Ar Gas Targets Research Articles

Subshell contributions to electron capture into the continuum in MeV/u collisions of deuterons with multielectron targets

The process of the capture of a target electron into the projectile continuum (ECC) is investigated both experimentally and theoretically for collisions of 1.25--6.00 MeV deuterons with multielectron gas targets. Double-differential cross sections (DDCS), ${d}^{2}\ensuremath{\sigma}/d\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}dE$, of the ECC cusp-shaped electron peak involving He, Ne, and Ar gas targets were measured at the emission angle of zero degrees with respect to the ion-beam velocity. Corresponding DDCS calculations, obtained using continuum distorted-wave (CDW) and continuum distorted-wave eikonal initial-state (CDW-EIS) theories, were critically compared to the measurements. The role of each atomic subshell of the multielectron Ne and Ar targets is examined employing the CDW-EIS theory with numerical target wave functions, which is found to best agree with the measurements.

Few-Cycle, μJ-Class, Deep-UV Source from Gas Media

Energetic, few-fs pulses in the deep-UV region are highly desirable for exploring ultrafast processes on their natural time scales, especially in molecules. The deep-UV source can be generated from gas media irradiated with few-cycle near-infrared laser pulses via a third-order frequency conversion process, which is a perturbative mechanism in a relatively weak field regime. In this work, we demonstrate that the deep-UV generation process is significantly affected by also even higher nonlinear processes, such as the ionization depletion of gas and plasma-induced spatiotemporal distortion of propagating light. In the experiment, by optimizing the deep-UV (3.6–5.7 eV) generation efficiency, the highest deep-UV energy of 1 μJ was observed from a moderately ionized 0.8-bar Ar gas target. The observed UV spectra exhibited frequency shifts depending on the experimental conditions—gas type, gas pressure, and the gas cell location—supporting the importance of the highly nonlinear mechanisms. The experimental observations were well corroborated by numerical simulations.

Production of C4+ (2s2p 3,1P) hollow states in collisions of 6-18 MeV C4+ (1s2, 1s2s 3S) mixed-state beams with gas targets

Normalized electron yields of the formation of 2s2p 3,1P states from the metastable states 1s2s 3,1S and from the ground state 1s21S were obtained in 6-18MeV C4+ collisions with H2, He, Ne and Ar gas targets. The method of zero-degree Auger projectile spectroscopy was used to detect electrons emitted in the Auger decay C4+ (2s2p 3,1P) → C5+ (1s) + e-with high resolution. These states are of particular importance in the detailed study of fundamental excitation mechanisms, i.e. electron-nucleus excitation, electron-electron excitation and electron-electron excitation with spin exchange. Currently, the role of the above mechanisms in the production of the 2s2p 3,1P states is investigated utilizing variable 1s2s 3S metastable fraction beams as a function of collision energy and target species. Our latest results are presented.

Production of photoionized plasmas in the laboratory with x-ray line radiation.

In this paper we report the experimental implementation of a theoretically proposed technique for creating a photoionized plasma in the laboratory using x-ray line radiation. Using a Sn laser plasma to irradiate an Ar gas target, the photoionization parameter, ξ=4πF/N_{e}, reached values of order 50ergcms^{-1}, where F is the radiation flux in ergcm^{-2}s^{-1}. The significance of this is that this technique allows us to mimic effective spectral radiation temperatures in excess of 1 keV. We show that our plasma starts to be collisionally dominated before the peak of the x-ray drive. However, the technique is extendable to higher-energy laser systems to create plasmas with parameters relevant to benchmarking codes used to model astrophysical objects.

Decay measurements of43K(β−)43Ca by HRS and TAS

In this work, decay spectroscopy of neutron-rich 43 K has been performed at the Radioactive Ion Beam (RIB) facility at Variable Energy Cyclotron Centre (VECC), Kolkata by producing it with an 18-MeV alpha beam on 40 Ar gas target. The beam was delivered from the K130 Cyclotron and the gas jet recoil transport system was used to move the products to a low-background site. The β-feeding intensities were measured by both high resolution γ-ray spectroscopy and total absorption γ-ray spectroscopy methods in order to address mainly the anomalies (if any) in the feeding to ΔJ ≥ 1 levels reported earlier. The decay half-life of 43 K was also measured to be 22.4(1) hrs.

