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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.


Related in: MedlinePlus

Absorption wavelength as a function of He atoms attached.Centre positions for the absorption spectra of HenC60+ around 958 nm (blue open circle, left y axis) and 964 nm (red open triangle, right axis) plotted as a function of n, the number of helium ad-atoms on the fullerene ion surface. The error bars indicate s.e.m. of the centre position of the Lorentzian profiles fitted to the ion signal depletion (see Fig. 2). The absorption wavelengths (corrected to vacuum) that were obtained for zero to a few helium atoms by Maier and colleagues9 are indicated by the bold symbols. The red arrows indicate the wavelengths at which the mass spectra shown in Fig. 1 were measured. The open grey squares represent calculated absorption wavelengths for HenC60+ including quantum effects, renormalized by a factor of 1.0008.
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f3: Absorption wavelength as a function of He atoms attached.Centre positions for the absorption spectra of HenC60+ around 958 nm (blue open circle, left y axis) and 964 nm (red open triangle, right axis) plotted as a function of n, the number of helium ad-atoms on the fullerene ion surface. The error bars indicate s.e.m. of the centre position of the Lorentzian profiles fitted to the ion signal depletion (see Fig. 2). The absorption wavelengths (corrected to vacuum) that were obtained for zero to a few helium atoms by Maier and colleagues9 are indicated by the bold symbols. The red arrows indicate the wavelengths at which the mass spectra shown in Fig. 1 were measured. The open grey squares represent calculated absorption wavelengths for HenC60+ including quantum effects, renormalized by a factor of 1.0008.

Mentions: The resulting line centre positions for the absorption spectra of HenC60+ (n=2–100) close to 958 and 964 nm are plotted in Fig. 3 as a function of the number of helium ad-atoms on the fullerene surface. The absorption wavelength (corrected to vacuum) that was obtained for no helium atoms by Campbell et al.9 is also indicated, together with their values for up to four attached He atoms (filled symbols). Data for the two weaker C60+ absorption features close to 937 and 943 nm are shown in Supplementary Fig. 3. For a growing number of helium atoms, we observe for the absorption wavelength a remarkably linear red shift of 0.072(1) nm per helium atom until n=32. Beyond n=32, a linear blue shift of 0.046(2) nm per helium atom is observed for the next 12 atoms. At ∼60 attached helium atoms we observe a local minimum in the red shift and then, again, a small increase up to 80 helium atoms. For larger clusters up to at least n=150, the absorption wavelength remains constant.


Atomically resolved phase transition of fullerene cations solvated in helium droplets
Absorption wavelength as a function of He atoms attached.Centre positions for the absorption spectra of HenC60+ around 958 nm (blue open circle, left y axis) and 964 nm (red open triangle, right axis) plotted as a function of n, the number of helium ad-atoms on the fullerene ion surface. The error bars indicate s.e.m. of the centre position of the Lorentzian profiles fitted to the ion signal depletion (see Fig. 2). The absorption wavelengths (corrected to vacuum) that were obtained for zero to a few helium atoms by Maier and colleagues9 are indicated by the bold symbols. The red arrows indicate the wavelengths at which the mass spectra shown in Fig. 1 were measured. The open grey squares represent calculated absorption wavelengths for HenC60+ including quantum effects, renormalized by a factor of 1.0008.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: Absorption wavelength as a function of He atoms attached.Centre positions for the absorption spectra of HenC60+ around 958 nm (blue open circle, left y axis) and 964 nm (red open triangle, right axis) plotted as a function of n, the number of helium ad-atoms on the fullerene ion surface. The error bars indicate s.e.m. of the centre position of the Lorentzian profiles fitted to the ion signal depletion (see Fig. 2). The absorption wavelengths (corrected to vacuum) that were obtained for zero to a few helium atoms by Maier and colleagues9 are indicated by the bold symbols. The red arrows indicate the wavelengths at which the mass spectra shown in Fig. 1 were measured. The open grey squares represent calculated absorption wavelengths for HenC60+ including quantum effects, renormalized by a factor of 1.0008.
Mentions: The resulting line centre positions for the absorption spectra of HenC60+ (n=2–100) close to 958 and 964 nm are plotted in Fig. 3 as a function of the number of helium ad-atoms on the fullerene surface. The absorption wavelength (corrected to vacuum) that was obtained for no helium atoms by Campbell et al.9 is also indicated, together with their values for up to four attached He atoms (filled symbols). Data for the two weaker C60+ absorption features close to 937 and 943 nm are shown in Supplementary Fig. 3. For a growing number of helium atoms, we observe for the absorption wavelength a remarkably linear red shift of 0.072(1) nm per helium atom until n=32. Beyond n=32, a linear blue shift of 0.046(2) nm per helium atom is observed for the next 12 atoms. At ∼60 attached helium atoms we observe a local minimum in the red shift and then, again, a small increase up to 80 helium atoms. For larger clusters up to at least n=150, the absorption wavelength remains constant.

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.


Related in: MedlinePlus