Population Densities Of Excited States Research Articles

On the population density of the argon excited levels in a high power impulse magnetron sputtering discharge

Population densities of excited states of argon atoms in a high power impulse magnetron sputtering (HiPIMS) discharge are examined using a global discharge model and a collisional-radiative model. Here, the ionization region model (IRM) and the Orsay Boltzmann equation for electrons coupled with ionization and excited states kinetics (OBELIX) model are combined to obtain the population densities of the excited levels of the argon atom in a HiPIMS discharge. The IRM is a global plasma chemistry model based on particle and energy conservation of HiPIMS discharges. OBELIX is a collisional-radiative model where the electron energy distribution is calculated self-consistently from an isotropic Boltzmann equation. The collisional model constitutes 65 individual and effective excited levels of the argon atom. We demonstrate that the reduced population density of high-lying excited argon states scales with (p*)−6, where p* is the effective quantum number, indicating the presence of a multistep ladder-like excitation scheme, also called an excitation saturation. The reason for this is the dominance of electron impact processes in the population and de-population of high-lying argon states in combination with a negligible electron–ion recombination.

Application of molecular convergent close-coupling cross sections in a collisional radiative model for the triplet system of molecular hydrogen

Collisional radiative (CR) models for molecular hydrogen are of high relevance for performing qualitative and quantitative analysis of excited-state population densities measured in plasmas or predicting the dependence of plasma emission on parameter variations. Although the development of such models for H2 started decades ago, major uncertainties still exist regarding the most important set of input parameters, namely the cross sections for electron-impact excitation. The deviations between cross sections from different datasets are particularly pronounced in the energy region close to the threshold energy, strongly increasing the uncertainty of CR models applied to low-temperature plasmas. This paper presents experimental validation of a set of newly calculated non ro-vibrationally resolved electron-impact cross sections calculated for the triplet system of H2 using the molecular convergent close-coupling method in the adiabatic-nuclei formulation. These cross sections are implemented into a CR model based on the flexible solver Yacora. A first comparison of CR calculations with the different datasets to experimentally-determined population densities is performed at a planar ICP discharge for varying pressure (between 1 and 10 Pa) and RF power (between 700 and 1100 W). For the experimentally-accessible electron temperature and density range (2.5–10 eV and 1.8–3.3 × 1016 m−3, respectively), very good agreement between the model and experiment is obtained using the new data set, in contrast to previously used cross sections.

A collisional radiative model of hydrogen plasmas developed for diagnostic purposes of negative ion sources.

A collisional radiative model of low-pressure hydrogen plasmas is elaborated and applied in optical emission spectroscopy diagnostics of a single element of a matrix source of negative hydrogen ions. The model accounts for the main processes determining both the population densities of the first ten states of the hydrogen atom and the densities of the positive hydrogen ions H(+), H2 (+), and H3 (+). In the calculations, the electron density and electron temperature are varied whereas the atomic and molecular temperatures are included as experimentally obtained external parameters. The ratio of the Hα to Hβ line intensities is calculated from the numerical results for the excited state population densities, obtained as a solution of the set of the steady-state rate balance equations. The comparison of measured and theoretically obtained ratios of line intensities yields the values of the electron density and temperature as well as of the degree of dissociation, i.e., of the parameters which have a crucial role for the volume production of the negative ions.

Excited State Distributions of Hydrogen Atoms in the Microwave Discharge Hydrogen Plasma and the Effect of Electron Energy Probabilistic Function

To understand the essentiality of the electron energy distribution function in a low-pressure discharge plasma, an experimental study is carried out on the diagnostics of microwave discharge hydrogen plasma with its discharge pressure ~1 torr in a cylindrical quartz tube. The electron kinetic temperature and density are measured by a Langmuir double probe. Number densities of electronically excited states of hydrogen atoms are experimentally examined by an optical emission spectroscopic (OES) measurement of line intensities of the Balmer series. The rotational and vibrational temperatures are observed for the Fulcher-α band spectrum of hydrogen molecule to understand the approximate value to the neutral gas temperature. The number density of the ground state of hydrogen atom is also experimentally estimated from the actinometry measurement. The electron energy probabilistic function (EEPF) is numerically calculated as a solution to the Boltzmann equation. Number densities of excited hydrogen atoms are calculated with the collisional-radiative (CR) model with experimentally measured data as input parameters. It is found that the population densities of excited states of hydrogen atoms become about one order or much larger than those determined by OES measurement if we assume Maxwellian EEPF. The CR model with the EEPF as a solution to the Boltzmann equation theoretically reproduce the experimentally measured values very well.

