew M Ellis,Jun Liu,Shengfu Yang

doi:10.1039/c1cp20653b

Benjamin Shepperson, Andrew M Ellis + Show 2 more

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https://doi.org/10.1039/c1cp20653b

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Abstract

Core-shell particles with water clusters as the core and surrounded by an atomic or molecular shell have been synthesized for the first time by adding water and a co-dopant sequentially to helium nanodroplets. The co-dopants chosen for investigation were Ar, O(2), N(2), CO, CO(2), NO and C(6)D(6). These co-dopants have been used to investigate the effect of an outer shell on the ionization of the core material by charge transfer in helium nanodroplets. The specific aim was to determine how the identity of the shell material affects the fragmentation of water cluster ions, i.e. whether it helps to stabilize parent ion ((H(2)O)(n)(+)) formation or increases fragmentation (to form (H(2)O)(n)H(+)). N(2), O(2), CO(2) and C(6)D(6) all show a marked softening effect, which is consistent with the formation of a protective shell around the water cluster core. For CO and NO co-dopants, the response is complicated by secondary reactions which actually favour water cluster ion fragmentation for some water cluster sizes.

Highlights

Helium droplets provide an unusual, nanoscale environment for exploring phenomena at very low temperatures
The ionization of a single dopant species in helium droplets is quite well studied, the ionization and subsequent chemistry of mixed dopants has attracted much less attention. This is somewhat surprising given the potential that helium droplets offer for studying ion–molecule reactions at very low temperature
The first specific attempt to study ion–molecule reactions in helium droplets involving two different molecular constituents was carried out by Farnik and Toennies, who characterized the reactions N2+ + D2, CH4+ + D2, and CH3+ + D2.9 These reactions were initiated by primary electron impact ionization of the doubly doped droplets and both reaction end-products and some reaction intermediates were identified

Summary

Introduction

Helium droplets provide an unusual, nanoscale environment for exploring phenomena at very low temperatures. The only previous attempt to explore a comparable core–shell effect in helium nanodroplets was reported by Lewis et al.[12] In their study Lewis and co-workers coated a triphenylmethanol (TPM) monomer with neon atoms and saw a clear reduction in the fragmentation of the parent ion as more neon was added This was interpreted as a softening effect caused by the formation of a protective shell around the TPM and the number of atoms in the first shell (B20 Ne atoms) could be inferred from the response of the parent ion to the mean number of neon atoms picked up by the droplets. The role of core–shell structures is established through reversal of the order of addition of the two dopants

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