Electrons In Xe Research Articles

Two- and three-dimensional extended solids and metallization of compressed XeF2

The application of pressure, internal or external, transforms molecular solids into extended solids with more itinerant electrons to soften repulsive interatomic interactions in a tight space. Examples include insulator-to-metal transitions in O(2), Xe and I(2), as well as molecular-to-non-molecular transitions in CO(2) and N(2). Here, we present new discoveries of novel two- and three-dimensional extended non-molecular phases of solid XeF(2) and their metallization. At approximately 50 GPa, the transparent linear insulating XeF(2) transforms into a reddish two-dimensional graphite-like hexagonal layered structure of semiconducting XeF(4). Above 70 GPa, it further transforms into a black three-dimensional fluorite-like structure of the first observed metallic XeF(8) polyhedron. These simultaneously occurring molecular-to-non-molecular and insulator-to-metal transitions of XeF(2) arise from the pressure-induced delocalization of non-bonded lone-pair electrons to sp(3)d(2) hybridization in two-dimensional XeF(4) and to p(3)d(5) in three-dimensional XeF(8) through the chemical bonding of all eight valence electrons in Xe and, thereby, fulfilling the octet rule at high pressures.

Absolute primary scintillation yield of gaseous xenon under low drift electric fields for 5.9 keV X-rays

A standard gas proportional scintillation counter equipped with a photomultiplier tube has been used to measure the primary scintillation yield in Xe at 800 Torr for 5.9 keV x-rays. The value m = 53±8 was measured for the number of primary scintillation photons produced per absorbed 5.9 keV x-ray photon, which gives a value Ws = 111±16 eV for the average energy expended to produce a primary scintillation photon in Xe for this x-ray energy. The experimental method is based on a comparison between the primary and secondary scintillation signals measured with a digital oscilloscope, where the distinction of the small primary signal from the noise is achieved by always triggering the oscilloscope with the secondary scintillation signal, and making a time average over 128 primary/secondary scintillation pulses. No evidence for recombination scintillation was found. A qualitative discussion of Ws in terms of the ratio between excitation and ionization cross sections for electrons in Xe is also presented.

Ratio of Transverse Diffusion Coefficient to Mobility of Electrons in High-Pressure Xenon

We constructed a parallel plate drift chamber for measuring the ratio of the transverse diffusion coefficient Dt to the mobility µ of electrons multiplied by the elementary charge e, eDt/µ, which is called the transverse characteristic energy, in high-pressure xenon gas. The characteristic energies of electrons in Xe were obtained at a high pressure of 1.0 MPa and the reduced electric field, E/N, from 0.77 to 7.7 ×10-18 V·cm2, where E denotes an electric field and N the number density of Xe atoms. At the same E/N, our results agreed well with the data at pressures from 107 to 193 kPa previously obtained by Koizumi et al. [J. Phys. B 19 (1986) 2331]. The characteristic energies of electrons in Xe were found to be constant as a function of E/N from a low pressure to 1.0 MPa.

Electron swarm and transport coefficients for the binary mixtures of SF6 with Ar and Xe

A pulsed Townsend technique was used to measure the electron drift velocity, the longitudinal diffusion, and the effective ionization coefficients, and the limiting field strength for the binary mixtures of SF6 with Ar and Xe. This paper covered a wide range of the density-reduced electric field strength E/N between 50 and 700 Td (1 Townsend (Td) = 10−17 V cm2). The content of SF6 in the gas mixtures was varied over the range 1–90%. For the SF6–Ar mixture, the electron drift velocities were found to be higher than those for pure SF6, and conversely for the SF6–Xe mixture. The above can be explained in terms of the larger momentum transfer cross-section for electrons in Xe than in Ar. The limiting field strength for the SF6–Xe mixture was found to be higher than that for the SF6–Ar one, but still lower than that for the SF6–N2 mixture.

Negative differential conductivity of electrons in electron-beam sustained discharge in pure xenon

The mechanism of negative differential conductivity (NDC) in an electron-beam sustained gas discharge of pure Xe is investigated by modeling. The drift velocity of electrons in Xe has been calculated using a self-consistent kinetic model. Model calculations involves the self-consistent solution of the Boltzmann equation for electron energy distribution function in the two-term approximation and a complete set of rate equations for the excited and ionic species in the discharge plasma. It is shown that the main factor responsible for the observation of the NDC effect is Coulomb interaction between electrons. Regions in experimentally achievable parameter space where NDC can be observed are determined.

Collective Excitations in Atomic Shells

Collective excitations in atomic shells are studied with Xe and Cs as an example. The starting point of our method is the active part of the self-consistent charge density of the 4d105s25p6 electrons in Xe and of the 4d105s25p66s1 electrons in Cs. The active charge density is homogeneously smeared out within the shell R1 < r < R2, where the radii R1 and R2 are found in a similar way as in the work by Mukhopadhyay and Lundqvist. Dipole-active collective excitations (atomic-shell surface plasmons) of the charge density within the shell are studied by sum rule techniques due to Lushnikov et al. Two dipole active surface plasmons are found whose peak positions and widths excellently agree with recent experimental data by West and Morton. This model result establishes an analog to the well-known plasmon pole approximation for electrons in metals.

Photoionization cross sections of the 5s electrons in Xe measured with synchrotron radiation

Measurements of the photoionization cross sections of the 5s electrons in Xe between 25 and 40 eV photon-energy using synchrotron radiation are reported. The cross section shows a minimum at 36 ± 2 eV in excellent numerical agreement with a RPAE calculation.

Parameters of streamer breakdown in xenon

It is known from experiments [1–3] that the velocity of streamers, induced in the center of the interelectrode gap and propagating to the electrodes under conditions when the streamer length is comparable with the distance between the electrodes, increases linearly as the streamer length increases. This relationship is in qualitative agreement with theory [4], Nevertheless, the velocity of streamers starting from the electrodes and propagating in a long interelectrode gap remains practically constant during the whole propagation process [5, 6], In the case of short gaps (2–5 cm), constancy of the velocity is observed during the stage of the process when the length of the streamer is much less (≃20%) than the length of the gap [7], Since the electric field at its end controls the streamer propagation, the constancy of the streamer velocity indicates that the controlling field is constant under these conditions. A number of theoretical models were proposed in [8–13] which describe uniformly moving anode- and cathode-directed streamers (henceforth called anode and cathode streamers). Comparison of experimental data with the corresponding theoretical model enables one to determine the streamer parameters: the electric field, the charged-particle density, the current density, the channel radius, etc. In the case of an anode streamer in Xe an attempt at such a comparison was made, in particular, in [6]. However, the lack of reliable data on the value of the drift velocity and the diffusion coefficient of electrons in Xe for E/p≃ (102 − 103) V/cm · mm Hg allowed only rough estimates to be made. In this paper a numerical calculation is made of the drift velocity, the diffusion coefficient of electrons in Xe, and the rate of excitation of Xe atoms in the resonance level in the range of values of E/p≃ (101−103) V cm · mm Hg, and the volt-ampere characteristic of the breakdown is measured under conditions described in [6] (p0=300 mm Hg and E ≃ 104−105 V/cm). Using these results, the formulas for the velocity of anode [12] and cathode [13] streamers, and experimental data [6], the parameters of the streamers studied in [6] are determined.