Understanding Energy Transfer Mechanisms for Tunable Emission of Yb3+-Er3+ Codoped GdF3 Nanoparticles: Concentration-Dependent Luminescence by Near-Infrared and Violet Excitation

Energy transfer (ET) is an important route to manage the population density of excited states, giving rise to spectrally tunable emission that is valuable for multicolor imaging and biological tracking. In this paper, a case study of GdF3 nanoparticles (NPs) codoped with Yb3+ and Er3+ was used to experimentally and theoretically investigate the ET mechanisms under near-infrared and violet excitation. Red-to-green ratio (RGR) is used as a primary evaluating protocol, and the power-dependent luminescence and Er3+ 4I13/2 luminescence behavior are used to identify the corresponding conjectures about ET mechanisms. Compared with the four common upconversion (UC) models, a joint effect of energy-back-transfer, multiphonon relaxation, and linear decay depletion mechanisms for the Er3+ 4I13/2 manifold was proposed for the UC process based on UC spectra for samples with different dopant concentrations. Meanwhile, the varying RGR could also be observed from downshifting (DS) emission spectra. The ET mechanism for t...

A collisional radiative model for low-pressure hydrogen–caesium plasmas and its application to an RF source for negative hydrogen ions

A collisional radiative (CR) model for caesium in low-temperature, low-pressure hydrogen–caesium plasmas is introduced. This model includes the caesium ground state, 14 excited states, the singly charged caesium ion and the negative hydrogen ion. The reaction probabilities needed as an input are based on literature data, using some scaling and extrapolations. Additionally, new cross sections for electron collision ionization and three-body recombination have been calculated.The relevance of mutual neutralization of positive caesium ions and negative hydrogen ions is highlighted: depending on the densities of the involved particle species, this excitation channel can have a significant influence on the population densities of excited states in the caesium atom. This strong influence is successfully verified by optical emission spectroscopy measurements performed at the IPP prototype source for negative hydrogen ions.As a consequence, population models for caesium in electronegative low-temperature, low-pressure hydrogen–caesium plasmas need to take into account the mutual neutralization process. The present CR model is an example for such models and represents an important prerequisite for deducing the total caesium density in such plasmas.

Collisional-Radiative Model for Spectroscopic Diagnostic of Optically Thick Helium Plasma

We have included the effect of radiation trapping in a collisional-radiative model of neutral helium atoms developed by Goto [M. Goto, JQSRT 76, 331 (2003)], which is used to determine the electron temperature and density in plasmas from visible emission line intensities of atoms. In addition to the electron temperature and density, photo-excitation events from the ground state 11S to the 21P, 31P, and 41P states per second per one atom are treated as fitting parameters to reproduce the population density obtained by spectroscopic measurement. The model has been applied to an RF plasma at Shinshu University, Japan. The electron temperature and density and the contribution of radiation trapping to the population density of excited states are evaluated.

Excited level populations and excitation kinetics of nonequilibrium ionizing argon discharge plasma of atmospheric pressure

Population densities of excited states of argon atoms are theoretically examined for ionizing argon plasma in a state of nonequilibrium under atmospheric pressure from the viewpoint of elementary processes with collisional radiative model. The dependence of excited state populations on the electron and gas temperatures is discussed. Two electron density regimes are found, which are distinguished by the population and depopulation mechanisms for the excited states in problem. When the electron impact excitation frequency for the population or depopulation is lower than the atomic impact one, the electron density of the plasma is considered as low to estimate the population and depopulation processes. Some remarkable characteristics of population and depopulation mechanisms are found for the low electron density atmospheric plasma, where thermal relaxation by atomic collisions becomes the predominant process within the group of close-energy states in the ionizing plasma of atmospheric pressure, and the excitation temperature is almost the same as the gas temperature. In addition to the collisional relaxation by argon atoms, electron impact excitation from the ground state is also an essential population mechanism. The ratios of population density of the levels pairs, between which exists a large energy gap, include information on the electron collisional kinetics. For high electron density, the effect of atomic collisional relaxation becomes weak. For this case, the excitation mechanism is explained as electron impact ladderlike excitation similar to low-pressure ionizing plasma, since the electron collision becomes the dominant process for the population and depopulation kinetics.

