Argon

Summary

Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abundant as water vapor (which averages about 4000 ppmv, but varies greatly), 23 times as abundant as carbon dioxide (400 ppmv), and more than 500 times as abundant as neon (18 ppmv). Argon is the most abundant noble gas in Earth's crust, comprising 0.00015% of the crust. Nearly all of the argon in Earth's atmosphere is radiogenic argon-40, derived from the decay of potassium-40 in Earth's crust. In the universe, argon-36 is by far the most common argon isotope, as it is the most easily produced by stellar nucleosynthesis in supernovas. The name "argon" is derived from the Greek word ἀργόν, neuter singular form of ἀργός meaning 'lazy' or 'inactive', as a reference to the fact that the element undergoes almost no chemical reactions. The complete octet

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Propriétés optiques de petits agrégats de métaux nobles en matrice de néon

Sylvain Lecoultre

The optical properties of small noble metal clusters have been studied by absorption and fluorescence for sizes between 1 and 10 atoms. The experimental technique consists in depositing size-selected clusters in a neon matrix and analyzing with a spectrometer the transmitted and fluorescing light through the sample in an energy range between 1.8 and 5.5 eV. A new experimental setup has been built. It allows refrigeration of an Aluminium support down to 6 K and condensation of the neon matrix. The design of the sample holder allows a precise measurement of the cluster current during deposition. In comparison to measurements in argon matrices, the optical transmission has been increased by two orders of magnitude on the entire UV-Visible range. Consequently, the signal to noise ratio is greatly improved. Absorption spectroscopy on silver, copper and gold clusters with sizes between 1 and 9 atoms have revealed for each case a large amount of optical transitions with peakwidth as narrow as 50 meV, typical for a molecular-like behavior. Fluorescence measurements by exciting the sample with a tunable laser source have been achieved. For the atomic and dimer gold systems, the experimental results show several new emission lines. The fluorescence spectra obtained for Ag8 and Ag9 in neon confirm the results obtained in argon and demonstrate that the interaction with the environment is relatively weak for larger clusters. A study on the efficient trapping of positively charged clusters inside argon and neon matrices has allowed the measurement of the absorption spectra of the silver trimer and pentamer in their cationic form. TD-DFT calculations have been performed in order to find the lowest energy structures and the excitation spectra for each system. The excellent agreement between theory and experiment allows us to assign with more certainty the measured data to given isomers for all the silver clusters and for about 50% of the copper and gold clusters. A Mulliken population analysis has allowed to find the s, p and d contributions for each calculated spectrum. This analysis shows that we have more or less pure s transitions up to 5 eV for silver clusters while the d electrons participate actively to the transitions even at low energies between 2 and 3 eV in copper and gold. A detailed discussion on the pentamers of silver, copper and gold is presented on the basis of the experimental and theoretical results.

EPFL2009

SWISSCASE

Michael Bodendorfer

This PhD thesis consists of planning, simulating, building, testing and characterizing a new electron cyclotron resonance (ECR) ion source, SWISSCASE. the Solar Wind Ion Source Simulator for the CAlibration of Space Experiments located at the University of Bern, Switzerland. The ion source will be operated in the existing CASYMS ultra high vacuum (UHV) facility and will extend the application field of the existing filament electron collision ion source inside CASYMS and the existing electron cyclotron resonance ion source MEFISTO, operated in its own UHV facility. The chosen ECR ionization concept operating at 10.88 GHz delivers high currents of highly charged ions of up to 2 µA for Ar8+ and a maximum identified charge state of Ar12+. In addition to argon, the ECR plasma of SWISSCASE has been operated with krypton, xenon and carbon dioxide gas, revealing all of the expected charge states and featuring a charge states distribution in favor of highly charged ions. Design constraints were given by the final application of SWISSCASE being implemented in CASYMS with limited space and power. The new ion source was realized with limited funding. Numerical simulations gave insight in unprecedented quality and detail about the complex three dimensional extent of the magnetic field distribution of the full permanent magnet confinement of both, SWISSCASE and MEFISTO, the second ECR ion source, operated since 1997 at the University of Bern. For both confinement systems, the isocontour surface, defined by the electron cyclotron resonance condition of the incident microwave, is visualized in 3D. In addition to the realization of the ECR ion source, a plasma Bremsstrahlung measurement revealed an average ECR electron temperature of 10 keV for an argon plasma. This temperature was used to perform numerical simulations of electron trajectories inside the SWISSCASE and the MEFISTO confinement. The simulated magnetic field distribution of MEFISTO and the simulated electron distribution of SWISSCASE are closely related to the observed triangle shaped surface coating patterns found in hexapole confined ECR ion sources.

EPFL2008

Physical and chemical characterization of gas-borne particles in the nanometer to sub-micrometer size range is important in many applications, such as nanoparticle related health studies, the monitoring of powder or dust distribution processes, assessing the release of engineered nanoparticles (ENP) from ENP reinforced materials due to abrasion or combustion, or exposure studies related to ENP containing spray products. Such investigations are also highly valuable in the development of technical processes, e.g. to determine distinct size fractions containing specific elements or isotopes in terms of dealing with nuclear waste, or to evaluate particulate matter in process gases and emissions from thermal waste treatment, gas turbines, or other combustion engines. Such particle speciation includes knowledge about particle size distribution and chemical composition, and if possible obtained with a high time resolution. However, most of the available techniques are offline methods and/or not able to provide such physical and chemical information simultaneously. The Scanning Mobility Particle Sizer (SMPS) is a well-established and widely used equipment for determining size distribution and number concentration of such particles, within scan durations of a few minutes. Inductively Coupled Plasma Mass Spectrometry (ICPMS) is a highly sensitive multi-element technique, which allows determining the elemental composition of normally liquid samples, with excellent detection limits and a wide dynamic concentration range. In this work, SMPS and ICPMS are coupled to one hyphenated system. A Rotating Disk Diluter (RDD) allows directing a well-defined flow of a diluted aerosol into subsequent measuring equipment, which besides is mainly argon based, due to the dilution effect. Therefore an RDD is implemented as sample introduction interface, and the SMPS flow concept is re-designed to fulfil the requirements of both different analytical methods. This coupling strategy allows achieving simultaneous information on particle size and elemental composition, with SMPS-typical time resolutions of a few minutes, and offers high flexibility in terms of dealing with different aerosols, loaded with particles in the nanometer to sub-micrometer size range. Proper SMPS operation under argon atmosphere instead of air is validated. The developed setup is tested with a model aerosol containing air-borne silver nanoparticles. The capabilities are then demonstrated on uniformly composited metal aerosols, as well as an aerosol mixture containing smaller gold and larger silver nanoparticles. Sensitivity and limit of detection, related to particle number and mass concentration, are determined for several elements and particle diameters. After the successful characterization, the instrumentation is applied in two research applications. The first is a study on aerosol particles, released by commercial consumer spray products. In the second application, particulate metal emissions from the thermal treatment of differently impregnated wood samples are investigated, with the focus on alkali metals.

EPFL2016