Submersion of rubidium clusters in helium nanodroplets

Abstract

Alkali atoms and small clusters are known to reside on the surface of a helium droplet rather than its inside as most other dopant species. A theoretical investigation suggested that alkali clusters (Li–Rb) exceeding a certain critical size can become submerged in the droplet, which was experimentally confirmed for sodium and potassium. Here, we report an analogous experimental study of rubidium cluster submersion by means of electron impact mass spectrometry. We recorded size distributions of Rb cluster ions at various electron energies between 8 and 160 eV. Our data suggest that Rb clusters attached to helium droplets undergo a gradual submersion transition similar to potassium, ultimately leading to the full submersion of clusters larger than $$\sim 100~\hbox {Rb}$$ ∼ 100 Rb atoms. Our findings are consistent with previous theoretical and experimental studies. Graphic abstract

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.…”

Helium Droplet Mass Spectrometry

“…Also shown is a similar transition for K clusters suggesting submersion into the HND around 80 K atoms [278]. Reproduced from [126]. Licensed under CC BY 4.0 ionization if they are located at or near the HND surface.…”

“…Whereas the authors observed these larger clusters using single-PI with photon energies of 3.3 and 4.1 eV, no ions larger than trimers were detected using multi-PI at 1.4 eV. Familiar magic numbers such as n = 5, 9 and 19 are found for both species, with additional anomalies at n = 13, 21 and 28 for Rb n + [126] as well as n = 15 for Cs n + . Barring explicit evidence, the authors consider several models of cluster formation to argue that neutral alkali clusters should be able to form in both high and low spin states on the larger HNDs (<N> ~20,000) used in the experiment.…”

“…Shortly after, An der Lan et al conducted studies utilizing EI MS of HNDs doped with sodium [277] and potassium [278] in an attempt to experimentally determine n c . The authors followed the suggestion given by Stark and Kresin [276] to exploit the fact that dopants can be selectively ionized via Penning [126]. The gradual submersion of Rb clusters into the HND is evident from the decrease in ion yield between 60 and 100 atoms in the 21.6 eV mass spectrum in (a).…”

“…Recently, Schiller et al expanded the experimental evidence of alkali cluster submersion with a study of rubidium [126]. The authors recorded EI mass spectra of clusters with up to 200 Rb atoms grown on or in HNDs at electron energies between 8 and 160 eV (Fig.…”