arXiv subjects

A. F. Borghesani

Publications and source records attributed to A. F. Borghesani.

33 records · Page 2Linked to original sources

arXiv2011

A battery-operated, stabilized, high-energy pulsed electron gun for the production of rare gas excimers

We report on the design of a new type of electron gun to be used for experiments of infrared emission spectroscopy of rare gas excimers. It is based on a filament heated by means of a pack of rechargeable batteries floated atop the high-voltage power supply. The filament current is controlled by a feedback circuit including a superluminescent diode decoupled from the high voltage by means of an optical fiber. Our experiment requires that the charge injection is pulsed and constant and stable in time. This electron gun can deliver several tens of nC per pulse of electrons of energy up to $100\,$keV into the sample cell. This new design eliminates ripples in the emission current and ensures up to 12 hrs of stable performance.

physics.ins-det↗

arXiv2010

Injection of photoelectrons into dense argon gas

The injection of photoelectrons in a gaseous or liquid sample is a widespread technique to produce a cold plasma in a weakly--ionized system in order to study the transport properties of electrons in a dense gas or liquid. We report here the experimental results of photoelectron injection into dense argon gas at the temperatureT=142.6 K as a function of the externally applied electric field and gas density. We show that the experimental data can be interpreted in terms of the so called Young-Bradbury model only if multiple scattering effects due to the dense environment are taken into account when computing the scattering properties and the energetics of the electrons.

physics.plasm-ph↗

arXiv2008

Mobility of O$_2^-$ ions in supercritical Ar: Experiment and Molecular Dynamics Simulations

A new analysis and new Molecular Dynamics (MD) simulations of the measurements of the mobility $μ_{i}$ of O$_{2}^{-} $ ions in dense supercritical Ar gas are reported. $μ_{i}$ shows a marked dependence on the distance from the critical temperature $T_{c}.$ A mobility defect appears as a function of the gas density and its maximum value occurs below the critical density. The locus of points of maximum mobility defect in the $P-T$ plane appears on the extrapolation of the coexistence curve into the single-phase region. MD simulations quantitatively reproduce the mobility defect near $T_{c}.$

physics.plasm-ph↗

arXiv2008

Dielectric screening--, multiple scattering--, and quantum indistinguishability effects in the IR emission spectrum of Xe$_2$ excimers in dense Xe gas

We report new measurements of the infrared emission spectrum of Xe$_{2}$ excimers produced by electron impact in dense gas at room temperature. These measurements extend the density range previously explored by a factor of two. We have now reached a pressure $P\approx 3.3 $MPa, corresponding to a gas density $N\approx 8\cdot 10^{26} $m$^{-3}$ in both pure Xe gas and in a Ar--Xe mixture. The analysis of the changes of the spectrum induced by the interaction with the gaseous environment shows how cooperative-- and many-body effects develop with increasing density. We %will also show that quantum effects related to the indistinguishability of identical particles are observed even at such high temperature.

physics.atom-ph↗

arXiv2007

Low- and high-density features of IR luminescence of Xe$_{2}$ excimers produced by electron impact

Electron--impact excitation of Xe atoms in pure Xe gas and in a Xe(10 %)--Ar(90 %) mixture has led to the discovery of infrared (IR) luminescence of Xe$_{2}$ excimers. The investigation of the emission spectrum at low gas density has allowed the identification of the molecular states involved in the transition. When the gas density is increased to values up to 40 times larger than the density of the ideal gas at standard temperature and pressure, the interaction of the excimer with the dense environment produces a strong red--shift of the spectrum that is interpreted in terms of many--body effects.

physics.atom-ph↗

arXiv2007

Infrared emission spectrum and potentials of $0_u^+$ and $0_g^+$ states of Xe$_2$ excimers produced by electron impact

We present an investigation of the Xe$_{2}$ excimer emission spectrum observed in the near infrared range about 7800 cm$^{-1}$ in pure Xe gas and in an Ar (90%) --Xe (10%) mixture and obtained by exciting the gas with energetic electrons. The Franck--Condon simulation of the spectrum shape suggests that emission stems from a bound--free molecular transition never studied before. The states involved are assigned as the bound $(3)0_{u}^{+} $ state with $6p [1/2]_{0}$ atomic limit and the dissociative $(1)0_{g}^{+}$ state with $6s [3/2]_{1}$ limit. Comparison with the spectrum simulated by using theoretical potentials shows that the dissociative one does not reproduce correctly the spectrum features.

