Microwave microplasma sources based on microstrip-like transmission lines

In this paper, we study two microwave sources based on a planar transmission line configuration, corresponding to linear resonators. In both sources, micro-plasmas are produced within the 50–200 m\mu m gap created between two metal electrodes placed at the open end of a microstrip-like transmission line. The study of the sources follows a complementary approach that uses simulation and experiment. Simulations analyze the electromagnetic behavior of the sources, using the commercial tool CST Microwave Studio^?, and characterize the plasmas produced, using a fluid-type code to describe the dynamics of charged particles. Experimentally, the return loss of the sources (hence their quality factors) is measured without and with plasma. Plasma diagnostics (in air and in argon), based on optical emission spectroscopy measurements, enable to obtain the typical plasma temperatures and the electron density (using Stark broadening measurements of the H_b_{\beta} line-emission profile). Results reveal that the sources have similar quality factors (~15–20), yielding high-density (~10¹⁴ cm^-3)^{-3}), low-power (~10–50 W), non-equilibrium micro-plasmas (with rotational temperatures of ~950–1400 K in air and ~550–630 K in argon, vibrational temperatures of ~5200–5800 K in air and excitation temperatures of ~5800 K in argon), over volumes of ~10^-4–10^-3 cm³.     

Sheath and electron density dynamics in the normal and self-pulsing regime of a micro hollow cathode discharge in argon gas

A microplasma is generated in the microhole (400 μm diameter) of a molybdenum-alumina-molybdenum sandwich (MHCD type) at medium pressure (30–200 Torr) in pure argon. Imaging and emission spectroscopy have been used to study the sheath and electron density dynamics during the stationary normal regime and the self-pulsing regime. Firstly, the evolution of the microdischarge structure is studied by recording the emission intensity of the Ar (5p[3/2]₁–4s[3/2]₁)_{1}) line at 427.217 nm, and Ar⁺ (4p′ ²P_3/2–4s′ ²D_5/2)_{5/2}) line at 427.752 nm. The maximum of the Ar⁺ line is located in the vicinity of the sheath-plasma edge. In both regimes, the experimental observations are consistent with the position of the sheath edge calculated with an ionizing sheath model. Secondly, the electron density is recorded by monitoring the Stark broadening of the H_b_\beta-line. In the self-pulsing regime at 150 Torr, the electron density reaches its maximum value of 4 × 10¹⁵ cm^-3, a few tens of ns later than the discharge current maximum. The electron density then decays with a characteristic decay time of about 2 μs, while the discharge current vanishes twice faster. The electron density in the steady-state regime is two orders of magnitude lower, at about 6–8 × 10¹³ cm^-3.     

Laser-based measurements of line strength, self- and pressure-broadening coefficients of the H35Cl R(3) absorption line in the first overtone region for pressures up to 1 MPa

A high-resolution spectrometer based on a vertical-cavity surface-emitting laser (VCSEL) was developed and used to determine the line strength S(T ₀)=12.53(11)×10⁻²¹ cm⁻¹/(molec cm⁻²) and the self-broadening coefficient g⁰_HCl=0.021787(61)\gamma^{0}_{\mathrm{HCl}}=0.021787(61)  cm⁻¹/atm of the R(3) absorption line in the first rovibrational overtone (2←0) band of H³⁵Cl. Furthermore, the first laser-based high-pressure study on the pressure broadening of HCl by He, N₂ and O₂(g⁰_N2=0.07292(5)\mathrm{O}_{2}(\gamma^{0}_{\mathrm{N}_{2}}=0.07292(5)  cm⁻¹/atm, g⁰_He=0.02113(1)\gamma^{0}_{\mathrm{He}}=0.02113(1)  cm⁻¹/atm, g⁰_O2=0.03978(6)\gamma^{0}_{\mathrm{O}_{2}}=0.03978(6)  cm⁻¹/atm) is presented covering pressures of up to 1 MPa. The results are compared to previously available low-pressure data.     

Visualization of focusing–refocusing cycles during filamentation in BaF<Subscript>2</Subscript>

Filamentation occurs within a 1.5 cm-long crystal of BaF₂ during the propagation of intense, ultrashort (40 fs) pulses of 800 nm light; a systematic study as a function of incident power enables us to extract quantitative information on laser intensity within the condensed medium, the electron density and the six-photon absorption cross section. At low incident power, a single filament is formed within the crystal; two or more filaments are observed along the direction transverse to laser propagation at higher incident powers. Further, due to fluorescence from six-photon absorption (6PA), we are able to map the intensity variation in the focusing–refocusing cycles along the direction of laser propagation. At still higher incident powers, we observe splitting of multiple filaments. By measuring the radius (L _min) of single filament inside BaF₂, we obtain estimates of peak intensities (I _max) and electron densities (ρ _max) to be 3.26×10¹³ W cm⁻² and 2.81×10¹⁹ cm⁻³, respectively. Use of these values enables us to deduce that the 6PA cross-section in BaF₂ is 0.33×10⁻⁷⁰ cm¹² W⁻⁶ s⁻¹.     

