Transitions In Argon Research Articles

High-precision,reference-free measurements of $$2p \rightarrow 1s$$ transitions in boron-like sulfur and argon

High-precision,reference-free measurements of $$2p \rightarrow 1s$$ transitions in boron-like sulfur and argon

Multisideband interference structures observed via high-order photon-induced continuum-continuum transitions in argon

We report a joint experimental and theoretical study of a three-sideband (3-SB) modification of the reconstruction of attosecond beating by interference of two-photon transitions (RABBIT) setup. The 3-SB RABBIT scheme makes it possible to investigate phases resulting from interference between transitions of different orders in the continuum. Furthermore, the strength of this method is its ability to focus on the atomic phases only, independent of a chirp in the harmonics, by comparing the RABBIT phases extracted from specific SB groups formed by two adjacent harmonics. We verify earlier predictions that the phases and the corresponding time delays in the three SBs extracted from angle-integrated measurements become similar with increasing photoelectron energy. A variation in the angle dependence of the RABBIT phases in the three SBs results from the distinct Wigner and continuum-continuum coupling phases associated with the individual angular-momentum channels. A qualitative explanation of this dependence is attempted by invoking a propensity rule. Comparison between the experimental data and predictions from an $R$ matrix (close-coupling) with time dependence calculation shows qualitative agreement in most of the observed trends.

Argon plume transition from a hollow swell to a diffuse swell with increasing amplitude of a trapezoidal voltage

Atmospheric pressure plasma jets can generate a remote plasma plume, which usually presents a conical or cylindrical morphology. Despite a few morphologies being observed, efforts should be made to obtain more plume structures because streamer dynamics may be revealed from them. For this purpose, an argon plasma plume excited by a trapezoidal voltage is investigated, which presents two kinds of swells (a hollow swell and a diffuse swell) with increasing voltage amplitude (V p). The results indicate that there are two positive discharges (Dp1 and Dp2) and one negative discharge (Dn) per voltage cycle for both of the swells. With increasing V p, the inception voltage and discharge intensity increase for every positive discharge, while they decrease for the negative discharge. Fast photography reveals that the positive streamer (Dp2) leaves different tracks in the two swells, which are curved in the hollow swell and randomly branched in the diffuse swell. The different tracks of Dp2 are explained with the consideration of applied field strength and residual positive ions of Dp1. The existence of residual positive ions is finally verified from optical emission spectra.

The Influence of liquid-liquid phase transition in molten argon on the controversial melting results of Mo, Ta and W

The long standing controversy between the melting of the bcc Mo,Ta,W and vanadium (V) metals, measured by dynamic shock experiments compared to the melting results obtained by diamond anvil cells has remained an enigma during the last two decades. The isobaric-like behavior of Mo,Ta,W and V under pressures is explained by the thermal expansion of these metals, which compress the liquid or solid argon transmitting medium. Argon, confined in the diamond anvil chamber, compressed by the thermal expansion of the Mo,Ta and W samples, shows evidence of a liquid-liquid phase transition observed by density (viscosity) change upon increasing pressure and the temperature. Two viscous regions are detected: below ~40 GPa where dTm/dP > 0, and above ~40 GPa where dTm/dP → 0. It is claimed that the discrepancy between the dynamic and the static measurements comes from the extremely compressibility of the liquid Ar pressure transmitting medium at pressures above 40(5) GPa and temperatures above 3000 K. The compressed liquid argon at these pressures and temperatures conditions seems to be the most reported compressible liquid in nature. The long standing enigmatic controversy mentioned above is thus solved.

Resonance-enhanced, rare-gas-assisted femtosecond-laser electronic-excitation tagging in argon/nitrogen mixtures.

Multiphoton-resonance enhancement of a rare-gas-assisted nitrogen femtosecond-laser electronic-excitation-tagging (FLEET) signal is demonstrated. The FLEET signal is ideal for velocimetric tracking of nitrogen gas in flow environments by virtue of its long-lived nature. By tuning to three-photon-resonant transitions of argon, energy can be more efficiently deposited into the mixture, thereby producing a stronger and longer-lived FLEET signal following subsequent efficient energy transfer from excited-state argon to the C (3Πu) excited state of nitrogen. Such resonant excitation exhibits as much as an order of magnitude increase in this rare-gas-assisted FLEET signal, compared to near-resonance excitation of seeded argon demonstrated in previous work, while reducing the required input excitation-pulse energies by two orders of magnitude compared to traditional FLEET.

