Full text: In recent experiments of our group on alkali-metal-doped helium droplets, it could be shown that the hyperfine structure of the dopants is strongly affected by the helium surrounding. A change of the hyperfine splitting of {sup 39}K is observed as a function of the nozzle temperature, indicating a dependence on the helium droplet size. Hence we started a theoretical investigation based on an open shell coupled cluster approach to study this size effect within molecular orbital theory. In our theoretical model we are restricted to very small He{sub N}X clusters (N=1- 5,X=Li,Na,K) due to the high computational demands of the chosen method. However, it is possible to identify two antagonistic effects of the helium surrounding on the electronic wavefunction of the alkali dopant: The first effect is a compression of the wavefunction which leads to higher spin densities |{Phi}{sub 0}|{sup 2} at the core, the second one is a reduction of |{Phi}{sub 0}|{sup 2} due to the Pauli repulsion, i.e. the deformation of the s-orbitals. Despite of the computational limitations we also try to give estimations for larger systems. Here we apply a combination of the quantum chemistry approach with a stochastic geometry optimization. Assuming only two-body-interactions we use the simulated annealing technique to obtain geometry predictions. (author)
Full text: In recent experiments of our group on alkali-metal-doped helium droplets, it could be shown that the hyperfine structure of the dopants is strongly affected by the helium surrounding. A change of the hyperfine splitting of {sup 39}K is observed as a function of the nozzle temperature, indicating a dependence on the helium droplet size. Hence we started a theoretical investigation based on an open shell coupled cluster approach to study this size effect within molecular orbital theory. In our theoretical model we are restricted to very small He{sub N}X clusters (N=1- 5,X=Li,Na,K) due to the high computational demands of the chosen method. However, it is possible to identify two antagonistic effects of the helium surrounding on the electronic wavefunction of the alkali dopant: The first effect is a compression of the wavefunction which leads to higher spin densities |{Phi}{sub 0}|{sup 2} at the core, the second one is a reduction of |{Phi}{sub 0}|{sup 2} due to the Pauli repulsion, i.e. the deformation of the s-orbitals. Despite of the computational limitations we also try to give estimations for larger systems. Here we apply a combination of the quantum chemistry approach with a stochastic geometry optimization. Assuming only two-body-interactions we use the simulated annealing technique to obtain geometry predictions. (author)
Spectroscopy of doped helium nanodroplets has become, in recent years, a popular tool for the study of bothfinite size samples of the helium quantum fluid and molecules isolated in this sub Kelvin environment. Whilemany dopants migrate to the center of the droplet, alkali (Ak) atoms in their ground electronic state, becauseof their so weak interaction with He, remain at the surface, the latter being in this way slightly distortedexhibiting a dimple. However, when in their first excited states, Ak atoms are forming exciplexes. These areanisotropic with the central atom surrounded by a ring of He atoms located in the nodal plane of the valenceelectron pz orbital. Structural properties, obtained by DMC and PIMC, are presented here for some Ak -Hen exciplexes.Ak2 dimers in helium environment have also gained interest in this last decade, in connexion with the generationof ultracold molecules. When attached to helium droplets, Ak atoms can float on the surface andform molecules in cold collisions. DMC and PIMC methods are used to compute energetics and structuralinformations for Rb2 in helium. In particular, the orientation of the dimer relative to the droplet surface isexamined.
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 ∼100Rb ∼ 100 Rb atoms. Our findings are consistent with previous theoretical and experimental studies. Graphic abstract
For the past 30 years, helium nano-droplets have been of particular interest to experimentalists.Indeed, this is a cold dense finite liquid environment which is an excellent matrix forspectroscopy applications, in atomic and molecular collision physics as well as in ultra coldchemistry with ability of isolating interesting reactions intermediates like HOCO in OH + CO→ CO2 + H or to synthesize new materials as metallic nano-wires. These phenomena areconsequences of the quantum collective behaviour of helium atoms at temperature T < 1 Kwhich classify helium nano-droplets as quantum liquids. In this thesis work we propose tomodel photo-dissociation [1] of immersed diatomic as molecular iodine, doping and rotationalexcitation of these droplets by using an home-made molecular dynamics developed by Pr.Marius Lewerenz, the CLUSTER code. To take the quantum nature of helium atoms intoaccount the methodology exploits the quantum effective potential technique [2] at experimentalwork temperature 0.37 K.With the photo-dissociation, an excitation above a molecular fragmentation limit leads to twopossible scenarios depending one the amortised energy by the helium droplets. The first one isthe fast recombination so-called ’cage effect’ [3] and the second is where photo-fragmentsreside in the helium bath. But with nano-meter scale of droplets a third scenario is possible withfragments leaving the cluster surrounded by a helium solvating shell. Results are shown forhelium nano-droplets containing thousands atoms (2500 to 9000 He) and statistics on size andvelocity of I@Hen photo-fragments have been computed and are similar to experimentaloutcomes for analogous system CH3I [4].According experiments using 1 fs X-ray flashes [5], the helium nano-droplets are geoids whichis a clue indicating a rotation on itself as the Earth. Our code can set precise angular momentumin order to study the pattern of distortion from fission to multiple splitting. To evaluate thenumbers of sub-cluster a Complete Clustering algorithm based on graph theory has beenprogrammed exhibiting a linear complexity.To conclude a study of xenon doping is presented where the recombination dynamics unveils ametastable state called Atkins’ snowball [6] and different patterns of aggregation as nano-crystals and nano-wires well known by experimentalists