Cavity enhanced absorption spectroscopy measurements of?pressure-induced broadening and shift coefficients in?the?υ1+υ3 combination band of ammonia

Cavity enhanced absorption spectroscopy is performed using an external cavity diode laser operating around 1516 nm. We demonstrate a sensitivity of 6×10⁻⁸ cm⁻¹ Hz^−1/2 and utilise a simple method to measure pressure-induced broadening and shift coefficients. The broadening and shift coefficients for six gases (helium, neon, argon, xenon, oxygen and nitrogen) have been determined at room temperature for four transitions in the υ ₁+υ ₃ combination band of ammonia. Comparisons of the broadening coefficients with previous work in this region, where it exists, show good agreement. The broadening and shift coefficients of nitrogen and oxygen are also in good agreement with calculated values using the Robert and Bonamy theory. Both the broadening and shift coefficients show a clear trend through the rare gases, which can be explained in terms of the varying magnitude of the long range attractive forces operating between the colliding partners. We also demonstrate the application of the Parmenter–Seaver formalism to estimate the potential well depth of the ammonia dimer from the obtained broadening coefficients. The obtained well depth agrees well with theoretical calculations.     

Spectroscopic measurements and electrical diagnostics of microhollow cathode discharges in argon flow at atmospheric pressure

This paper is dedicated to the study of the electrical and optical characteristics of direct current microhollow cathode discharges (MHCD) in argon flow. Experiments have been carried out in order to determine the so-called Paschen’s curves in a static open MHCD. Current-voltage characteristics were obtained as a function of the pressure and hole diameter. MHCD enable stable direct current discharge operation, which could be ignited for pressures ranging from 12 to 800 Torr, in a very wide range of current densities and electrodes materials. Optical emission spectroscopy and analysis of the spectral line broadening of plasma line emissions were performed in order to measure parameters such as electron number density (2–4 × 10¹⁴ cm^-3)^{-3}), gas temperature (460–640 K), excitation temperature (~ 7000 K) and electron temperature (~ 8500 K), for current ranging from 7 to 15 mA. Lower gas temperature was measured compared to the static MHCD ones.     

Nonlinear transformation of a spectrum of femtosecond laser pulses in a gas-filled microcapillary

Features of light pulse propagation and nonlinear optical transformation of the spectrum generated by titanium-sapphire laser pulses (τ_0.5 = 27 fs, λ₀ = 790 nm) have been studied experimentally in a 50-cm cylindrical hollow waveguide (microcapillary with 280-μm diameter core) filled with gaseous molecular nitrogen and helium. Stable guided propagation of light pulses with an intensity of ~1.5⋅10¹⁴ W/cm² in the fundamental EH₁₁ mode of the gas-filled capillary has been demonstrated. Exact focusing of the laser light made it possible to obtain rather high relative (≥95%) and absolute (~60%) energy transmission efficiencies for the pulses at gas pressures equal to or lower than 760 Torr. A method to determine the nonlinear phase shift of the pulses has been proposed. Values of the nonlinear refractive index n₂ ≈ 4.5⋅10^–23 cm²/(W⋅Torr) (N₂) and n₂ ≈ 2.8⋅10^–23 cm²/(W⋅Torr) (He) have been found. A short-wavelength shift in addition to the Kerr nonlinearity has been shown to be contributed by the generated electron plasma at high pulse intensities (≥10¹⁴ W/cm²).     