High-resolution x-ray spectroscopy to probe quantum dynamics in collisions of Ar17+,18+ ions with atoms and solids, towards clusters

We report on studies of projectile excited states produced by electron capture in both low and high velocity regimes, and when highly charged ions (HCIs) collide either with dilute or dense matter. Quantum effects in the interaction dynamics are probed via high-resolution x-ray spectroscopy for Ar17+ at 7 keV u−1 and for Ar18+ at 13.6 MeV u−1 on Ar, N2 or CH4 gas targets and on carbon solid foils. Relevant comparison between those two collision velocity regimes, and between gaseous and solid targets reveal specific features. In particular, the effect of multiple capture process occurring within a single-collision with gaseous target can be compared with the consequence of multistep collisions arising at surfaces and in solid-bulk at low velocity. At high velocity, beside evidence for collective response of the target electrons due to the wake field induced by HCI passing through the solid-bulk, we demonstrate that excitation and ionization collision processes damp the populations of projectile excited states for long ion transit times. The evolution of the np population as a function of n in solid is at variance from the 1/n3 law found in gas, and the disagreement increases with solid target thickness. We have also tackled studies of HCIs in collision with clusters showing that x-ray spectroscopy provides a powerful tool to sign the presence of clusters in a supersonic gas jet.

Formation of very-low-energy states crossing the ionization threshold of argon atoms in strong mid-infrared fields

Atomic ionization by intense mid-infrared (mid-IR) pulses produces low-electron-energy features that the strong-field approximation, which is expected to be valid in the tunneling ionization regime characterized by small Keldysh parameters $(\ensuremath{\gamma}\ensuremath{\ll}1)$, cannot describe. These features include the low-energy structure (LES), the very-low-energy structure (VLES), and the more recently found zero-energy structure (ZES). They result from the interplay between the laser electric field and the atomic Coulomb field which controls the low-energy spectrum also for small $\ensuremath{\gamma}$. In the present joint experimental and theoretical study we investigate the vectorial momentum spectrum of photoelectrons emitted from an Ar gas target at very low energies. Using a reaction microscope optimized for the detection of very-low-energy electrons, we have performed a thorough study of the three-dimensional momentum spectrum well below 1 eV. Our measurements are complemented by quantum and classical simulations, which allow for an interpretation of the LES and VLES and of the ZES in terms of two-dimensional Coulomb focusing and recapture into Rydberg states, respectively.

Electron radiography using a table-top laser-cluster plasma accelerator

We explore the use of a laser-based electron gun for applications in transmission electron radiography and microscopy at electron energies up to 2 MeV. This new approach holds the promise to overcome some limitations of existing conventional electron guns at high beam energies especially for ultrafast applications. Our laser-electron gun is based on titanium-sapphire, ultrashort pulse lasers to drive electron acceleration in a plasma. The focused laser pulse travels in a tailored Ar gas target and accelerates electrons to MeV energy in less than a millimetre. As a first application, we use this electron beam to perform contact transmission electron radiography of cm-scale thin and thick samples. We obtain transmission electron radiography of organic and inorganic dense objects over a field of view more than 50 mm wide. The images are well exposed and show details of both thick and thin samples. The spatial resolution for the current geometrical configuration was found to be approximately 60 µm and was limited by geometrical effects combined with the intrinsic detector resolution and diffusion in the sample.

Target Dependence of Single-Electron-Capture Cross Sections for Slow Be, B, C, Fe, Ni, and W Ions Colliding with Atomic and Molecular Targets

Single-electron-capture cross sections σ10 for W+ projectile ions on Ar and Kr atomic gas targets at 10 keV (55 eV/u) and on H2, D2, CH4, C2H6, and C3H8 molecular gas targets at between 5.0 and 10 keV (27 and 55 eV/u) were experimentally derived for the first time. With our published single-electron-capture cross sections σq q-1 for Beq+, Bq+, Cq+, Feq+, Niq+, and Wq+ (q = 1 for Fe; q = 1, 2 for the others) ions in low energy, an attempt was made to draw scaling behavior of single-electron-capture cross sections for such slow low-q ions on target species. Established scaling formulas are found to reproduce the measured cross sections generally within a magnitude and with higher precision for specific initial charge state and target species.