Reference-free measurements of the1s2s2p2P1/2,3/2o→1s22s2S1/2and1s2s2p4P5/2→1s22s2S1/2transition energies and widths in lithiumlike sulfur and argon ions

We have measured the widths and energies of the 1s2s2p 2 P 1/2,3/2 → 1s 2 2s 2 S 1/2 transitions in lithiumlike sulfur and argon, as well as the energies of the forbidden 1s2s2p 4 P 5/2 → 1s 2 2s 2 S 1/2 M2 transition in both elements. All measurements were performed with a double-flat crystal spectrometer without the use of any reference line. The transition energy measurements have accuracies ranging from 2.3 ppm to 6.4 ppm depending on the element and line intensity. The widths and the intensity ratios of the 1s2s2p 2 P 1/2,3/2 → 1s 2 2s 2 S 1/2 lines have also been measured. These are the first reference-free measurements of transitions in core-excited lithiumlike ions, and have an accuracy comparable to the best relative measurements. We have also performed multi-configuration Dirac-Fock calculations of the widths, energies and intensity ratios. Extensive comparison between existing experimental results and theory is performed, and Bayesian techniques employed to extract the energy of the 1s 2p 2 4 P 1/2 → 1s 2 2p 2 P 1/2 transition in sulfur and identify contaminant transitions.

Photonic crystal nanobeam cavities with optical resonances around 800 nm

We report on the design and the fabrication of 1D photonic crystal (PhC) nanobeam cavities with optical resonances around 800 nm, compatible with Rubidium, Cesium or Argon atomic transitions. The cavities are made of Indium Gallium Phosphide (InGaP) material, a III-V semi-conductor compound which has a large index of refraction ($n \simeq 3.3$) favoring strong optical confinement and small mode volumes. Nanobeam cavities with inline and side coupling have been designed and fabricated, and quality factors up to $2\times 10^4$ have been measured.

Liquid–gas phase transition of Ar inside graphene nanobubbles on the graphite substrate

Graphene nanobubbles (GNBs) are formed when a substance is trapped between a graphene sheet (a 2D crystal) and an atomically flat substrate. The physical state of the substance inside GNBs can vary from the gas phase to crystal clusters. In this paper, we present a theoretical description of the gas–liquid phase transition of argon inside GNBs. The energy minimization concept is used to calculate the equilibrium properties of the bubble at constant temperature for a given mass of captured substance. We consider the total energy as a sum of the elastic energy of the graphene sheet, the bulk energy of the inner substance and the energy of adhesion between this substance, the substrate and graphene. The developed model allows us to reveal a correlation between the size of the bubble and the physical state of the substance inside it. A special case of a GNB that consists of argon trapped between a graphene sheet and a graphite substrate is considered. We predict the ‘forbidden range’ of radii, within which no stable GNBs exist, that separates bubble sizes with liquid argon inside from bubble sizes with gaseous argon. The height-to-radius ratio of the bubble is found to be constant for radii greater than 200 nm, which is consistent with experimental observations. The proposed model can be extended to various types of trapped substances and 2D crystals.

Diagnostics of Ar/N2 mixture plasma with detailed electron-impact argon fine-structure excitation cross sections

A collisional-radiative model is developed for the Ar/N2 mixture plasma using reliable fully relativistic electron impact excitation cross sections of argon. We consider several fine structure transitions of argon and their corresponding cross sections used in the model have been obtained using fully relativistic distorted wave (RDW) theory. Processes, which account for the coupling of argon with nitrogen molecules, have been further added in the model. The applicability of the model with the RDW cross sections is tested by coupling it to the recent available spectroscopic measurements for a low pressure electron beam generated Ar/N2 plasma. The plasma parameters viz. electron density (ne) and electron temperature (Te) are evaluated by optimizing the model simulated intensities with the eight emission lines (out of 3p54p (2pi) → 3p54s (1si) transitions) observed through the measurements. We find from our study that as the concentration of N2 increases in the Ar/N2 mixture from 0 to 10%, our extracted electron density varies from 2.3 × 1011 to 2.8 × 1011 cm−3 and the electron temperature decreases from 1.0 to 0.43 eV which are in good agreement with the available probe measurement. Using present collisional radiative model, the individual contributions from the ground and 1si states to the radiating states are also evaluated as a function of N2 concentrations.

