Paschen Notation Research Articles

Investigation on the kinetics of a diode-pumped plasma-jet type metastable argon laser

Diode-pumped rare gas lasers (DPRGLs) have the potential for high-energy laser output with high beam quality. The metastable concentration >1014 cm−3 and pump intensity >10 kW/cm2 are required in developing high-power and efficient DPRGLs. However, the kinetics of 1s and 2p levels (Paschen notation) under these conditions is unclear yet, particularly regarding the adverse effects of interference energy levels (e.g., 1s4 and 2p1–2p8). This study addresses this gap by measuring the number densities of 1s and 2p levels in a diode-pumped plasma jet metastable argon laser. The experiment was conducted with a much higher number density of 1014 cm−3 and a higher pump intensity of 12 kW/cm2 than our previous works. The results indicate that the adverse effect of 1s4 accumulation could be partially compensated, and the relative population distribution of 2p energy levels remained unchanged. However, the high gas temperature (>1000 K) caused a significant negative impact of high 2p levels (2p1–2p8), and the air environment weakened the effect of He collision relaxation.

Ultra-fast development of Ar 2p relative densities in nanosecond-pulsed barrier discharge in argon

A time-correlated single photon counting based optical emission spectroscopy technique is applied for an investigation of the nanosecond-pulsed barrier discharge in atmospheric pressure argon gas. We record the development of the light intensities originating from all ten Ar 2p (Paschen notation) radiative states with high spatio-temporal resolution. We identify different stages of the discharge development: an early rapid electron avalanching, streamer propagation phase, generation of discharge channel and surface streamer propagation over the dielectric surface. The quantified relative 2p densities for identified discharge phases enable a well-resolved insight into the ultra-fast discharge kinetics and open a possibility for the development of future detailed diagnostics of the rapidly ionized argon plasmas. Moreover, the electrical characterization of the discharge is made using the simplest equivalent circuit and it reveals new findings for the application of such an approach for similar discharges.

Kinetics of Metastable Argon Optical Excitation and Gain in Ar/He Microplasmas.

The optically pumped rare-gas metastable laser is capable of high-intensity lasing on a broad range of near-infrared transitions for excited-state rare gas atoms (Ar*, Kr*, Ne*, Xe*) diluted in flowing He. The lasing action is generated by photoexcitation of the metastable atom to an upper state, followed by collisional energy transfer with He to a neighboring state and lasing back to the metastable state. The metastables are generated in a high-efficiency electric discharge at pressures of ∼0.4 to 1 atm. The diode-pumped rare-gas laser (DPRGL) is a chemically inert analogue to diode-pumped alkali laser (DPAL) systems, with similar optical and power scaling characteristics for high-energy laser applications. We used a continuous-wave linear microplasma array in Ar/He mixtures to produce Ar(1s5) (Paschen notation) metastables at number densities exceeding 1013 cm-3. The gain medium was optically pumped by both a narrow-line 1 W titanium-sapphire laser and a 30 W diode laser. Tunable diode laser absorption and gain spectroscopy determined Ar(1s5) number densities and small-signal gains up to ∼2.5 cm-1. Continuous-wave lasing was observed using the diode pump laser. The results were analyzed with a steady-state kinetics model relating the gain and the Ar(1s5) number density.

Extended reaction kinetics model for non-thermal argon plasmas and its test against experimental data

An extended reaction kinetics model (RKM) suitable for the analysis of weakly ionised, non-thermal argon plasmas with gas temperatures around 300 K at sub-atmospheric and atmospheric pressures is presented. It considers 23 different species including electrons as well as the ground state atom, an atomic and molecular ion, four excited molecular states, and 15 excited atomic states of argon, where all individual 1s and 2p states (in Paschen notation) are included as a separate species. This 23-species RKM involves 409 collision processes and radiative transitions and recent electron collision cross section data. It is evaluated by means of results of time- and space-dependent fluid modelling of argon discharges and their comparison with measured data for two different dielectric barrier discharge configurations as well as a micro-scaled atmospheric-pressure plasma jet setup. The results are also compared with those obtained by use of a previously established 15-species RKM involving only the two lumped 2p states 2p10…5 and . It is found that the 23-species RKM shows generally better agreement with experimental data and provides more options for direct comparison with measurements than the frequently used 15-species RKM.