Electron loss of fast heavy projectiles in collision with neutral targets

The multiple electron loss of heavy projectiles in fast ion-atom collisions has been studied in the framework of the sudden perturbation approximation. Especially, a model is developed to calculate the cross sections for the loss of any number of electrons from the projectile ion, including the ionization of a single electron and up to the complete stripping of the projectile. For a given collision system, that is specified by the (type and charge state of the) projectile and target as well as the collision energy, in fact, the experimental cross sections for just three final states of the projectile are required by this model in order to predict the loss of any number, $N$, of electrons for the same collision system, or for any similar system that differs only in the energy or the initial charge state of the projectile ion. The model is simple and can be utilized for both, the projectile and target ionization, and without that large computer resources are requested. Detailed computation have been carried out for the multiple electron loss of Xe$^{18+}$ and U$^{6+,\, 10+,\, 28+}$ projectiles in collision with neutral Ar and Ne gas targets.

Tarnishing resistance of silver–palladium thin films

Ag–Pd decorative coatings are supposed to provide better tarnishing resistance as compared to pure Ag, while keeping good optical (brightness) and mechanical properties (ductility). Palladium has a similar optical appearance as silver, and forms a thin protective oxide surface layer. Thus, a small incorporation of Pd in Ag could improve its tarnishing resistance. Thin Ag–Pd films were deposited by magnetron co-sputtering from Pd and Ag targets. The Ar gas pressure, target power and substrate temperature were varied to modify the chemical composition of the films and optimise their properties. The optical properties of the films were evaluated by spectroscopic ellipsometry and colorimetry. Increasing Ar gas pressure and substrate temperature results in a drastic decrease of the specular reflectivity of the films. At constant deposition conditions the reflectivity of the Ag–Pd films decreases with increasing Pd content. The film hardness, evaluated by nanoindentation, increases with Pd content. The tarnishing resistance of the films was evaluated by sulphidation tests. The colour change of the films due to the sulphidation was measured by colorimetry. The nature of chemical bonds of the tarnished products was evidenced by XPS. The results suggest an improvement of tarnishing resistance of the Ag–Pd films with increasing Pd content.

スパッタＰｄ‐Ａｇ合金薄膜の構造・組成に及ぼす熱処理の影響

Pd-Ag alloy thin films deposited on silicon substrates by means of rf magnetron sputtering under various deposition parameters, such as rf power, Ar gas pressure and target configuration, were studied using FESEM, XPS and XRD. The surface morphology, chemical composition, chemical state and crystal structure of the films was influenced by various deposition parameters. Two different heat treatments were carried out on the films. In the oxidation atmosphere, the films were almost oxidized, while in the reduction atmosphere, Ag content and lattice constant of the films were increased.

Electron capture and ionization of 33-TeV Pb ions in gas targets

We have measured the total cross sections for electron capture by bare ${\mathrm{Pb}}^{82+}$ ions and for the ionization of hydrogenlike ${\mathrm{Pb}}^{81+}(1s)$ ions at $158\mathrm{G}\mathrm{e}\mathrm{V}/\mathrm{A},$ $\ensuremath{\gamma}=168,$ in Ar, Kr, and Xe gas targets. At this energy, the total capture cross sections are dominated by electron capture from pair production. The capture measurements are compared with the results of several theoretical calculations and with similar measurements made with solid targets. The ${\mathrm{Pb}}^{81+}(1s)$ ionization cross sections obtained, which are substantially lower than those measured in solids, agree well with recent calculations that predict saturation at high energies from target screening effects.

Angular characteristics of processes of electron detachment from negative ions and hydrogen atoms in gases

The results of experimental measurement of spatial-angular distributions of hydrogen particles (H−, H0, H+) obtained in scattering of a collimated ribbon beam of H− ions and H0(1s) atoms in He, Ar, Kr, Xe, H2, O2, and CO2 gas targets for certain values of energy in the range from 0.6 to 15 MeV are reported. The experimental setup and the measurement procedure with an angular resolution of 5×10−6 rad are described. The angular characteristics of measured distributions, i.e., full width at half maximum and standard deviation, were determined. It is shown that the shape of distribution for a beam of hydrogen atoms produced by neutralization of H− ions in a gas target varies with the type and thickness of the target, and the angular spread is smallest for the H2 target. The variations in the shape of distribution are due to the contribution of scattering processes without changing the charge of particles.

Wavelengths and energy levels of Xe V and Xe VI obtained by collision-based spectroscopy

We have utilized collision-based spectroscopy to investigate the spectra of Xe V and Xe VI. The radiation emitted following electron capture by 50 keV Xe5+ and 60 keV Xe6+ ions impinging on a He (Ar) gas target has been recorded in the 350–6000 (1200–2500) Å wavelength region. A number of new energy levels of Xe V and Xe VI have been established from lines identified by us. In particular, we have observed and identified transitions from the 5s25p4f (5s24f) configuration of Xe V (Xe VI). The analysis was supported by Hartree–Fock calculations.

