Chemistry and physics of dopants embedded in helium droplets

Abstract

Helium droplets represent a cold inert matrix, free of walls with outstanding properties to grow complexes and clusters at conditions that are perfect to simulate cold and dense regions of the interstellar medium. At sub-Kelvin temperatures, barrierless reactions triggered by radicals or ions have been observed and studied by optical spectroscopy and mass spectrometry. The present review summarizes developments of experimental techniques and methods and recent results they enabled.

Electron diffraction as a structure tool for charged and neutral nanoclusters formed in superfluid helium droplets

“…With the invention of superfluid helium droplets, a new method of cluster formation has been discovered; the high pickup efficiency of large superfluid helium droplets is ideal for forming molecular and atomic clusters. 32–38 For volatile species, the doping pressure can be controlled by the temperature or the opening time of a sample-containing pulse valve (SPV). For non-volatile species including refractive metals, heating ovens have been used to supply the sample molecules or atoms.…”

Efficient Formation of Size-Selected Clusters upon Pickup of Dopants into Multiply Charged Helium Droplets

“…Droplets containing up to several billion helium atoms [ 16 , 17 , 18 ] can be formed via expansion of precooled pressurized helium through small nozzles into an ultra-high vacuum [ 19 , 20 ]. Evaporative cooling leads to an isothermal temperature of these droplets of about 0.4 K [ 21 ] and collisions with dopants lead to pickup and aggregation of these species to clusters [ 22 , 23 ] and nanoparticles [ 24 ] inside (heliophilic dopants) or on dimples at the surface of the helium droplets (heliophobic dopants) [ 25 , 26 ]. A convolution of the size distribution of the helium droplets and the Poisson pickup statistics lead to a broad log-normal size distribution of the dopant clusters.…”

Complexes with Atomic Gold Ions: Efficient Bis-Ligand Formation

“…Magic numbers and shell closures in cluster size distributions are the result of fragmentation where less stable cluster sizes are depleted and decay into more stable ones that more likely survive [30]. For heliophilic dopants such as gold, charge transfer from He + or a small He n + ionic core to a dopant cluster is the dominant ionization mechanism upon electron irradiation of doped HNDs at sufficiently high electron energies [27,[31][32][33][34]. The high ionization energy of helium compared to all dopants makes this reaction highly exothermic and the excess energy released into the internal degrees of freedom of the ionized dopant cluster is expected to lead to fragmentation and subsequently to the observed intensity anomalies in the cluster size distributions.…”