There are myriad
ions that are deemed too short-lived to be experimentally
accessible. One of them is SF
6
+
. It has never
been observed, although not for lack of trying. We demonstrate that
long-lived SF
6
+
can be formed by doping charged
helium nanodroplets (HNDs) with sulfur hexafluoride; excess helium
is then gently stripped from the doped HNDs by collisions with helium
gas. The ion is identified by high-resolution mass spectrometry (resolution
m
/Δ
m
= 15000), the close agreement
between the expected and observed yield of ions that contain minor
sulfur isotopes, and collision-induced dissociation in which mass-selected
He
n
SF
6
+
ions collide
with helium gas. Under optimized conditions, the yield of SF
6
+
exceeds that of SF
5
+
. The procedure
is versatile and suitable for stabilizing many other transient molecular
ions.
Helium clusters attaching the recently experimentally observed sulphur hexafluoride SF6+ and sulphur pentafluoride SF5+ ions are investigated in a combined experimental and theoretical effort. Mass spectrometry ion yields are obtained...
Collisional excitation of CaPh−, formed in helium nanodroplets, produces Ca− but no phenanthrene anions. We conclude that the EA of Ph is below that of Ca, i.e. below 24.55 meV.
The adsorption of helium or hydrogen on cationic triphenylene (TPL, C18H12), a planar polycyclic aromatic hydrocarbon (PAH) molecule, and of helium on cationic 1,3,5-triphenylbenzene (TPB, C24H18), a propeller-shaped PAH, is studied by a combination of high-resolution mass spectrometry and classical and quantum computational methods. Mass spectra indicate that HenTPL+ complexes are particularly stable if n = 2 or 6, in good agreement with the quantum calculations that show that for these sizes, the helium atoms are strongly localized on either side of the central carbon ring for n = 2 and on either side of the three outer rings for n = 6. Theory suggests that He14TPL+ is also particularly stable, with the helium atoms strongly localized on either side of the central and outer rings plus the vacancies between the outer rings. For HenTPB+, the mass spectra hint at enhanced stability for n = 2, 4 and, possibly, 11. Here, the agreement with theory is less satisfactory, probably because TPB+ is a highly fluxional molecule. In the global energy minimum, the phenyl groups are rotated in the same direction, but when the zero-point harmonic correction is included, a structure with one phenyl group being rotated opposite to the other two becomes lower in energy. The energy barrier between the two isomers is very small, and TPB+ could be in a mixture of symmetric and antisymmetric states, or possibly even vibrationally delocalized.
In this Letter, we report the experimental detection of likely the largest ordered structure of helium atoms surrounding a monatomic impurity observed to date using a recently developed technique. The mass spectrometry investigation of He N Ca 2+ clusters, formed in multiply charged helium nanodroplets, reveals magic numbers at N = 12, 32, 44, and 74. Classical optimization and path integral Monte Carlo calculations suggest the existence of up to four shells surrounding the calcium dication which are closed with well-ordered Mozartkugellike structures: He 12 Ca 2+ with an icosahedron, the second at He 32 Ca 2+ with a dodecahedron, the third at He 44 Ca 2+ with a larger icosahedron, and finally for He 74 Ca 2+ , we find that the outermost He atoms form an icosidodecahedron which contains the other inner shells. We analyze the reasons for the formation of such ordered shells in order to guide the selection of possible candidates to exhibit a similar behavior.
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