Optically enhanced discharge excitation and trapping of Ar39

We report on a two-fold increase of the $^{39}Ar$ loading rate in an atom trap by enhancing the generation of metastable atoms in a discharge source. Additional atoms in the metastable $1s_5$ level (Paschen notation) are obtained via optically pumping both the $1s_4$ - $2p_6$ transition at 801 nm and the $1s_2$ - $2p_6$ transition at 923 nm. By solving the master equation for the corresponding six-level system, we identify these two transitions to be the most suitable ones and encounter a transfer process between $1s_2$ and $1s_4$ when pumping both transitions simultaneously. We calculate the previously unknown frequency shifts of the two transitions in $^{39}Ar$ and confirm the results with trap loading measurements. The demonstrated increase in the loading rate enables a corresponding decrease in the required sample size, uncertainty and measurement time for $^{39}Ar$ dating, a significant improvement for applications such as dating of ocean water and alpine ice cores.

Role of excimer formation and induced photoemission on the Ar metastable kinetics in atmospheric pressure Ar–NH3 dielectric barrier discharges

Tunable diode laser absorption spectroscopy was used to record the space-and time-resolved number density of argon metastable atoms, Ar(1s3) (Paschen notation), in plane-to-plane dielectric barrier discharges (DBDs) operated in a Penning Ar–NH3 mixture at atmospheric pressure. In both low-frequency (LF 650 V, 50 kHz) discharges and dual LF–radiofrequency (RF 190 V, 5 MHz) discharges operated in α–γ mode, the density of Ar(1s3) revealed a single peak per half-period of the LF voltage, with rise and decay times in the sub-microsecond time scale. These results were compared to the predictions of a 1D fluid model based on continuity and momentum equations for electrons, argon ions (Ar+ and Ar2 +) and excited argon 1s atoms as well electron energy balance equation. Using the scheme commonly reported for Ar-based DBDs in the homogeneous regime, the Ar metastable kinetics exhibited much slower rise and decay times than the ones seen in the experiments. The model was improved by considering the fast creation of Ar2 * excimers through three-body reactions involving Ar(1s) atoms and the rapid loss of Ar2 * by vacuum ultraviolet light emission. In optically thin media for such photons, they can readily reach the dielectric barriers of the DBD electrodes and induce secondary electron emission. It is shown that Ar2 * and photoemission play a significant role not only on the Ar metastable kinetics, but also on the dominant ionization pathways and possible α–γ transition in dual frequency RF–LF discharges.

Диэлектрический барьерный разряд в смеси He-Ne низкого давления. Спектроскопия послесвечения на переходах 2p-=SUP=-5-=/SUP=-5s-> 2p-=SUP=-5-=/SUP=-3p

The results of a spectroscopic study and simulation of the processes of population and destruction of the levels of the 2p55s configuration of a neon atom in a decaying plasma of a low-frequency barrier discharge in a He-Ne mixture are presented. Experimental conditions: helium pressure 0.08-22 Torr, neon pressure ≤ 3mTorr, electron density less than 1011 cm-3. On the basis of data on the evolution of the populations of the 3si levels (in Paschen's notation) with a change in helium pressure, data on the rate constants of collisional processes that determine the kinetics of these levels in He-Ne plasma are obtained more accurate than those available in the literature.

Dielectric barrier discharge in a low-pressure He-Ne mixture. Afterglow spectroscopy of 2p-=SUP=-5-=/SUP=-5s-> 2p-=SUP=-5-=/SUP=-3p transitions

The results of a spectroscopic study and simulation of the processes of population and destruction of the levels of the 2p55s configuration of a neon atom in a decaying plasma of a low-frequency barrier discharge in a He-Ne mixture are presented. Experimental conditions: helium pressure 0.08-22 Torr, neon pressure ≤ 3 mTorr, electron density less than 1011 cm-3. On the basis of data on the evolution of the populations of the 3si levels (in Paschen's notation) with a change in helium pressure, data on the rate constants of collisional processes that determine the kinetics of these levels in He-Ne plasma are obtained more accurate than those available in the literature. Keywords: dielectric barrier discharge, helium-neon plasma, excitation transfer, afterglow, inelastic collisions.

Development of a lumping methodology for the analysis of the excited states in plasma discharges operated with argon, neon, krypton, and xenon