Superfluid helium droplets represent a unique state of matter, which are large clusters of helium typically containing approximately 103-1011 helium atoms and exhibit remarkable properties such as superfluidity, a very low temperature (0.37 K) and high thermal conductivity. This PhD project investigates two novel aspects of superfluid helium droplets: the use of superfluid helium nanodroplets as the nanoreactors to grow magnetic nanoparticles and the generation and exploration of quantum vortices in a controlled manner. In the first part, we exploit the very low temperature and rapid cooling to develop a new approach for fabricating magnetic nanoparticles. For the very low temperature and the ultrahigh thermal conductivity, superfluid helium can suppress thermal effects during the atom-by-atom growth of magnetic nanoparticles, making the relatively weak exchange interactions (compared with metallic bonding) the driving force. As a result, the atomic spins align for ferromagnetic elements and thus the magnetic moments of nanoparticles are maximized. In particular, we focus on iron nanoparticles coated by a gold shell and investigate their properties by electron microscopy (for structural investigation) and x-ray circular dichroism (XMCD), for magnetic property measurement) at the Diamond Light Source. We first study mass spectrometry of small iron clusters and observed abnormal behaviours. Unlike other molecular clusters formed in helium droplets such as water, gold and silver, which typically follow a Poisson distribution, Fe+ channel was found to be far greater than that of FeN+ (N = 2-8) clusters. We postulate this as an indicator for the formation of high-spin iron clusters inside superfluid helium and attempt to provide an interpretation based on DFT calculations. However, XMCD showed an expected low magnetisation for Fe/Au core-shell nanoparticles which is even lower than iron oxide nanoparticles, indicating that the neutral Fe atoms are oxidized into Fe2+ within the nanoparticles which is magnetically inert. This is accounted by the very high electron negativity of Au atoms and the alloying effect during the growth of nanoparticles, which dismisses the magnetic properties. Our work shows that the choice of protective shell is important to maintain the magnetic properties of iron nanoparticles and points the direction for the next-step research. The second part presents a breakthrough in quantum vortex research. We demonstrate a novel method for generating controlled vortex arrays in superfluid helium droplets through collisions with cesium ions. Subsequential addition of metal atoms (Ag and Au) to helium droplets and the formation of nanodroplets allow the vortex lattices to be imaged after the nanoparticles are deposited onto a solid surface. By this approach we have revealed a record-high vortex density of 5.6×10¹⁴ m⁻², exceeding previous observations in bulk superfluid helium by more than six orders of magnitude. This unprecedented vortex density opens new possibilities for studying quantum hydrodynamics at extreme angular momenta and investigating quantum turbulence in previously inaccessible regimes. Through detailed theoretical analysis and experimental characterization, this work establishes superfluid helium droplets as a versatile platform for both materials synthesis and fundamental research. Our findings not only advance the understanding of superfluidity but also provide a new pathway for developing high-performance magnetic nanomaterials that can potentially revolutionize biomedical science and technologies.
Abstract Helium–alkali tetramers 4 He 3 A , 3 He 4 He 2 A and 3 He 2 4 HeA , where A is one of the alkali atoms 6,7 Li, 23 Na, 39,40,41 K, 85,87 Rb or 133 Cs, were studied using diffusion Monte Carlo method. Besides ground-state energies, exact distribution functions were evaluated. Spatial arrangements of atoms in clusters qualify He–A tetramers as transitional systems, which reveal halo-like spatial extension. On the one hand, swapping 4 He by 3 He atom in a He–Li tetramer, one can trace how quasi-halo structure transforms into halo structure. On the other hand, swapping 3 He by 4 He in He–A tetramer reveals behavior previously noticed in helium droplets doped with an alkali atom. Helium atoms start grouping on one side of an alkali atom, which is clearer in He–A clusters with more 4 He atoms that have stronger self-binding. The effect is the more pronounced the bigger alkali atom is, since it is then harder for the remaining He atoms to surround an alkali.
Diffusion and path integral Monte Carlo methods are currently, and successfully, used to describe structures and binding energies of helium clusters doped with some impurity. For diatomic dopants, by considering the He atoms as "electrons" and the dopant as "nuclei" within a Hartree/Hartree-Fock framework, our group has developed a complementary tool which, in addition to the above mentioned properties, provides us with wave-functions. It allows to perform spectral simulations that can be compared with the experiment. This paper reviews the fundamental aspects of such quantumchemistry-like methodology in which, together with masses, Coulomb interactions are replaced by proper molecular ones. Extensions towards more accurate ab initio methodologies are outlined. Finally, the challenging scenario arising when the impurity is not immersed but attached to the nanodroplet, giving rise to an unusual "solar" system in which the planets no longer move around the dopant as a "sun", is addressed.