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.
In 2015, Campbell et al. (Nature 523, 322) presented spectroscopic laboratory gas phase data for the fullerene cation, C + 60 , that coincide with reported astronomical spectra of two diffuse interstellar band (DIB) features at 9633 and 9578 Å. In the following year additional laboratory spectra were linked to three other and weaker DIBs at 9428, 9366, and 9349 Å. The laboratory data were obtained using wavelength-dependent photodissociation spectroscopy of small (up to three) He-tagged C + 60´H e n ion complexes, yielding rest wavelengths for the bare C
Threshold photodetachment spectroscopy has been performed on the molecular anion CN − at both 16(1) K and 295(2) K in a 22-pole ion trap and at 295(2) K from a pulsed ion beam. The spectra show a typical energy dependence of the detachment cross section yielding a determination of the electron affinity of CN to greater precision than has previously been known at 31 163(16) cm −1 [3.864(2) eV]. Allowed s-wave detachment is observed for CN − , but the dependence of the photodetachment cross section near the threshold is perturbed by the long-range interaction between the permanent dipole moment of CN and the outgoing electron. Furthermore, we observe a temperature dependence of the cross section near the threshold, which we attribute to a reduction of the effective permanent dipole due to higher rotational excitation at higher temperatures.
Cryogenic 22-pole ion traps have found many applications in ion-molecule reaction kinetics and in high resolution molecular spectroscopy. For most of these applications it is important to know the translational and internal temperatures of the trapped ions. Here, we present detailed rotational state thermometry measurements over an extended temperature range for the two ion/buffer gas systems OH − /He and OD − /HD with ion-to-neutral mass ratios of 4.25 and 6 respectively. The measured rotational temperatures show a termination of the thermalisation with the buffer gas around 25 K, independent of mass ratio and confinement potential of the trap. Different possible explanations for this incomplete thermalisation have been investigated, among them the thermalisation of the buffer gas and the heating due to room temperature blackbody radiation and room temperature gas entering the trap.
Helium
is considered an almost ideal tagging atom for cold messenger
spectroscopy experiments. Although helium is bound very weakly to
the ionic molecule of interest, helium tags can lead to shifts and
broadenings that we recorded near 963.5 nm in the electronic excitation
spectrum of C60+ solvated with up to 100 helium
atoms. Dedicated quantum calculations indicate that the inhomogeneous
broadening is due to different binding energies of helium to the pentagonal
and hexagonal faces of C60+, their dependence
on the electronic state, and the numerous isomeric structures that
become available for intermediate coverage. Similar isomeric effects
can be expected for optical spectra of most larger molecules surrounded
by nonabsorbing weakly bound solvent molecules, a situation encountered
in many messenger-tagging spectroscopy experiments.
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