In this paper, a methodology is presented to compute the plasma properties (e.g.,, density and temperature) accounting for the dynamics of the excited states. The proposed strategy applies to both zero-dimensional (0D) models and multidimensional fluid and hybrid codes handling low-pressure (<50 mTorr) plasma discharges filled with argon, neon, krypton, and xenon gases. The paper focuses on two main aspects: (i) a lumping methodology is proposed to reduce the number of reactions and species considered in order to keep at bay the computational cost without a major loss of accuracy; (ii) the influence that different datasets of cross sections have on the results has been assessed. First, the lumping methodology has been implemented in a 0D model accounting for singly charged ions, neutrals, along with 1s and 2p excited states (Paschen notation). Metastable and resonant are treated as two separate species within the 1s energy level (1sM and 1sR, respectively). The results have been benchmarked against those obtained treating each energy level of the excited states as an individual species. Differences lower than 1% have been obtained. Second, the results of the 0D model have been compared against measurements of electron density and temperature performed on an inductively coupled plasma. Numerical predictions and experiments present a disagreement up to 20%–30%, which is comparable to the uncertainty band of the measurements. Finally, the lumping strategy has been implemented in a 2D fluid code to assess its computational affordability, and the results have been compared against the experiments as well. A variance up to 30% in electron density and temperature is registered adopting different datasets of cross sections.

Ultra-high-resolution optical absorption spectroscopy of DC plasmas at low pressure using a supercontinuum laser combined with a laser line tunable filter and a HyperFine spectrometer

Optical absorption spectroscopy of non-equilibrium plasmas using a supercontinuum laser combined with a laser line tunable filter and a HyperFine spectrometer is examined. Owing to the <2 pm spectral resolution of the HyperFine system, details on the absorption line intensity, position and width can be obtained with very high precision using a broadband light emission source. As an example, absorption spectroscopy measurements were recorded in a reduced-pressure, nominally pure argon DC plasma column and allowed for the determination of the neutral gas temperature and the Ar 1s2 and Ar 1s4 (Paschen notation) number densities. At a pressure of 1 Torr, a discharge current of 20 mA, and an absorption length of 18 cm, the analysis of the Ar 2p3-1s2 and Ar 2p8-1s4 absorption lines at respectively 840.8 nm and 842.5 nm resulted in number densities of 5.5 1015 m−3 and 1.1 1016 m−3 and a neutral gas temperature of T g = 340 K. These values are typical of DC discharges operated under similar experimental conditions. While the fractional absorption of both lines decreased with decreasing optical absorption path, the number densities remained the same, as expected. Finally, the number densities increased with increasing discharge current, a result also coherent with the literature.

Enhancement of the Kr81 and Kr85 count rates by optical pumping

We report an increase of up to 60% on the count rates of the rare $^{81}\mathrm{Kr}$ and $^{85}\mathrm{Kr}$ isotopes in the Atom Trap Trace Analysis method by enhancing the production of metastable atoms in the discharge source. Additional atoms in the metastable $ 1s_5 $ level (Paschen notation) are obtained via optically pumping the $1s_4-2p_6$ transition at 819 nm. By solving the master equation for the system, we identify this transition to be the most suitable one and can describe the measured increase in metastable population as a function of the 819-nm laser power. We calculate the previously unknown isotope shifts and hyperfine splittings of the $1s_4-2p_6$ transition in $^{81}\mathrm{Kr}$ and $^{85}\mathrm{Kr}$, and verify the results with count rate measurements. The demonstrated count-rate increase enables a corresponding decrease in the required sample sizes for $^{81}\mathrm{Kr}$ and $^{85}\mathrm{Kr}$ dating, a significant improvement for applications such as dating of ocean water and deep ice cores.

Influence of N2, O2, and H2 admixtures on the electron power balance and neutral gas heating in microwave Ar plasmas at atmospheric pressure

A combination of optical emission and absorption spectroscopy of argon 2p –1s transitions (Paschen notation) combined with collisional-radiative (CR) modelling of argon 2p states was used to characterize microwave argon plasmas at atmospheric pressure in presence of N2, O2, and H2 admixtures. In particular, the neutral gas temperature (obtained from the broadening of argon 2p 2–1s2 and 2p 3–1s2 emission lines), the number density of argon 1s5 atoms (obtained from absorption spectroscopy of the argon 2p 9–1s5 transition using a tunable laser diode), the electron temperature (obtained from the comparison between measured and simulated argon 2p -to-1s relative line emission intensities), and the electron density (obtained from the Stark broadening of the Hβ line and argon relative line emission intensities) were recorded as a function of the axial distance along the microwave plasma column. The results show that, for a given position in the plasma and a higher amount of admixture in the nominally pure argon plasma, the neutral gas temperature increases and the electron number density decreases, while the electron temperature and the population of argon metastable atoms first decreases and then increases at higher concentrations. With such information, a detailed analysis of the electron power balance was performed. It is found that less than 1% of the admixture in the argon plasma already absorbed more than 80% of the microwave power. Part of this energy is used for neutral gas heating, mostly through electron-impact excitation of rotational levels.