Study of impurity behaviour for first magnetic configuration changes in W7-X plasmas by means of PHA spectra

Transport properties of impurities in stellarators are sensitive to potential variations on flux-surfaces, and thermodynamic forces from the bulk plasma which might be affected by changes in magnetic field properties. In this contribution, the effect of planar coil current changes on the core impurity behavior in the first operating phase of Wendelstein 7-X (W7-X) is described. Hydrogen plasmas, at different heating power levels were investigated showing Core-Electron-Root-Confinement (CERC) conditions, characterized by peaked Te profiles and positive radial electric fields in the plasma core. Impurity spectra in the x-ray emission range between 1 and 10 keV were measured by the Pulse Height Analysis (PHA) diagnostic. The line-integrated measurements of transitions in sulphur, chlorine and argon are in the focus of this analysis. First observations indicate that the scans of the magnetic configuration did not affect strongly the impurity concentration in the core. Clear differences in the impurity content, however, have been observed when the ECRH power was changed, which affects the bulk plasma properties.

High-precision measurements of n=2→n=1 transition energies and level widths in He- and Be-like argon ions

We performed a reference-free measurement of the transition energies of the $1s 2p\,^1P\_1\to 1s^2 \,^1S\_0$ line in He-like argon, and of the $1s 2s^2 2p\,^1P\_1\to 1s^2 2s^2\,^1S\_0$ line in Be-like argon ions. The highly-charged ions were produced in the plasma of an Electron-Cyclotron Resonance Ion Source. Both energy measurements were performed with an accuracy better than 3 parts in $10^6$, using a double flat-crystal spectrometer, without reference to any theoretical or experimental energy. The $1s 2s^2 2p\,^1P\_1\to 1s^2 2s^2\,^1S\_0$ transition measurement is the first reference-free measurement for this core-excited transition. The $1s 2p\,^1P\_1\to 1s^2 \,^1S\_0$ transition measurement confirms recent measurement performed at the Heidelberg Electron-Beam Ion Trap (EBIT). The width measurement in the He-like transition provides test of a purely radiative decay calculations. In the case of the Be-like argon transition, the width results from the sum of a radiative channel and three main Auger channels. We also performed Multiconfiguration Dirac-Fock (MCDF) calculations of transition energies and rates and have done an extensive comparison with theory and other experimental data. For both measurements reported here, we find agreement with the most recent theoretical calculations within the combined theoretical and experimental uncertainties.

The influence of gas pressure on E↔H mode transition in argon inductively coupled plasmas

Considering the gas pressure and radio frequency power change, the mode transition of E↔H were investigated in inductively coupled plasmas. It can be found that the transition power has almost the same trend decreasing with gas pressure, whether it is in H mode or E mode. However, the transition density increases slowly with gas pressure from E to H mode. The transition points of E to H mode can be understood by the propagation of electromagnetic wave in the plasma, while the H to E should be illustrated by the electric field strength. Moreover, the electron density, increasing with the pressure and power, can be attributed to the multiple ionization, which changes the energy loss per electron-ion pair created. In addition, the optical emission characteristics in E and H mode is also shown. The line ratio of I750.4 and I811.5, taken as a proxy of the density of metastable state atoms, was used to illustrate the hysteresis. The 750.4 nm line intensity, which has almost the same trend with the 811.5 nm line intensity in H mode, both of them increases with power but decreases with gas pressure. The line ratio of 811.5/750.4 has a different change rule in E mode and H mode, and at the transition point of H to E, it can be one significant factor that results in the hysteresis as the gas pressure change. And compared with the 811.5 nm intensity, it seems like a similar change rule with RF power in E mode. Moreover, some emitted lines with lower rate constants don’t turn up in E mode, while can be seen in H mode because the excited state atom density increasing with the electron density.

