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.
Deep Dive into 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.
In bulk cryocrystals of heavy rare gases like argon, krypton, and xenon, excitonic states have been observed both in absorption and luminescence spectra [1]. Structure defects, surface, impurities make it much more difficult to register excitonic features in luminescence spectra, which suggests strong damping of free excitons on such lattice imperfections. In clusters of these gases, the excitonic states of both bulk and surface type have been observed so far only in absorption spectra [2,3], e.g., a dependence of energy position of n>1 exciton levels on cluster radius (confinement effect) was reported in Ref. [3].
The motivation of this paper grew out of the electron-diffraction and cathodoluminescence results of our previous works [4,5] and is to detect excitonic luminescence in free xenon clusters. In Ref. [4] it was shown that condensation of argon-xenon mixtures into clusters can be accompanied by a complete phase segregation into a xenon cluster core and an argon outer shell with a sharp interface between the components. At the same time, Ref. [5] demonstrated that an increase in heavy-impurity concentration in the primary gas mixture can cause formation of pure krypton clusters of a lowered temperature, argon serving only as a carrying gas. Similar results obtained earlier using X-ray photoelectron spectroscopy were reported in Ref. [6] for a phase segregation in argon-xenon clusters and Ref. [7] for elimination of argon clusters in argon-krypton mixtures.
Temperature is an important factor in observation of excitonic bands in luminescence spectra, since cooling intensifies strongly the excitonic luminescence in bulk xenon below T≈60 K [8]. Using argon-xenon mixtures with the pressure ≥1 atm and the initial xenon concentration over 2 at.% allowed us to obtain pure xenon clusters with a temperature lowered down to about 40 K, as well as xenon clusters of the same temperature, but covered by a thin argon shell. It was in these experiments that we detected for the first time luminescence bands being due to bulk and surface excitons in xenon. Some of the results obtained were reported briefly in Ref. [9]. Here we present a more complete experimental picture with an extended analysis of the observed features. We believe that our results can help to open new opportunities in the studies of excitonic states, including those of exciton-related quantum size effects, in substrate-free rare-gas clusters.
Experiments were carried out on free clusters obtained by the method of isentropic expansion of a supersonic gas jet (see, for example, Ref. [10]). A beam of a mixture of argon and xenon gases exhausted into vacuum through a metallic conical nozzle with the throat diameter 0.34 mm, cone angle 8.6°, and exit-to-throat cross-sectional area ratio 36.7. The vacuum chamber was evacuated with a condensation pump cooled by liquid hydrogen. The pressure p 0 and temperature T 0 of the gas mixture at the entrance to the nozzle was varied from 0.5 to 2.5 atm and from 160 to 250 K, respectively. The concentration of xenon in a mixture C Xe was varied from 1 to 3 at.%. Cluster beams were probed at the distance of 30 mm from the nozzle exit. Depending on the initial parameters C Xe , T 0 , and p 0 , it was possible to obtain xenon clusters with practically no argon and those covered by an argon shell. Increase in pressure and/or decrease in temperature of the gas mixture favored formation of larger clusters.
Electron diffraction measurements allowed us to observe diffraction patterns of clusters up to the diffraction vector values s ≈ 7 Å -1 , |s| = λ -1 ⋅4π sinν, where ν is diffraction angle, λ is electron wavelength (electron energy was 50 keV). In the case of mixed argon-xenon clusters, the relative quantity of the chemical components was estimated from the ratio of argon and xenon diffraction peak intensities. The difference between the atomic factors of electron scattering by these substances was taken into account. The cluster size was estimated by broadening of the diffraction peaks. Electron-diffraction measurements also made it possible to determine the cluster temperature T cl . With this purpose we measured lattice parameters of argon or xenon to compare them with the values derived from the relevant well-known temperature dependences. The background resulting from incoherent (nonelastic) and gas scattering of electrons was subtracted from the diffraction patterns.
Clusters were optically studied by their cathodoluminescence spectra measured in the region 8-8.5 eV, where the atomic emission lines of Xe ( 3 P 1 ) and emission bands from bulk and surface excitons in massive xenon samples are located. We also measured luminescence spectra in the vicinity of 7 eV, where the molecular Xe 2 * emission band is observed. All measurements were made for constant values of electron energy (1 keV) and beam current (20 mA), which allowed us to make quantitative comparisons between the spectra obtained under different init
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