Three-dimensional simulation of laser–plasma-based electron acceleration

A sequential three-dimensional (3D) particle-in-cell simulation code PICPSI-3D with a user friendly graphical user interface (GUI) has been developed and used to study the interaction of plasma with ultrahigh intensity laser radiation. A case study of laser–plasma-based electron acceleration has been carried out to assess the performance of this code. Simulations have been performed for a Gaussian laser beam of peak intensity 5 × 10¹⁹ W/cm² propagating through an underdense plasma of uniform density 1 × 10¹⁹ cm^− 3, and for a Gaussian laser beam of peak intensity 1.5 × 10¹⁹ W/cm² propagating through an underdense plasma of uniform density 3.5 × 10¹⁹ cm^− 3. The electron energy spectrum has been evaluated at different time-steps during the propagation of the laser beam. When the plasma density is 1 × 10¹⁹ cm^− 3, simulations show that the electron energy spectrum forms a monoenergetic peak at ~14 MeV, with an energy spread of ±7 MeV. On the other hand, when the plasma density is 3.5 × 10¹⁹ cm^− 3, simulations show that the electron energy spectrum forms a monoenergetic peak at ~23 MeV, with an energy spread of ±7.5 MeV.     

Fabrication of a nanoparticle TiO<Subscript>2</Subscript> photoelectrochemical cell utilizing a solid polymeric electrolyte of PAN–PC–LiClO<Subscript>4</Subscript>

A nanoparticle TiO₂ solid-state photoelectrochemical cell utilizing as a solid electrolyte of poly(acrylonitrile)–propylene–carbonate–lithium perchlorate (PAN–PC–LiClO₄) has been fabricated. The performance of the device has been tested in the dark and under illumination of 100-mW cm⁻² light. A nanoparticle TiO₂ film was deposited onto indium tin oxide-covered glass substrate by controlled hydrolysis technique assisted with spin-coating technique. The average grain size for the TiO₂ film is 76 nm. LiClO₄ salt was used as a redox couple. The room temperature conductivity of the electrolyte is 4.2 × 10⁻⁴ S cm⁻¹. A graphite electrode was prepared onto a glass slide by electron beam evaporation technique. The device shows the rectification property in the dark and shows the photovoltaic effect under illumination. The best J _sc and V _oc of the device were 2.82 μA cm⁻² and V _oc of 0.58 V, respectively, obtained at the conductivity of 4.2 × 10⁻⁴ S cm⁻¹ and intensity of 100 mW cm⁻². The J _sc was improved by about three times by introducing nanoparticle TiO₂ and by using a solid electrolyte of PAN–PC–LiClO₄ replacing PVC–PC–LiClO₄ in the device. The current transport mechanism of the cell is also presented in this paper.     

Optical investigation of LaMnO<Subscript>3</Subscript> thin films: a study of the 2-eV band

We measure the transmittance of LaMnO₃ (LMO) thin films in the spectral range from 10 000 to 38 000 cm^-1 as a function of temperature. The optical conductivity obtained from the transmittance data is decomposed into five Drude-Lorentz contributions: three of them show evident changes below the Néel temperature T _N @\simeq 140 K. We discuss the band assignment in the framework of the tight-binding and of the orbitally degenerate Hubbard models. Our results ascribe the overall low-frequency spectrum of LMO to charge-transfer mechanisms involving the t _2g and e _g electrons of the Mn ions, without invoking the presence of excitonic contributions. This finding definitely promotes a Mott-Hubbard scenario for LMO and provides an estimate of the Hamiltonian parameters.     

Al-doped zinc oxide films grown by successive chemical solution deposition

Films of ZnO were grown and doped by the successive chemical solution deposition technique from a zincate solution containing aluminum (Al/Zn molar ratio in the range 2–10%). To our knowledge, this is the first report of its kind. A post-deposition heat treatment in argon (500 °C, 20 min) was performed for the activation of incorporated Al donors. Films were characterized by using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, optical transmittance, photoluminescence and Hall-effect measurement. Films with a resistivity 0.2 Ω cm, carrier concentration 1.2×10¹⁸ cm^-3 and mobility 26 cm²/V s can be grown from a solution with Al/Zn = 10%. Further reduction in resistivity is plausible by using more heavily doped solutions and by the optimization of annealing parameters. Although the doping process does not change the films’ structure and surface morphology, it slightly lowers the optical transparency in the visible region and blue shifts the room-temperature photoluminescence peak to 378 nm. PACS  81.16.Be; 73.61.Ga; 78.55.Et; 79.60.-i; 68.55.Jk     

The low-lying level structure of atomic lawrencium (Z = 103): energies and absorption rates

The low-lying level structure of atomic lawrencium has been calculated by using medium- to large-scale multiconfiguration Dirac-Fock wave functions. From these computations, an overview on the excitation energies and absorption rates is compiled for the 30 lowest levels of the neutral atom with regard to its 7s² 7p ²P_1/2 ^○ ground state. For many of these levels, an accuracy of ∼1200 cm^-1 is estimated by performing analogue computations for the homologous element lutetium. From the predicted level scheme, the excitation energies of about 15 levels fall into the spectroscopically relevant region between 20 000 and 30 000 cm^-1 and, hence, might help in the set-up and interpretation of forthcoming experiments on the resonant excitation of atomic Lr.     

