arXiv subjects

V. L. Vakula

Publications and source records attributed to V. L. Vakula.

13 recordsLinked to original sources

arXiv2026

Features of clustering in a supersonic argon jet when using a conical nozzle

The article presents an original spectroscopic method for determining the initial stage of the clustering process in supersonic jets. The technique has been tested on a supersonic argon jet excited by electrons at a distance of 30 millimetres from the nozzle outlet, where the clustering process is practically complete, and the influence of secondary processes on the intensity of the observed emissions is significantly suppressed. It is proposed to use the continuum emitted by neutral excited clusters with a wavelength of 127 nm as an indicator of cluster formation in a supersonic argon jet under various flow conditions. Analysis of the temperature dependence of the intensity of this continuum recorded at multiple argon pressures at the nozzle inlet allowed us to establish that the parameters of supersonic jet flow corresponding to the onset of crystallisation are related by the empirical expression. The calculated value of the constant for argon in the studied range of pressures and temperatures was 0.011.

physics.atm-clus↗

arXiv2026

Experimental determination of emission cross sections for electron induced processes in a supersonic argon jet

The article discusses an original spectroscopic method for the study of supersonic argon jets in atomic and cluster flow regimes. The method is based on absolute measurements of the VUV radiation flux from a supersonic argon jet induced by a 1 keV electron beam. The integral radiation flux and spectral distribution of the radiation flux density of a supersonic argon jet in the range of 50-150 nanometers were measured, which made it possible to determine the density of the non-condensed atomic component, the fraction of condensate, and the density of clusters in a supersonic argon jet at given parameters of its flow into vacuum. The obtained parameters also allowed us to estimate the emission cross sections for the argon resonance lines ArI (λ=104.8 nm, λ=106.7 nm) and the cluster continuum λ=127 nm.

physics.atm-clus↗

arXiv2025

Diagnostics of the condensate fraction in a clustered supersonic argon jet

A new method for determining the condensate fraction and cluster density in absolute units has been proposed and tested for a supersonic argon jet, which can also be applied to supersonic jets of other gases. The method is based on measuring the absolute intensities of the Ar II resonance lines (93.2 and 92.0 nm) when the supersonic jet is excited by an electron beam with constant current density. Knowing the absolute intensities of the lines and their emission cross sections, we determined the density of the noncondensed atomic component in the supersonic argon jet and the evolution of the condensate fraction over the whole temperature range investigated.

physics.atm-clus↗

arXiv2025

Size effect on polarization bremsstrahlung emission from xenon clusters

We measured the spectral distribution of the absolute differential cross section of both ordinary and polarization ultra-soft X-ray bremsstrahlung for 0.7 keV electrons scattered on substrate-free nanoclusters of xenon. Clusters were produced in a supersonic gas jet expanding adiabatically into a vacuum. An original method based on absolute measurements of the intensity of the atom and cluster emission in the vacuum ultraviolet and ultra-soft X-ray spectral regions was used to determine the cluster density in the scattering area. The bremsstrahlung arising from scattering of electrons on clusters had a polarization component which dominated the differential cross section. For the first time, cluster size effect on the formation of the polarization bremsstrahlung was found for xenon.

physics.atm-clus↗

arXiv2022

Size effect on polarization bremsstrahlung induced by electrons scattered on xenon clusters

We measured spectral distribution of absolute differential cross section of both ordinary and polarization bremsstrahlung for 0.3-1.0 keV electrons scattered on atoms and substrate-free nanoclusters of xenon. Clusters were produced in a supersonic gas jet expanding adiabatically into a vacuum. An original method based on absolute measurements of intensity of the atom and cluster emission in the VUV and USX spectral regions was used to determine the cluster density in the scattering area. The bremsstrahlung arising from scattering on clusters had a polarization component which dominated the differential cross section. For the first time cluster size effect on the formation of the polarization bremsstrahlung was found for xenon.