Multiple-electron ionization, capture, and loss by 19-MeV Fq+ (q=2-9) in collisions with Ne and Ar.

Charge-exchange processes in fast collisions of F{sup {ital q}+} ({ital q}=2--9) on Ne and Ar gas targets have been studied using the projectile--recoil-ion coincidence method. The target ionization without a projectile charge change increases with increasing projectile charge approximately as {similar_to}{ital q}{sup 1.4}. This dependence is weaker than the {ital q}{sup 2} dependence predicted by the first Born approximation because of the large ionization probabilities involved. The single-electron-capture cross sections follow the known {ital q}{sup 3} scaling law, while the double-electron-capture cross sections display a {ital q}{sup 6} dependence, as predicted by the independent-electron approximation for small capture probabilities. Single- and double-electron loss from the {ital L} shell decreases with increasing {ital q} mainly because of the decrease in the number of {ital L}-shell electrons, while binding-energy effects play only a minor role. In contrast, the large sudden decrease in electron-loss cross sections once {ital K}-shell electrons must be removed is due to the large increase in binding energy.

Harmonic generation in the VUV region at high repetition rate

We describe here a source of subpicosecond, coherent, partially tunable radiation operating at 500 Hz repetition rate in the VUV region (200−100nm), based on a Ti:S pump laser delivering 500 μJ, 130 fs pulses and producing harmonics both in Xe and in Ar gas targets. The gas target is injected in the interaction chamber through a high repetition rate piezoelectric valve. In particular, we have analyzed the 3rd (265 nm) and the 5th (159 nm) harmonic yields versus laser intensity in different experimental conditions. We also present a study of the optimization of the phase-matching conditions in the gas medium. In virtue of its coherence, partial tunability and high repetition rate characteristics, i.e. short data acquisition times, this laser source lends itself as a very promising tool for carrying out atomic physics experiments in the VUV spectral range.

Wavelengths and energy levels of Xe VII and Xe VIII obtained by collision-based spectroscopy

The Xe VII and Xe VIII spectra have been investigated by collision-based spectroscopy. The radiation emitted following electron capture by 10q keV Xeq+ ions (q = 6-8) impinging on a He (Ar) gas target has, with some exceptions, been recorded in the 350-8000 (1200-2500) Å wavelength region. The xenon ions were provided by the Uppsala University ECR ion source. Many of the observed, previously unreported spectral lines have been identified. In total, nine new energy levels of Xe VII and Xe VIII have been established, of which two are tentative. The analysis was supported by Hartree-Fock calculations.

Formation and mean lifetime of the metastable doubly charged rare-gas dimer NeAr2+

The observation of a long-lived ${\mathrm{NeAr}}^{2+}$ molecular ion is reported. This ion was obtained by charge stripping of a 900-keV ${\mathrm{NeAr}}^{+}$ beam in an Ar gas target, and its mean lifetime was determined to be 275\ifmmode\pm\else\textpm\fi{}25 nsec. This is a direct measurement of the mean lifetime of a doubly charged rare-gas dimer. The ion decay is decribed well by a single exponential decay, suggesting that only one metastable state is populated in the collision. The cross section for the ${\mathrm{NeAr}}^{2+}$ production was determined to be (3\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ ${\mathrm{cm}}^{2}$. The existence of this weakly bound molecular ion and the fact that its mean lifetime is known can serve to motivate better theoretical calculations of molecular structure and the dissociation process.

Absolute doubly differential cross sections for electron emission in collisions of 3.5-MeV/u Fe17+ and Fe22+ ions with He and Ar gas targets.

Absolute doubly differential cross sections have been measured as a function of electron energy and angle of observation for electron emission in collisions of 3.5-MeV/u ${\mathrm{Fe}}^{17+}$ and ${\mathrm{Fe}}^{22+}$ ions with He and Ar gas targets under single-collision conditions. The measured electron emission cross sections are compared to theoretical and scaled cross sections based on the Born approximation. The results using intermediate-mass ions are discussed with reference to previously reported cross sections from collisions with highly charged lighter- and heavier-ion species at MeV/u projectile energies. The continuum--distorted-wave--eikonal-initial-state approximation shows good agreement with experiments except in the ``binary-encounter peak'' where the interaction between projectile electrons and target electrons could play an important role.