physics.atom-ph↗

arXiv2005

Electron and ion transport in dense rare gases

A review of the research on electron and ion transport in dense rare gases is presented. The investigation of the transport properties of electrons in dense rare gases aims at understanding the dynamics and energetics of electron states in a dense medium and at elucidating how changes of the environment influence their nature and scattering properties. The quantum nature of electrons couples them to the environment is such a way to produce a density-dependent shift of their energy that is the key to rationalize the observed phenomena.

physics.atom-ph↗

arXiv2005

Environment influence on the IR fluorescence of Xe$_{2}^{*}$ molecules in electron beam excited Ar--Xe mixture at high density

We report new measurements of the near infrared (NIR) Xe$_2^*$ excimer fluorescence in an electron--beam--excited Ar (90%)--Xe (10 %) mixture at room temperature. Previous measurements up to a density $N\approx 2\times 10^{26}$ m$^{-3}$ discovered a broad excimer fluorescence band at $\approx 7800$ cm$^{-1},$ whose center is red--shifted by increasing $N$ (A. F. Borghesani, G. Bressi, G. Carugno, E. Conti, and D. Iannuzzi, {\em J. Chem. Phys.}, {\bf 115}, 6042 (2001)). The shift has been explained by assuming that the energy of the optical active electron in the molecule is shifted by the density--dependent Fermi shift and by accounting for the solvation effect due to the environment. We have extended the density range up to $N\approx 6\times 10^{26}$ m$^{-3}, $ confirming the previous measurements and extending the validity of the interpretative model. A detailed analysis of the width of the fluorescence band gives a value of 2.85 nm for the size of the investigated excimer state. Such a large value lends credence to the validity of the proposed explanation of the experimental findings.

physics.atom-ph↗

arXiv2002

Electron Mobility in Dense Argon Gas at Several Temperatures

The mobility $μ$ of excess electrons in dense Argon gas has been measured as a function of the applied electric field $E$ and of the gas density N at several temperatures in the range $142.6 $T_c,$ shows the well known anomalous positive density effect, i.e., m0N increases with increasing N, confirming previous results obtained for T=300 K and 162.7 K. Below Tc, however, $μ_0 N$ does not show the expected anomalous positive density effect, but it rather features a broad maximum. This appears to be a crossover behavior between the positive density effect shown for T>Tc and the small negative effect previously observed for T$\approx$ 90 K. In any case, the data at all temperatures confirm the interpretation of the anomalous density effect as being essentially due by the density-dependent quantum shift of the electron ground state kinetic energy in a disordered medium as a result of multiple scattering (MS) processes, although other MS processes do influence the outcome of the experiment.

physics.plasm-ph↗

arXiv2002

Inhomogeneous gas model for electron mobility in high density neon gas

Experimental studies of electron mobilities in Neon as a function of the gas density have persistently shown mobilities up to an order of magnitude smaller than expected and predicted. A previously ignored mechanism (gas in--homogeneity which is negligible in the thermal mobilities for He and other gases) is found to reproduce the observed Neon mobilities accurately and simply at five temperatures with just one variable parameter. Recognizing that a gas is not a homogeneous medium, a variation in local density combined with the quantum multi--scattering theory, shifts the energy and cross section -- which in turn changes the collision probability and finally the mobilities. A lower density where a momentum transfer interaction occurs moves the mobility strongly in the same direction as the anomalous experiments. By going backwards from the observed mobilities, the collision frequency at each temperature and density is made to reproduce the experimental data by looking for the local (as opposed to average) density at which the (rare) momentum transfer interactions occur. These density deviations give a picture of the size and behavior of the wave packets for electron motion which looks very much like the often discussed wave function collapse.