Characterization of a microwave argon plasma column at atmospheric pressure by optical emission and absorption spectroscopy coupled with collisional-radiative modelling

Optical emission and absorption spectroscopy of argon 2p-1s transitions (Paschen notation) combined with collisional-radiative (CR) modeling of argon 2p states are developed and used to determine the neutral gas temperature, the Ar 1s number density, and the electron temperature along a microwave argon plasma column at atmospheric pressure. The CR model, designed specifically for atmospheric-pressure and optically thick plasma conditions, is fully detailed and validated by comparing the relative line emission intensities of argon 2p-to-1s transitions measured experimentally with the ones predicted by the CR model using the electron temperature as the only adjustable parameter. Subsequently, the neutral gas temperature (∼1300–1600 K; obtained from the broadening of argon 2p2-1s2 and 2p3-1s2 emission lines), the Ar 1s5 number density (1–2 × 1018 m−3; obtained from absorption spectroscopy of the argon 2p9-1s5 transition using a tunable laser diode), and the electron temperature (∼1.4 eV; obtained from the comparison between the measured and simulated 2p-to-1s emission line intensities) are reported as a function of the axial distance along the microwave plasma column. The values and behaviors reveal a good agreement with those reported in previous experimental and modeling studies.

Dissociation Energy and Dissociative Recombination of $${\text{Ne}}_{2}^{ + }$$ and HeNe+ Ions

We present the results of our study of a weakly ionized decaying helium plasma with a small admixture of neon by means of kinetic spectroscopy. The experimental conditions are a helium pressure of 10–38 Torr, an electron density [e] ≤ 2 × 1011 cm–3, and a neon density [Ne] ≤ 2 × 10–4 [He]. The recombination processes of $${\text{Ne}}_{2}^{ + }$$ , $${\text{He}}_{2}^{ + }$$ , HeNe+, and Ne+ ions with electrons have been identified by the decaying-plasma radiation. The neon afterglow spectrum at [e] ≤ 1011 cm–3 is shown to be formed mainly by dissociative recombination of $${\text{Ne}}_{2}^{ + }$$ and HeNe+ ions in the ground vibrational state $${v}$$ = 0. The early afterglow of a number of atomic neon lines is formed by the well-known process of excitation transfer from helium atoms in metastable states He(23S1, 21S0). At [e] > 1011 cm–3, the afterglow spectrum is enriched due to the collisional-radiation recombination of Ne+ ions. In the recombination stage of the afterglow at room electron temperature, the existence of a clear upper bound on the excitation energy of the atomic neon levels being populated is characteristic of the dissociative recombination of both molecular ions. For $${\text{Ne}}_{2}^{ + }$$ this is the 3p1 level (in Paschen notation) with an energy of 20.369 eV, with the levels closest to $${\text{Ne}}_{2}^{ + }$$ ( $${v}$$ = 0) in energy being 3p3 and 3p2 with energies of 20.26 and 20.3 eV, respectively; for HeNe+, this is the 5 $$d_{1}^{'}$$ level with an energy of about 21.02 eV. These values are proposed to be taken as the energies of $${\text{Ne}}_{2}^{ + }$$ and HeNe+ ions in the ground vibrational state.

Диссоциативная рекомбинация молекулярных ионов Ne-=SUB=-2-=/SUB=--=SUP=-+-=/SUP=- с электронами. Заселение атомов конфигурации 2p-=SUP=-5-=/SUP=-4p в распадающейся плазме

The paper presents the results of the study of decaying neon plasma by the method of kinetic spectroscopy. Experimental conditions: neon pressure 0.2–152 Torr, electron density in the initial stage of decay [e] ≤ 5 × 1010 cm-3. The plasma was created by a pulsed barrier discharge with electrodes on the outer surface of a cylindrical glass tube. Discharge frequency 40–160 Hz. On the basis of a comparative analysis of the dependences of the intensities of spectral lines on the time and temperature of electrons in the afterglow, it was shown that in terms of the mechanisms of population, the levels of 2p54p are clearly divided into two groups. Lower levels: from 3p10 to 3p3 (Paschen notation) are associated with the dissociative recombination of molecular ions with electrons and, up to pressures of 0.6 Torr, the lines emitted by them behave identically to the lines of 2p53s ← 2p53p and 2p53p ← 2p53d transitions. The behavior of the upper levels (3p5, 3p2, 3p4, and 3p1) was more complex, and at low pressures, the populations of all 2p54p levels turned out to be related to the impact-radiation recombination of Ne + ions. Significant differences in the dependences of the relative populations of 3d and 4p levels on the neon pressure are discussed.

Interatomic potentials for ground and excited states of Ar+He.