Plasma-filled rippled wall rectangular backward wave oscillator driven by sheet electron beam

Performance of the backward wave oscillator (BWO) is greatly enhanced with the introduction of plasma. Linear theory of the dispersion relation and the growth rate have been derived and analysed numerically for plasma-filled rippled wall rectangular waveguide driven by sheet electron beam. To see the effect of plasma on the TM₀₁ cold wave structure mode and on the generated frequency, the parameters used are: relativistic factor γ = 1.5 (i.e. v/c = 0.741), average waveguide height y ₀ = 1.445 cm, axial corrugation period z ₀ = 1.67 cm, and corrugation amplitude ε = 0.225 cm. The plasma density is varied from zero to 2 ×10¹² cm^− 3. The presence of plasma tends to raise the TM₀₁ mode cut-off frequency (14 GHz at 2 ×10¹² cm^− 3 plasma density) relative to the vacuum cut-off frequency (5 GHz) which also causes a decrease in the group velocity everywhere, resulting in a flattening of the dispersion relation. With the introduction of plasma, an enhancement in absolute instability was observed.     

Line strength and collisional broadening studies of hydrogen sulphide in the 1.58 μm region using diode laser spectroscopy

High resolution diode laser spectroscopy has been applied to the detection of hydrogen sulphide at ppm levels utilizing different transitions within the region of the ν ₁+ν ₂+ν ₃ and 2ν ₁+ν ₂ combination bands around 1.58 μm. Suitable lines in this spectral region have been identified, and absolute absorption cross sections have been determined through single-pass absorption spectroscopy and confirmed in the Doppler linewidth regime using cavity enhanced absorption spectroscopy (CEAS). The desire for a sensitive system potentially applicable to H₂S sensing at atmospheric pressure has led to an investigation on suitable transitions using wavelength modulation spectroscopy (WMS). The set-up sensitivity has been calculated as 1.73×10⁻⁸ cm⁻¹ s^1/2, and probing the strongest line at 1576.29 nm a minimum detectable concentration of 700 ppb under atmospheric conditions has been achieved. Furthermore, pressure broadening coefficients for a variety of buffer gasses have been measured and correlated to the intermolecular potentials governing the collision process; the H₂S–H₂S dimer well depth is estimated to be 7.06±0.09 kJ mol⁻¹.     

EUV and fast ion emission from cryogenic liquid jet target laser-generated plasma

A liquid jet of either nitrogen or argon of 20 μm diameter was exposed to intense laser fields with pulse durations between 70 fs and 250 ps, leading to intensities of 10¹⁶ W cm^-2 and 10¹³ W cm^-2, respectively. The emission of extreme UV light and soft X-rays shows the characteristic lines of hydrogen-like nitrogen and carbon-like argon. For nitrogen the emitted photon flux at 250 ps was about two orders of magnitude higher than for 70 fs pulses. A weak dependence on the laser polarization with respect to the liquid jet axis was found. The kinetic energy of the emitted ions easily exceeded 100 keV for nitrogen and 200 keV for argon for a pulse duration close to 2 ps. Received: 21 August 2000 / Revised version: 20 December 2000 / Published online: 22 March 2001     

Radiation defects introduced by MeV electrons in argon implanted MOS structures

The influence of MeV electron irradiation on the interface states of argon implanted thin oxide MOS samples has been studied by the thermally stimulated current (TSC) method. The oxide thickness of the structures is 18 nm. Two groups of n-type MOS structures non-implanted and implanted with 20 keV Ar⁺ ions and a dose of 5×10¹² cm⁻² are examined. Both groups are simultaneously irradiated by 23 MeV electrons with doses of 1.2×10¹⁶, 2.4×10¹⁶ or 6.0×10¹⁶ el/cm². The energy position and density of the interface states (generated by electron irradiation, ion implantation or both treatments of the samples) are determined. It is shown that MeV electron irradiation decreases the concentration of interface states (like an oxygen-vacancy and di-vacancy) slightly and creates additional interface states (like an impurity-vacancy) at the Si–SiO₂ interface of argon implanted MOS structures.     