physics.atom-ph↗

arXiv2002

Effect of CH$_{4}$ addition on excess electron mobility in liquid Kr

The excess electrons mobility $μ$ has been measured recently in liquid mixtures of Kr and CH$_{4}$ as a function of the electric field up to $E\approx 10^{4} V/cm $ and of the CH$_{4}$ concentration $x$ up to $x \approx 10 % ,$ at temperatures $T\approx 130 K,$ fairly close to the normal boiling point of Kr $(T_{b}\approx 120 K)$(folegani). We present here new data which extend the previous set in the region of low electric field. The experimental results are interpreted in terms of a kinetic model previously proposed to explain the concentration dependent behavior of $μ$ in liquid Ar--Kr and Ar--Xe mixtures. The main result is that CH$_{4}$ is more effective in enhancing energy--transfer rather than momentum--transfer in comparison with mixtures of liquified noble gases. The field dependence of $μ$ is quite complicate. In particular, at intermediate values of the field, there appears to be a crossover between two different electric--field dependent behaviors of $μ.$ The electric field strength at crossover is well correlated with the concentration of CH$_{4}.$ This fact suggests that different excitations of the molecular solute might be involved in the momentum-- and energy--transfer processes for different values of the mean electron energy.

physics.atom-ph↗

arXiv2001

High Temperature Electron Localization in dense He Gas

We report new accurate mesasurements of the mobility of excess electrons in high density Helium gas in extended ranges of temperature $[(26\leq T\leq 77) K ]$ and density $[ (0.05\leq N\leq 12.0) {atoms} \cdot {nm}^{-3}]$ to ascertain the effect of temperature on the formation and dynamics of localized electron states. The main result of the experiment is that the formation of localized states essentially depends on the relative balance of fluid dilation energy, repulsive electron-atom interaction energy, and thermal energy. As a consequence, the onset of localization depends on the medium disorder through gas temperature and density. It appears that the transition from delocalized to localized states shifts to larger densities as the temperature is increased. This behavior can be understood in terms of a simple model of electron self-trapping in a spherically symmetric square well.

physics.plasm-ph↗

arXiv2001

Electron mobility maximum in dense argon gas at low temperature

We report measurements of excess electron mobility in dense Argon gas at the two temperatures $T=152.15$ and 162.30 K, fairly close to the critical one ($T_c =150.7$ K), as a function of the gas density $N$ up to 14 atoms$\cdot$nm$^{-3}$ ($N_{c}=8.08$ atoms$\cdot$nm$^{-3}$). For the first time a maximum of the zero-field density-normalized mobility $μ_{0}N$ has been observed at the same density where it was detected in liquid Argon under saturated vapor pressure conditions. The existence of the $μ_{0}N$ maximum in the liquid is commonly attributed to electrons scattering off long-wavelength collective modes of the fluid, while for the low-density gas a density-modified kinetic model is valid. The presence of the $μ_{0}N$ maximum also in the gas phase raises therefore the question whether the single scattering picture valid in the gas is valid even at liquid densities.

physics.plasm-ph↗

arXiv2000

Infrared Fluorescence of Xe2 Molecules in beam-excited Xe Gas at high Pressure

We report experimental results of proton- and electron-beam-induced near-infrared (NIR) fluorescence in high-pressure Xe gas at room temperature. The investigated wavelength band spans the range $0.7 \leqλ\leq 1.8 μ{m}.$ In the previously unexplored range for $λ> 1.05 μ{m}$ we have detected a broad continuum NIR fluorescence at $λ\approx 1.3 μ{m}$ that shifts towards longer wavelengths as pressure is increased up to $1.5 {MPa}.$ We believe that this continuum is produced in a way similar to the VUV continua of noble gas excimers and that the pressure-dependent shift can be explained by the interaction of the outer electron of the excimer with the gas.

physics.plasm-ph↗

arXiv1999

Parametric Resonance in the Drift Motion of an Ionic Bubble in near critical Ar Gas

The drift mobility of the negative oxygen ion in dense Argon gas near the liquid-vapor critical point has been measured as a function of the density. Near the critical temperature the zero-field density-normalized ion mobility shows a deep minimum at a density smaller than the critical one. This phenomenon was previously intepreted as the result of the formation of a correlated cluster of Argon atoms around the ion because of the strong electrostriction exerted by the ion on the highly polarizable and compressible gas. We now suggest that a possible alternative explanation is related to the onset of a parametric resonance of a bubble surrounding the ion. At resonance a large amount of energy is dissipated by sound waves in addition to viscous dissipation processes, resulting in the large mobility drop observed.

physics.plasm-ph↗