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 He_nC₆₀⁺ 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 He_nC₆₀⁺ 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.

Alkali metal atoms and small alkali clusters are classic heliophobes and when in contact with liquid helium they reside in a dimple on the surface. Here we show that alkalis can be induced to submerge into liquid helium when a highly polarizable co-solute, C₆₀, is added to a helium nanodroplet. Evidence is presented that shows that all sodium clusters, and probably single Na atoms, enter the helium droplet in the presence of C₆₀. Even clusters of cesium, an extreme heliophobe, dissolve in liquid helium when C₆₀ is added. The sole exception is atomic Cs, which remains at the surface.

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.

We report the first experimental observation of negatively charged hydrogen and deuterium cluster ions, ${\mathrm{H}}_n^{-}$ and ${\mathrm{D}}_n^{-}$, where $n\ge 5$. These anions are formed by an electron addition to liquid helium nanodroplets doped with molecular hydrogen or deuterium. The ions are stable for at least the lifetime of the experiment, which is several tens of microseconds. Only anions with odd values of $n$ are detected, and some specific ions show anomalously high abundances. The sizes of these “magic number” ions suggest an icosahedral framework of ${\mathrm{H}}_2$ (${\mathrm{D}}_2$) molecules in solvent shells around a central ${\mathrm{H}}^{-}$ (${\mathrm{D}}^{-}$) ion. The first three shells, which contain a total of 44 ${\mathrm{H}}_2$ or ${\mathrm{D}}_2$ molecules, appear to be solidlike, but thereafter a more liquidlike arrangement of the ${\mathrm{H}}_2$ (${\mathrm{D}}_2$) molecules is adopted.

The reactions of carbon atoms with dihydrogen have been investigated in liquid helium droplets at T = 0.37 K. A calorimetric technique was applied to monitor the energy released in the reaction. The barrierless reaction between a single carbon atom and a single dihydrogen molecule was detected. Reactions between dihydrogen clusters and carbon atoms have been studied by high-resolution mass spectrometry. The formation of hydrocarbon cations of the type with m = 1–4 and n = 1–15 was observed. With enhanced concentration of dihydrogen, the mass spectra demonstrated the main “magic” peak assigned to the cation. A simple formation pathway and the high stability of this cation suggest its high abundance in the interstellar medium.

In order to adequately care for an increasingly aging population, there is a need for novel technical solutions to enhance the standard of living for patients and elderly people. Thus, gathering of health-related data and information on usage to improve said solutions is of uttermost importance. In order to allow a seamless integration for use in hospitals and retirement or private homes, these technical aides have to be the first and foremost easy to use and install. To this end, we present a sensor mat that is able to detect urinary incontinence as well as bed occupancy by patients and can be positioned underneath the bed sheet for easy usage. Thus, it can be applied fast and reliable to use by nursing staff or caretakers. The focus of this paper lies on the external unit, which consists of auxiliary electronics for data recording and processing, as well as either a proprietary radio frequency or a WiFi-Module for data transmission. Wetness is detected by measuring the resistance of embroidered electrodes, while the occupancy status is determined by the capacitance of these electrodes. The electronics can be powered by three standard AA batteries, and assuming a battery capacity of 1500 mAh, the sensor mat can be used all day for up to 2.8 years. Within this work, the electronic monitoring and alert system is presented and characterized by sensory data obtained during a variety of different tests. Due to its low-power consumption and, apart from cleaning, maintenance free operation, the presented sensor mat is an ideal tool to employ in various scenarios.