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Atomically resolved phase transition of fullerene cations solvated in helium droplets

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ABSTRACT

Helium has a unique phase diagram and below 25 bar it does not form a solid even at the lowest temperatures. Electrostriction leads to the formation of a solid layer of helium around charged impurities at much lower pressures in liquid and superfluid helium. These so-called ‘Atkins snowballs' have been investigated for several simple ions. Here we form HenC60+ complexes with n exceeding 100 via electron ionization of helium nanodroplets doped with C60. Photofragmentation of these complexes is measured by merging a tunable narrow-bandwidth laser beam with the ions. A switch from red- to blueshift of the absorption frequency of HenC60+ on addition of He atoms at n=32 is associated with a phase transition in the attached helium layer from solid to partly liquid (melting of the Atkins snowball). Elaborate molecular dynamics simulations using a realistic force field and including quantum effects support this interpretation.

No MeSH data available.


Mass spectrum.Comparison of a mass spectrum where HenC60+ is transparent (962.21 nm) with mass spectra for three different laser wavelengths at the electronic transition of bare C60+ near 964 nm. Different parts of the mass spectrum are depleted depending on the laser wavelength. The pronounced intensity drops at n=32 and n=60 can be assigned to shell closures of the He adsorbate layer.
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f1: Mass spectrum.Comparison of a mass spectrum where HenC60+ is transparent (962.21 nm) with mass spectra for three different laser wavelengths at the electronic transition of bare C60+ near 964 nm. Different parts of the mass spectrum are depleted depending on the laser wavelength. The pronounced intensity drops at n=32 and n=60 can be assigned to shell closures of the He adsorbate layer.

Mentions: Figure 1 shows one mass spectrum off-resonant with the HenC60+ ions at 962.21 nm compared with three mass spectra for laser wavelengths between 964.55 and 965.65 nm, close to the bare C60+ electronic excitation around 964 nm. The red circles indicate the peak of Hen−12C60+ determined via a fitting routine26. Supplementary Fig. 1 shows the detailed result of the analysis of the mass spectrum for the ion He17C60+. A movie showing the changes in a section of the mass spectrum throughout the scanned wavelengths is available as Supplementary Movie 1. Different parts of the mass spectrum are depleted, depending on the laser wavelength. This depletion can be regarded as hole burning of the cluster ion signals by the laser. For 964.55 nm, clusters around n=15 are depleted. At longer wavelengths around 965 nm, two regions of the mass spectrum are diminished, which correspond to ∼20 and 55 physisorbed helium atoms. For 965.65 nm in particular, low-ion signals are observed at n=30 and 34.


Atomically resolved phase transition of fullerene cations solvated in helium droplets
Mass spectrum.Comparison of a mass spectrum where HenC60+ is transparent (962.21 nm) with mass spectra for three different laser wavelengths at the electronic transition of bare C60+ near 964 nm. Different parts of the mass spectrum are depleted depending on the laser wavelength. The pronounced intensity drops at n=32 and n=60 can be assigned to shell closures of the He adsorbate layer.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5121423&req=5

f1: Mass spectrum.Comparison of a mass spectrum where HenC60+ is transparent (962.21 nm) with mass spectra for three different laser wavelengths at the electronic transition of bare C60+ near 964 nm. Different parts of the mass spectrum are depleted depending on the laser wavelength. The pronounced intensity drops at n=32 and n=60 can be assigned to shell closures of the He adsorbate layer.
Mentions: Figure 1 shows one mass spectrum off-resonant with the HenC60+ ions at 962.21 nm compared with three mass spectra for laser wavelengths between 964.55 and 965.65 nm, close to the bare C60+ electronic excitation around 964 nm. The red circles indicate the peak of Hen−12C60+ determined via a fitting routine26. Supplementary Fig. 1 shows the detailed result of the analysis of the mass spectrum for the ion He17C60+. A movie showing the changes in a section of the mass spectrum throughout the scanned wavelengths is available as Supplementary Movie 1. Different parts of the mass spectrum are depleted, depending on the laser wavelength. This depletion can be regarded as hole burning of the cluster ion signals by the laser. For 964.55 nm, clusters around n=15 are depleted. At longer wavelengths around 965 nm, two regions of the mass spectrum are diminished, which correspond to ∼20 and 55 physisorbed helium atoms. For 965.65 nm in particular, low-ion signals are observed at n=30 and 34.

View Article: PubMed Central - PubMed

ABSTRACT

Helium has a unique phase diagram and below 25 bar it does not form a solid even at the lowest temperatures. Electrostriction leads to the formation of a solid layer of helium around charged impurities at much lower pressures in liquid and superfluid helium. These so-called ‘Atkins snowballs' have been investigated for several simple ions. Here we form HenC60+ complexes with n exceeding 100 via electron ionization of helium nanodroplets doped with C60. Photofragmentation of these complexes is measured by merging a tunable narrow-bandwidth laser beam with the ions. A switch from red- to blueshift of the absorption frequency of HenC60+ on addition of He atoms at n=32 is associated with a phase transition in the attached helium layer from solid to partly liquid (melting of the Atkins snowball). Elaborate molecular dynamics simulations using a realistic force field and including quantum effects support this interpretation.

No MeSH data available.