Photoassociative spectroscopy of ultracold argon and krypton

We report photoassociative spectroscopy performed separately on Kr84 and Ar40. The spectra were obtained as a function of laser intensity with the spectroscopy laser tuned around the ns[3/2]2→np[5/2]3 trapping transition, where n=4 for argon and n=5 for krypton. The resulting spectra are the first ever obtained for krypton on this important transition. This transition in argon was previously studied in our group. At that time, discrete features were observed in the spectrum, but their origin was not conclusively identified. We have now identified these features as artifacts from laser sidebands. We discuss the origin of these sidebands, how they imprint their presence on the spectra, and present spectra for argon with the artifacts removed. For both krypton and argon, while we observe photoassociation, we do not observe any resolved vibrational structure. We present a theoretical analysis of the expected vibrational level spacings based on calculated potential curves and show that the absence of resolved structure in the spectra of our experiments (as configured) is not surprising. Finally, we discuss the requirements of future experiments in which resolution of vibrational structure in this system might be obtained.

Time-Resolved Electron Temperature in an Argon Theta Pinch by Line Ratio Methods

Pulsed inductive argon plasma in an 80-J, 15-kV, 490-kHz theta pinch is interrogated using spectroscopic methods. Time-resolved electron temperature is obtained by coupling line intensity ratios with steady-state corona and collisional-radiative (CR) models. Neutral excited state argon transitions from the 2p to 1s subshells were utilized. The backfill pressures of 50 and 100 mtorr are of primary focus. Time-resolved electron temperature estimates are presented ranging from 1 to 11.1 eV for the corona model up to an excess of 80 eV for the CR model. Near-IR spectral emission is seen to increase rapidly near the first zero crossing of the oscillating discharge current while electron temperature increase lags by roughly one full discharge cycle later near the third zero crossing. An analysis of long exposures provides an account of substantial second-order diffracted spectra. Weak spectral signal quality for the short exposures of 0.25-μs yielded time-resolved spectral intensity trend lines of a low accuracy with an average percent difference of 69% between raw data and trend lines.

OES characterization of streamers in a nanosecond pulsed SDBD using N2 and Ar transitions

The characterization of non-thermal homogeneous plasmas is possible using optical emission spectroscopy (OES), notably by estimating the reduced electric field. This method was applied to characterize streamers generated by a nanosecond pulsed surface dielectric barrier discharge (SDBD) operated in quiescent air at atmospheric pressure and also at 0.5 atm. The average reduced electric field associated with the surface streamers was determined using four different sets of transitions occurring in air plasmas, the first negative system (FNS) of , the first positive system (FPS) and second positive system (SPS) of and argon transitions . The analysis of the results allowed to critically assess the validity of the estimated reduced electric field for the present conditions. It is shown experimentally that the inhomogeneous nature of the streamer head influences significantly the estimation of the reduced electric field. Moreover, the estimated reduced electric field is not sufficient to characterize the processes taking place in the streamer head, due to the steep variation of both the reduced electric field E/N and the electron density ne in space and time. To overcome this limitation, a new method is proposed to take into account the spatial structure of a streamer head. The applicability of the new method is demonstrated for these experimental conditions and shows a very good agreement for the transitions tested.

Pressure broadening and shift rates for Ar (s–p) transitions observed in an Ar–He discharge

The pressure broadening and shift rates have been measured for the 801.699nm (s5→p8), 800.836nm (s4→p6) and 795.036nm (s3→p4) transitions in argon perturbed by 10–200Torr of helium and argon at a temperature of 440±20K using a radio-frequency, capacitively coupled discharge. For a 10% Ar in He plasma the pressure broadening and shift rates were measured as 14.18±0.65 and 1.81±0.30MHz/Torr, 17.85±0.78 and 0.72±0.32MHz/Torr, and 16.59±1.22 and 2.94±0.48MHz/Torr for the 801.699nm, 800.836nm and 795.036 nm transitions, respectively. The influence of the slightly varying gas temperature on the broadening and shift rates is less than 1%. Stark broadening and shifting by electrons in the discharge are not measurable due to low electron densities and temperatures. Phase-changing collisional cross-sections in the literature decrease with temperature, in good agreement with the Lindholm–Foley T−0.2 trend based on the Lennard–Jones potential. Further investigation of the Ar⁎–Ar and Ar⁎–He interaction potentials is necessary to understand the behavior of the cross sections.

