O. G. Danylchenko

Electron diffraction study of the structural transformations in free argon clusters

An electron diffraction technique is used to study the structure of clusters formed in an isentropically expanding supersonic argon jet. The formation of the hcp phase with increasing cluster size is reliably detected for the first time. Observations are made for mean cluster sizes N in the range from 1×103 to 8×104 atoms/cluster. An analysis of the shape of the diffraction peaks is carried out. It is found that in the range N⩽2×103 atoms/cluster, where the clusters are icosahedral, the profiles of the diffraction peaks are well approximated by a Lorentzian. For fcc clusters with N⩾3×103 atoms/cluster a better approximation is the standard Gaussian function. In the case N⩾1×104 atoms/cluster one observes peaks of the hcp phase in addition to the fcc peaks. The intensity of the hcp peaks increases with increasing cluster size, and for N≈8×104 atoms/cluster, the (110), (101), (103), and (202) peaks, characteristic only for the hcp phase, are clearly registered in addition to the fcc peaks. A possible m...

Electron diffraction study of two-component clusters Ar–Kr: Features of the nucleation, growth mechanisms, and structural states

The structure and growth processes of clusters formed in adiabatically expanding supersonic jets of gaseous mixtures of rare gases are investigated for the first time. Ar–Kr mixtures containing 0.1, 0.5, 2.5, and 20% Kr atoms are studied at temperatures and pressures of the gaseous mixtures at the nozzle entrance T0=100 and 120K and P0=0.15–2.5atm. The experiments give a number of results that shed light on the picture of cluster formation in Ar–Kr gaseous mixtures. It is established that the mechanisms of cluster formation depend substantially on the krypton impurity concentration and the temperature of the gaseous mixture at the nozzle entrance. The evolution of the character of the cluster formation, from homogeneous to heterogeneous, is traced. The intensification of rare-gas cluster growth by an impurity is revealed for the first time, and the concentration dependence of the transition from a process of formation of solid solutions to the formation of clusters containing only impurity krypton is traced.

Experimental verification of the Hagena relation for large clusters formed in a conical nozzle

The dependence of the size of inert gas clusters formed in a conical nozzle on the gas pressure and temperature at the nozzle input was studied for free clusters with a total number of atoms above 103. An analysis of the experimental data allowed a relationship between the number of atoms in a cluster and the Hagena parameter in the case of large clusters to be checked for the first time and the value of this parameter to be refined. It is shown that the established relations are also quite well obeyed in the presence of a small amount of impurities in supersonic jets of inert gases.

Structure of mixed clusters formed in supersonic jets of Ar–N2 gas mixtures

An electron diffraction study is done on substrate-free clusters formed from nanodrops in expanding supersonic jets of Ar–N2 gas mixtures. The mean characteristic linear dimension of the clusters varies from 20to200A, and the temperature of the clusters is 40K. The content of the gas mixtures spanned the entire concentration interval. These studies revealed that the mixed Ar–N2 clusters are enriched with Ar, and a method for determining the absolute concentrations of the cluster constituents is proposed. The structure of the clusters in relation to their size and the component concentrations is established for the first time. The diagram of the structural states is compared with published data for bulk samples grown from the liquid. A transformation from the fcc to the hcp structure on increasing cluster size is revealed, and also a transformation from the hcp to an hcp+fcc structure with increasing argon concentration; this is not observed in the bulk samples. In clusters the mutual transformations of th...

Size-induced structural transformations in argon clusters with a fivefold symmetry axis

An electron-diffraction study of the structure of free clusters of Ar is carried out in the interval of mean sizes N≈600–1500 atoms/cluster. The existence of an amorphous (polyicosahedral) phase in large rare gas clusters with N≈600–800 atoms/cluster is observed for the first time. It is shown that with increasing number of atoms in a cluster a transition to a multilayer icosahedral (ordered) phase occurs in a rather narrow size interval.