Static and dynamic structure factors with account of the ion structure for high-temperature alkali and alkaline earth plasmas*

The electron-electron, electron-ion, ion-ion and charge-charge static structure factors are calculated for alkali (at T = 30 000 K, 60 000 K, n _e = 0.7 × 10²¹ ÷ 1.1 × 10²² cm^-3) and Be²⁺ (at T = 20 eV, n _e = 2.5 × 10²³ cm^-3) plasmas using the method described by Gregori et al. The dynamic structure factors for alkali plasmas are calculated at T = 30 000 K, n _e = 1.74 × 10²⁰, 1.11 × 10²² cm^-3 using the method of moments developed by Adamjan et al. In both methods the screened Hellmann-Gurskii-Krasko potential, obtained on the basis of Bogolyubov's method, has been used taking into account not only the quantum-mechanical effects but also the repulsion due to the Pauli exclusion principle. The repulsive part of the Hellmann-Gurskii-Krasko (HGK) potential reflects important features of the ion structure. Our results on the static structure factors for Be²⁺ plasma deviate from the data obtained by Gregori et al., while our dynamic structure factors are in a reasonable agreement with those of Adamyan et al.: at higher values of k and with increasing k the curves damp down while at lower values of k, and especially at higher electron coupling, we observe sharp peaks also reported in the mentioned work. For lower electron coupling the dynamic structure factors of Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺ do not differ while at higher electron coupling these curves split. As the number of shell electrons increases from Li⁺ to Cs⁺ the curves shift in the direction of low absolute value of ω and their heights diminish. We conclude that the short range forces, which we take into account by means of the HGK model potential, which deviates from the Coulomb and Deutsch ones, influence the static and dynamic structure factors significantly.     

A novel PEO-based composite polymer electrolyte with NaAlOSiO molecular sieves powders

Ion-conducting thin film polymer electrolytes based on poly(ethylene oxide) (PEO) complexes with NaAlOSiO molecular sieves powders has been prepared by solution casting technique. X-ray diffraction, scanning electron microscopy, differential scanning calorimeter, and alternating current impedance techniques are employed to investigate the effect of NaAlOSiO molecular sieves on the crystallization mechanism of PEO in composite polymer electrolyte. The experimental results show that NaAlOSiO powders have great influence on the growth stage of PEO spherulites. PEO crystallization decrease and the amorphous region that the lithium-ion transport is expanded by adding appropriate NaAlOSiO, which leads to drastic enhancement in the ionic conductivity of the (PEO)₁₆LiClO₄ electrolyte. The ionic conductivity of (PEO)₁₆LiClO₄-12 wt.% NaAlOSiO achieves (2.370 ± 0.082) × 10⁻⁴ S · cm⁻¹ at room temperature (18 °C). Without NaAlOSiO, the ionic conductivity has only (8.382 ± 0.927) × 10⁻⁶ S · cm⁻¹, enhancing 2 orders of magnitude. Compared with inorganic oxide as filler, the addition of NaAlOSiO molecular sieves powders can disperse homogeneously in the electrolyte matrix without forming any crystal phase and the growth stage of PEO spherulites can be hindered more effectively.     

An ab initio molecular dynamics study of ionic conductivity in hexagonal lithium lanthanum titanate oxide La0.5Li0.5TiO3

To date, the fastest lithium ion-conducting solid electrolytes known are the perovskite-type ABO₃ oxide, with A = Li, La and B = Ti, lithium lanthanum titanate (LLTO) Li_3x La_{( 2 \mathord}

Study of optical properties and molecular structure of plasma polymerized diethylene glycol dimethyl ether

This paper deals with plasma polymerization processes of diethylene glycol dimethyl ether. Plasmas were produced at 150 mtorr in the range of 10 W to 40 W of RF power. Films were grown on silicon and quartz substrates. Molecular structure of plasma polymerized films and their optical properties were analyzed by Fourier transform infrared spectroscopy (FTIR) and ultraviolet-visible spectroscopy. The IR spectra show C–H stretching at 3000–2900 cm^-1, C=O stretching at 1730–1650 cm^-1, C–H bending at 1440–1380 cm^-1, C–O and C–O–C stretching at 1200–1000 cm^-1. The concentrations of C–H, C–O and C–O–C were investigated for different values of RF power. It can be seen that the C–H concentration increases from 0.55 to 1.0 au (arbitrary unit) with the increase of RF power from 10 to 40 W. The concentration of C–O and C–O–C decreases from 1.0 to 0.5 au in the same range of RF power. The refraction index increased from 1.47 to 1.61 with the increase of RF power. The optical gap calculated from absorption coefficient decreased from 5.15 to 3.35 eV with the increase of power. Due to its optical and hydrophilic characteristics these films can be applied, for instance, as glass lens coatings for ophthalmic applications.