physics.atm-clus↗

arXiv2020

Quasicrystalline and crystalline rare-gas clusters produced in supersonic jets: Impact of the jet clustering level on cathodoluminescence spectra

The paper proposes a new approach to studying cathodoluminescence spectra of substrate-free rare-gas clusters produced in supersonic jets exhausting into a vacuum. The approach, which takes into account the fraction of the clustered substance in the jet, is applied to quantitatively analyze integrated intensities of the luminescence bands of the neutral and charged excimer complexes (Rg2)* and (Rg4+)* measured for nanoclusters of three rare gases (Rg = Ar, Kr, and Xe) with average sizes ranging from 100 to 18000 atoms per clusters (diameters varying from 2 to 13 nm). The amount of the clustered substance, which affects the absolute values of integrated intensity of the bands, is shown to be proportional to the logarithm of the average size of clusters in the jet. Analysis of normalized intensities allowed us to spectroscopically find two ranges of average sizes of Ar, Kr, and Xe nanoclusters which, in accordance with the electron diffraction studies, can be assigned to quasicrystalline icosahedral and crystalline fcc structures in clusters, as well as to find the cluster size range in which both structures coexist. We show that in fcc clusters the luminescence of the neutral molecules (Rg2)* comes from within the volume of the cluster, while the charged excimer complexes (Rg4+)* emit mostly from subsurface layers.

physics.atm-clus↗

arXiv2016

Analysis of luminescence spectra of substrate-free icosahedral and crystalline clusters of argon

We propose a new approach to analysis of cathodoluminescence spectra of substrate-free nanoclusters of argon produced in a supersonic jet expanding into a vacuum. It is employed to analyze intensities of the luminescence bands of neutral and charged excimer complexes (Ar2)* and (Ar4+)* measured for clusters with an average size of 500 to 8900 atoms per cluster and diameters ranging from 32 to 87 Å. Concentration of the jet substance condensed into clusters, which determines the absolute values of the integrated band intensities, is shown to be proportional to the logarithm of the average cluster size. Analysis of reduced intensities of the (Ar2)* and (Ar4+)* bands in the spectra of crystalline clusters with an fcc structure allows us to conclude that emission of the neutral molecules (Ar2)* comes from within the whole volume of the cluster, while the charged complexes (Ar4+)* radiate from its near-surface layers. We find the cluster size range in which the jet is dominated by quasicrystalline clusters with an icosahedral structure and demonstrate that the transition from icosahedral to fcc clusters occurs when the average cluster size is N = 1400+-400 atoms per cluster.

physics.atm-clus↗

arXiv2011

Observation of (N2)2 dimers in free nitrogen and argon-nitrogen clusters

Supersonic-jet luminescence spectroscopy was applied to study vibronic transitions in icosahedral N2 and Ar-N2 clusters having from 100 to 400 particles per cluster. In the case of mixed Ar-N2 clusters, the w->X transitions were observed to occur in single N2 molecules in an Ar environment, that is very much in the same way as in Ar-N2 bulk samples. In N2 clusters, however, a band series was detected which was never observed earlier. In the spectra of Ar-N2 clusters, this new series coexisted with the 'bulk' w->X transitions. Our analysis demonstrated that the series should be assigned to emission of van der Waals (N2)2 dimers from inside clusters. Earlier, such dimers were only observed in molecular beams and gaseous nitrogen, this paper reports their observation in the solid phase of nitrogen suggesting that they may be the basic building blocks of small N2 clusters. Our results can be of interest from the viewpoint of producing polymeric nitrogen since (N2)2 dimers can be considered to be a starting species for its synthesis.

physics.atm-clus↗

arXiv2009

Exciton-impurity luminescence from noncrystalline xenon-argon clusters

For the first time in binary mixtures of solid rare gases exciton-impurity luminescence is observed from a xenon-argon system containing argon as impurity. An exciton-impurity emission band is registered for binary clusters with the structure of multilayer icosahedron. The optical transition occurs from an energy level lying very close to the lowest level of volume excitons in bulk xenon samples. The results demonstrate the possibility of probing excitonic levels in noncrystalline condensed media.

