Spatial structure of helium trimers as confirmation of structurelessness in quatarons

In December, 2014 in the journal “Nature. Communications” there appeared an article of a large international team (Germany, China, Italy) headed by J. Voigtsberger on the structure of trimers ⁴He 3 and ³He⁴Не 2 [1]. This work is of interest as it has principle importance for understanding the original physical conditions of small (van der Waals) cluster forms of matter, including peculiarities of the structural organization of specific cluster forms of "hidden" phase (quatarons). In fact cluster helium forms in this case act as modelling objects for studying quatarons, though the authors had absolutely another goals and objectives.

Initially the work was motivated by disputes on spatial structure of molecules 4Не3 proceeding for more than twenty years. Some works prove the linear character of arrangement of atoms in this molecule, while others affirm that the molecule has the form of a regular triangle [2]. The work above [1] really finishes these disputes. There it is shown that actually the molecule 4Не3 has no certain structure. Atoms in 4Не3 form a chaotic cloud. They with identical probability are distributed in spherical volume.

New experimental data in the quoted work [1] were obtained on setting «COLTRIMS». Clusters (molecules) of 4Не3 were formed at escape of gaseous helium from the nozzle, separated on weights and under the influence of femtosecond laser impulses the atoms in molecules were ionized. Scattering at the expense of Coulomb repulsion, the molecules were registered in special detectors. In these experiments it was possible to define distance and angles between atomic bonds and by that to make conclusions on the sizes and the form of initial molecules. As a result the unexpected absence of structure in 4He3 trimer was established.

Thus, the possibility of existence of molecules (trimers) without the ordered structure is proved. This result, obviously, can also be considered as the first experimental proof of the reality of existence of nonstructured quatarons and, accordingly, of the quataron form of the atomic-molecular organization of matter in the nanoworld.

Earlier, our ideas on dynamic structure of precrystallization clusters were based on the general theoretical calculations, or on indirect experimental data. In confirmation of our ideas we referred to A.Ziveil’s researches [3] who had established presence of transitive state in the process of formation of molecules from separate atoms. As a result of special experiments [4] carried out for the solution of the problem on formation of fullerenes from separate atoms or fragments of graphite layers, the assumption on the liquid clusterprecursor was made. As in these experiments graphite layers consisted of carbon 12C, and separate atoms from 13C, then at assemblage of fullerenes from fragments 12C the taking place mixing of atoms of carbon could not be.

The idea about dynamism of the structure of precrystallization clusters was first stated by the author more than 15 years ago within the new ideas on cluster selforganization of matter at nanolevel (the quataron concept). Fundamentals of this concept are given in [5, 6].

The key element of the quataron concept is the existence of a special form of the atomic-molecular organization of matter as nano-size clusters of the so-called “hidden” phase that were named quatarons. These clusters are not particles of a new (crystal) phase and were interpreted as pre-crystallization (prenucleation) formations.

It was shown that transformation of quatarons into crystalline nuclei is possible only at achievement of the sizes r≥4δ where δ approximately corresponds to diameter of cluster-forming atoms (molecules). Qua-tarons, despite small binding energy (mainly van der Waals bonds) retain their integrity, but represent the dynamic structures similar to molecules of ⁴He 3 . It is interesting that the dynamic structure of pre-nucleation clusters is reflected in the name «DOLLOP» (Dynamically Ordered Liquid - Like Oxyonion Polymers) which was given to these clusters by the authors of the work [7]. Ability to continuous reorganization of structure makes the definition of their equilibrium structure or prediction of the instant form impossible and senseless, though sometimes they try to interpret quatarons as regular or semiregular polyhedrons. As a whole in the range of the sizes from δ to 4δ the structure of qua-tarons, depending on number and type of bonds between atoms, evolves from almost full lack of structure to more or less ordered objects close to 4δ.

In the process of establishment of chemical bonds between the increasing number of atoms qua-tarons become more and more similar to rigid structures. The limiting case of such evolution for hollow quatarons is their full transformation into fullerenes [8]. Quatarons, as a rule, exist in such conditions when formation of bonds and their rupture occurs continu- ously. In rare cases, when inter-atomic bond is very strong, as for example, covalent bonds between atoms of carbon, there appears a chance of fixing of corresponding rigid structure of fullerene type.

The easy change of distances between atoms and angles of bonds between them, characteristic of quatarons, allows to consider quatarons as the ideal building units at growth of crystals. They easily (practically without kinetic resistance) incorporate in a crystal lattice (are reconstructed on growing faces). On this basis the modern quataron theory of crystals formation [9] is built according to which quatarons are the basic building units in growth of crystals. The most probable form of reorganization of quatarons on a crystal face is the formation of the two-dimensional nuclei providing layer-by-layer growth of crystals. Disintegration of weakly-bound quatarons on a growing face to separate atoms with the subsequent atomic growth of a crystal is thus quite probable.

Thus, we can say that after work [1] discussions about spatial structure of quatarons, as well as helium trimers, are coming to an end. However the problem on the nature of surprising similarity of behaviour of atoms in quatarons and helium clusters is still unclear. It is especially surprising that absence of structure is not the characteristic property of molecules of all inert gases. For example, Ne 3 represents rigid enough triangle [10]. In this connection there is the assumption of the possible superfluidity of quataron nanophase.

In conclusion we should say that the established absence of structure in cluster forms demands leaving from the settled tradition of their description as energetically minimized and geometrically optimized (equilibrium) structures towards the recognition of high energy state and morphological variability of quatarons, existing in nonequilibrium conditions. This in turn demands working out of new theoretical approaches and experimental methods with the high spatial-temporal resolution for studying not got in “a chemical trap” small cluster forms of matter and avoided hit in “symmetrical trap” nano-cluster amorphous matter. When and under what conditions the atoms manage to avoid hit in these "traps" is the fundamental question which still remains without the satisfactory answer.

This work is supported by RFBR (14-05-00592а) and Grant of the President of the Russian Federation for support of Leading Science School (SC-4795.2014.5).