Abstract
The effects of interactions between He(-) and clusters of fullerenes in helium nanodroplets are described. Electron transfer from He(-) to (C60)n and (C70)n clusters results in the formation of the corresponding fullerene cluster dianions. This unusual double electron transfer appears to be concerted and is most likely guided by electron correlation between the two very weakly bound outer electrons in He(-). We suggest a mechanism which involves long range electron transfer followed by the conversion of He(+)into He2 (+), where formation of the He-He bond in He2 (+) releases sufficient kinetic energy for the cation and the dianion to escape their Coulombic attraction. By analogy with the corresponding dications, the observation of a threshold size of n ≥ 5 for formation of both (C60)n (2-) and (C70)n (2-) is attributed to Coulomb explosion rather than an energetic constraint. We also find that smaller dianions can be observed if water is added as a co-dopant. Other aspects of He(-) chemistry that are explored include its role in the formation of multiply charged fullerene cluster cations and the sensitivity of cluster dianion formation on the incident electron energy.
Highlights
The helium monoanion, He−, was first detected in experiments in 1939.1 He− is an unusual ion because atomic helium in its 1s2 1S ground state has a negative electron affinity, on account of its compact closed-shell electronic structure
We suggest a mechanism which involves long range electron transfer followed by the conversion of He+ into He2+, where formation of the He–He bond in He2+ releases sufficient kinetic energy for the cation and the dianion to escape their Coulombic attraction
He− is metastable with respect to autodetachment since it is embedded in a continuum of states formed from the ground state of neutral atomic helium and a free electron
Summary
The helium monoanion, He−, was first detected in experiments in 1939.1 He− is an unusual ion because atomic helium in its 1s2 1S ground state has a negative electron affinity, on account of its compact closed-shell electronic structure. With the ability to produce He− in close proximity with a dopant molecule or cluster inside a liquid helium droplet, it becomes possible to explore the chemistry of this unusual anion This can be seen rather clearly if we assume that the He− bubble can travel at the Landau velocity for superfluid 4He A remarkable finding was that in addition to monoanions produced by direct electron attachment, dianions were formed This was the first report of dianions produced inside helium droplets and the dependence of the dianion signal on the electron energy was found to possess the same resonance behavior as for He− formation, showing that the dianions were produced by twoelectron transfer from He−. Any ions produced were extracted into a high resolution and high repetition rate reflectron time-of-flight mass spectrometer (Tofwerk)
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