The absolute optical oscillator strengths of the 3p54 s and excitations of argon measured by the dipole (γ, γ) method

The optical oscillator strengths of the 3p54s and excitations of argon are of crucial importance in determining the elemental abundance of argon from astronomical observations. Substantial efforts have been made to determine the optical oscillator strengths of the 3p54s and transitions of argon, however, significant discrepancies among the reported values still exist. In this work, the absolute optical oscillator strengths of the 3p54s and transitions of argon have been measured by a newly developed high-resolution dipole (γ, γ) method. A comprehensive comparison with the previous experimental and theoretical data was then made as a cross-check.

Modeling the gain of inner-shell X-ray laser transitions in neon, argon, and copper driven by X-ray free electron laser radiation using photo-ionization and photo-excitation processes

Using an X-ray free electron laser (XFEL) at 960 eV to photo-ionize the 1s electron in neutral neon followed by lasing on the 2p-1s transition in singly-ionized neon, an inner-shell X-ray laser was demonstrated at 849 eV in singly-ionized neon gas several years ago. It took decades to demonstrate this scheme, because it required a very strong X-ray source that could photo-ionize the 1s (K shell) electron in neon on a timescale comparable to the intrinsic Auger lifetime in neon of 2 fs. In this paper, we model the neon inner shell X-ray laser under similar conditions to those used in the XFEL experiments at the SLAC Linac Coherent Light Source (LCLS), and show how we can improve the efficiency of the neon laser and reduce the drive requirements by tuning the XFEL to the 1s-3p transition in neutral neon in order to create gain on the 2p-1s line in neutral neon. We also show how the XFEL could be used to photo-ionize L-shell electrons to drive gain on n = 3–2 transitions in singly-ionized Ar and Cu plasmas. These bright, coherent, and monochromatic X-ray lasers may prove very useful for doing high-resolution spectroscopy and for studying non-linear process in the X-ray regime.

E‐H Transition in Argon/Oxygen Inductively Coupled RF Plasmas

AbstractThis contribution presents results of comprehensive investigation of the E‐H transition in an inductively coupled radio frequency argon/oxygen discharge at 13.56 MHz. For the characterization of the discharge arrangement, the spatial magnetic and electric field components of the planar coil in vacuum are calculated and provide information about the electron heating region. The used double coil leads to a radial symmetric field distribution. Further, voltage and current probe measurements reveal the inductivity of the coil and the phase shift between the coil voltage and the current. As a result, the coil current and the RF voltage are directly proportional. The phase shift is nearly constant in the E‐mode and decreases slightly in the H‐mode. The positive ion saturation current as well as the line integrated electron density are measured by means of Langmuir probe and 160 GHz microwave interferometer, respectively, to study the mode transition in argon with different oxygen contents. The positive ion saturation current and the electron density decrease with increasing oxygen admixture at fixed pressure and RF power. The onset and the way of the E‐H transition strongly depend on the oxygen content. The mode transition of a pure argon discharge is step like and changes to a continuous mode transition with the admixture of oxygen. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Coherent Forward Scattering Spectrometer as a Wavelength Tuning Indicator for Diode Laser Atomic Absorption Spectrometry

A coherent forward scattering spectrometer was used as a wavelength tuning indicator for diode laser atomic absorption spectrometry. Atomic absorption transition of excited argon atoms at 842.46 nm was investigated. Argon atoms were excited in a glow discharge. Coherent forward scattering signal attained maximum at 0.1 T of external magnetic field and at 12 mA discharge current in 266 Pa (2 Torr) of argon. When the lasing wavelength crossed an atomic absorption line, a spike-like signal was observed. When a continuous coherent forward scattering signal was observed, the lasing wavelength was confirmed to approach within about ±3 pm from the absorption transition of argon.