physics.atm-clus↗

arXiv2007

Luminescence evidence for bulk and surface excitons in free xenon clusters

Cathodoluminescence spectra of free xenon clusters produced by condensation of xenon-argon gas mixtures in supersonic jets expanding into vacuum were studied. By varying initial experimental parameters, including xenon concentration, we could obtain clusters with a xenon core (300-3500 atoms) covered by an argon outer shell as well as shell-free xenon clusters (about 1500 atoms). The cluster size and temperature (about 40 K for both cases) were measured electronographically. Luminescence bands evidencing the existence of bulk and surface excitons were detected for shell-free xenon clusters. The emission from bulk excitons in small clusters is supposed to be due to processes of their multiple elastic reflections from the xenon-vacuum interface. A presence of an argon shell causes extinction of the excitonic bands. In addition, some new bands were found which have no analogs for bulk xenon cryosamples.

physics.atm-clus↗

arXiv2006

Direct observation of free excitons in luminescence spectra of xenon clusters

Luminescence of surface and free bulk excitons is detected in xenon for the first time for substrate-free rare-gas clusters. Xenon clusters were produced by the method of gas condensation in a supersonic jet emitted into vacuum. Optical study was accompanied by electron diffraction measurements to determine the structure of clusters.

physics.atm-clus↗

arXiv2003

Dynamical Dimension Reduction in Underdoped Hihg Temperature Superconductors

Dynamical Dimension Reduction in Underdoped High Temperature Superconductors G.G.Sergeeva (National Science Center "Kharkov Institute of Physics and Technology",Ukraine), V.L.Vakula (B.Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine) Abstract For underdoped cuprate HTSC we discuss the supposition that both the pseudogap (PG) and supercoducting (SC) states result from dynamical dimension reduction when HTSC behaves on cooling if its dimensionality is changed. It is shown that the transition to the PG state occurs at the temperature T* as dimensional crossover when charges motion changes from the three dimensional (3D) to two-dimensional (2D). Namely two-dimensionality at T*>T is responsible for the crucial role of Jahn-Teller (JT) distortions which bound up with holes and form delocalized JT polarons and the localized three spin polarons in copper-oxygen planes that removes the competition between pairing carriers and their localization. At the cooling T*>T the SC transition occurs at Tc as a sequence of the crossovers from zero-dimensional to 2D SC fluctuations, and then to the 3D SC fluctuations. Some experimental evidence of the local hole-JT polaron pairs and dynamical dimension reduction in the PG and SC states are discussed.

cond-mat.supr-con↗

arXiv2002

Three Component Model of Pseudogap State for Hihg Temperature Superconductors

Three Component Model of Pseudogap State for Hihg Temperature Superconductors G.G.Sergeeva (Kharkov Institute of Physics and Technology, Ukraine), V.L.Vakula (Institute for Low Temperature Physics and Engineering, Ukraine) Abstract For underdoped cuprate HTS three component model of pseudogap state is proposed where it is shown that at T<T* the zero-dimensional superconducting fluctuations are developing in copper-oxygen planes in the addition to Jahn-Teller small polarons (with total spin 1/2) and holes. These superconducting fluctuations are the local "hole-Jahn-Teller small polaron" pairs with BCS pairing. At T<T* (T* is the pseudogap temperature) the crossover of three-dimensional charge to "two-dimensional" one occurs, and polaron shift of the energy leads to the compensation of Coulomb repulsion between the polaron and the hole, and to their on-site attraction in in copper-oxygen planes. Some experimental evidences of the local "hole-Jahn-Teller small polaron" pairing at T<T* are discussed.

cond-mat.supr-con↗