Diagnostics of composition and size of clusters formed in supersonic jets of Ar–Kr gas mixtures

The electron diffraction technique is employed to study the component composition of binary Ar–Kr clusters with N¯=1×103−9×104 atoms/cluster as a function of cluster size and the composition of the feed gas mixture. A diagram showing the conditions under which homogeneous Kr clusters rather than heterogeneous Ar–Kr clusters are generated in the jet is presented for the first time. It is found that the key parameters to analyze the composition of clusters are the critical cluster radius Rcr and the critical Kr gas concentration Ccr. For the parameter range of heterogeneous cluster formation ( CKrgas Rcr), an analytical expression is provided which describes quantitatively the enrichment of such clusters with krypton. The results obtained allow one to predict the composition of clusters produced in experiments with binary jets of noble and simple molecular gases, as well as to control simultaneously their size and composition.

Dimensions of heterogeneous clusters formed during condensation of Ar-Kr mixtures in supersonic jet

The average number of atoms in mixed atomic clusters formed during the adiabatic expansion of Ar-Kr mixtures with various compositions into vacuum has been determined using the method of electron diffraction. A previously unreported phenomenological relation between the average cluster size and the total pressure and temperature of a gas mixture at the input of a supersonic nozzle has been found. This relation has a rather general character and can be applied to other binary clusters of atomic and molecular gases. A method is proposed for establishing the boundaries of existence of the homogeneous and heterogeneous clusters depending on the pressure, temperature, and composition of gas mixtures.

Observation of exciton luminescence from icosahedral xenon-argon clusters

Exciton-impurity luminescence is observed for the first time in binary mixtures of solidified rare gases, in the xenon-argon system with argon as the impurity. An intense exciton-impurity emission band is observed in binary clusters having the structure of a multilayered icosahedron. The optical transition occurs from an energy level lying very close to the lowest bulk exciton level in bulk xenon samples. The results demonstrate the potential for probing exciton levels in disordered condensed rare gas media.

Composition of Ar–Kr, Kr–Xe, and N2–Ar Clusters Produced by Supersonic Expansion of Gas Mixtures

Size and composition of clusters produced by adiabatic expansion of binary gas mixtures (Ar-Kr, Kr-Xe, and N2-Ar) with various component concentrations are studied by using electron-diffraction technique. The resulting homogeneous and heterogeneous clusters are shown to have compositions substantially different from those of the primary gas mixtures and dependent on cluster size. We have found that the key parameters needed for an analysis of cluster composition are the critical cluster radius and the heavier component concentration in the gas mixture that can be used to establish the regions of existence of homogeneous and heterogeneous clusters. These critical values determine the coefficient of the enrichment of clusters with the heavier component with respect to its concentration in the primary gas mixture. Theoretical relations are obtained which provide a good quantitative description of the experimental results and can be expected to be also valid for finding the composition of binary clusters of other van der Waals systems.AbstractAn electron-diffraction study is made of the composition of binary Ar–Kr, Kr–Xe, and N2–Ar clusters of various size produced by expansion through a supersonic nozzle of gas mixtures with various component concentrations. The resulting clusters are shown to have compositions substantially different from those of the primary gas mixtures and dependent on cluster size. We have found that the key parameters needed for an analysis of cluster composition are the critical cluster radius and the heavier component concentration in the gas mixture which can be used to establish the regions of existence of homogeneous and heterogeneous clusters. These critical values determine the coefficient of the enrichment of clusters with the heavier component with respect to its concentration in the primary gas mixture. Theoretical relations are obtained which provide a good quantitative description of the experimental results and can be expected to be also valid for finding the composition of binary clusters of other van der Waals systems.

Electron diffraction data on nucleation and growth of an hcp phase in homogeneous (Ar) and heterogeneous (Ar-Kr) clusters

The nucleation and growth of the hcp phase in homogeneous (Ar) and heterogeneous (Ar-Kr) clusters formed in adiabatically expanding supersonic jets of the inert gases are studied by electron diffraction. The average size of the clusters ranges from 2 × 103 to 1 × 105 atoms/cluster. A threshold size of the clusters is found at which an hcp phase forms along with the fcc structure. The relative amount of the hcp phase in the single crystal clusters increases with their size. The relative volume of the hcp phase in the heterogeneous clusters exceeds that in homogeneous clusters of the same size. A correlation is established between the relative volume of the hcp phase in the clusters and the number of “defect” planes contained in the fcc matrix from which hcp phase nucleates. It is found that in very large (δ ≥ 150 A) polycrystalline aggregations the fraction of the hcp phase reaches a maximum and does not increase as the clusters become larger. A mechanism is proposed for the nucleation and growth of the hc...