The potential energy curves (PECs) of the ground and excited states that correlate in the atomic limit with Ar([Ne]3s 23p 6,1S), Ar([Ne]3s 23p 54s 1, 3P, 1P), and Ar([Ne]3s 23p 54p 1, 3D, 3P, 3S, 1D, 1P, 1S) are calculated at the multireference configuration interaction (MRCI+Q) theoretical level with extrapolations to the complete basis set limit using all-electron correlation consistent triple-, quadruple-, and quintuple-zeta basis sets. Scalar relativistic corrections are calculated using second-order Douglas-Kroll-Hess Hamiltonian with the corresponding basis sets contracted for scalar relativistic Hamiltonians. For these calculations, the 3s orbitals of the Ar atom are not included in the active space but are correlated through single and double excitations. Spin-orbit eigenstates are computed by diagonalizing the Breit-Pauli matrix between internal configurations with no electrons in external orbitals and added to the scalar relativistic results. A total of 32 molecular PECs are computed with spin-orbit contributions, which correlate with 1s1, 1s5-2, and 2p10-1 atomic Ar energies in Paschen notation. Important features of the PECs and system crossings are discussed.

Modeling of a dual-wavelength pumped metastable argon laser

Optically pumped metastable argon laser is an attractive research topic of innovative gas lasers, but the slow collisional relaxation rates of 1s4–1s5 (in Paschen notation) may form a bottleneck on the cycling of active atoms and decrease the laser output at room temperature. Here, by employing a method of a dual-wavelength pump, we demonstrate the removal of accumulation on the 1s4 level and the improvement of output power in the simulation. The simulated results show that a large increase in laser output is possible with a relatively weak assistant pump intensity. This method offers a feasible way to scale the laser to higher gain.

Argon metastable production in argon-helium microplasmas

Microwave resonator-driven microplasmas are a promising technology for generating the high density of rare-gas metastable states required for optically pumped rare gas laser systems. We measure the density of argon 1s5 states (Paschen notation) in argon-helium plasmas between 100 Torr and atmospheric pressure using diode laser absorption. The metastable state density is observed to rise with helium mole fraction at lower pressures but to instead fall slightly when tested near atmospheric pressure. A 0-D model of the discharge suggests that these distinct behaviors result from the discharge being diffusion-controlled at lower pressures, but with losses occurring primarily through dissociative recombination at high pressures. In all cases, the argon metastable density falls sharply when the neutral argon gas fraction is reduced below approximately 2%.

Cyclic evolution of the electron temperature and density in dusty low-pressure radio frequency plasmas with pulsed injection of hexamethyldisiloxane

Optical emission spectroscopy was used to analyze the very-low-frequency cyclic evolution of the electron energy and density caused by repetitive formation and loss of dust nanoparticles in argon plasmas with pulsed injection of hexamethyldisiloxane (HMDSO, [CH3]6Si2O). After elaborating a Boltzmann diagram for Ar high-lying levels and a collisional-radiative model for Ar 2p (Paschen notation) states, temperatures characterizing the low- and high-energy parts of the electron population were calculated. Relative electron densities were also estimated from relative line emission intensities. Both temperatures increase when the dust occupation increases, and then decrease when dust is lost. The opposite trend was observed for the electron density. Such cyclic behaviors of the electron energy and electron density in the HMDSO-containing plasmas are in good agreement with the evolution processes in dusty plasmas, in which the formation of negative ions followed by an electron attachment on the surfaces of the nanoparticles is a critical phenomenon driving dust growth.

Resonance broadening of argon lines in a micro-scaled atmospheric pressure plasma jet (argon μAPPJ)

Optical emission from atmospheric pressure micro-jet operating with pure argon (argon μAPPJ) flow has been detected with a moderate resolution spectrometer. Large broadening of the several argon (Ar) lines has been observed in the near infrared spectral region. This effect was attributed to resonance broadening of the s2 (Paschen notation) level in 3p54s configuration. In the present work, corresponding line profiles are suggested for plasma diagnostics. For this, a general case of resonance broadening coefficient of noble gases is discussed. As broadening reflects the Ar density, and the static gas pressure of the jet is in equilibrium with the ambient, the local gas temperature can be inferred. An estimation of gas temperature from the width of the 750 nm Ar line is in agreement with rotational temperature of OH radicals determined from the A2Σ+ → X2Π (0, 0) band. At low temperatures (300–600 K) and at partial Ar pressure near atmospheric, the resonance width of the suggested lines is very sensitive to small temperature variations. High temperature sensitivity and large width make the resonance broadened lines very attractive for diagnostics of low temperature discharges at elevated pressure, e.g., as they are used in plasma-medicine.