Formation of excited states of atoms and ions in laser plasma from CuInS2

Emission spectra and the energy distribution of the excited-state population density of atoms and ions in erosion laser plasma from CuInS2 with various crystal-structure orderings are analyzed. It is shown that increased ordering of the target crystal structure causes the excited-state energies of indium atoms generated in the laser erosion plume to increase and that sulfur atoms always emit only in transitions from highly excited states. The ratio of relative ion concentrations in the laser plasma plume is Cu+/In+/S+ = 0.3/0.08/2, which corresponds neither to the atomic ratio of Cu/In/S (1/1/2) in the target nor to the ratio of ionization energies. The results are explained by recombination processes for ions and by the atomization specifics of the CuInS2 target exposed to long-wavelength radiation. The atomization consists essentially of dissociative processes expressed by CuInS2 → CuInS + S and CuInS2 → Cu + InS + S. The electron temperature of polycrystal (single-crystal) plasma at a distance of 1 mm from the target is 0.3 eV (0.4 eV) for atoms and 1.3 eV (2.7 eV) for ions and varies negligibly for plasma up to a distance of 7 mm from the target.

Optical actinometry of the N-atom density in nitrogen plasma

Trace-rare-gas optical actinometry is used to measure the N-atom density in 50 Hz pulsed dc nitrogen plasma as functions of the discharge parameters. The excited-state population density of N atoms is extracted from their respective optical transitions and is related to the ground-state population density by considering the changes in the electron energy distribution function. This technique provides reliable information on the atomic N concentration in the gas discharge, which is vital in the synthesis of nitrides owing to its high chemical reactivity.

Light amplification at 501nm and large nanosecond optical gain in organic dye-doped polymeric waveguides

We report on the results of a gain measurement on a substituted stilbene-dye-doped polymer waveguide operating in the blue-green spectral region. By monitoring amplified spontaneous emission from the waveguides under nanosecond photopumping, we deduced net modal gain as large as 84±6cm−1 at 501nm, the peak of the gain spectrum, when pumped with a fluence of 1.1mJ∕cm2. Moreover, the effective stimulated emission cross section of 2.7×10−16cm2 and the loss coefficient of 0.3cm−1 at 501nm are extracted. The transparency pump fluence and the corresponding excited state population density are found to be 17μJ∕cm2 and 8.8×1015cm−3 which is approximately 0.1% of total population, respectively. Comparison is made with other organic and inorganic gain media operating in the short wavelength (violet-green) region of the spectrum. We demonstrate that small-molecule-doped polymers show large gain, comparable with those in conjugated polymers and dendrimers.

Diagnostics of nitrogen plasma by trace rare-gas–optical emission spectroscopy

Trace rare-gas–optical emission spectroscopy is carried out to characterize the nitrogen plasma as a function of discharge parameters. The functional dependence of N2(CΠu3) and N2+(BΣu+2) excited states is monitored by measuring the emission intensities of the bandheads of second positive and first negative systems. The excited-state population density of N atoms and N2 molecules, extracted from their optical emission, is related to the ground-state population density after normalizing the changes for excitation cross section and electron energy distribution function by optical actinometry. The electron temperature is determined from the plasma-induced optical emission of trace rare gas by the line-to-line method. The obtained data may help us to adjust the optimum discharge conditions for the production of active species, which are considered to be important for the desired treatment of the samples.

On line-ratio analysis for helium–argon microwave discharges

Regarding spectroscopy diagnostics via line-intensity ratio of high-density diffusion-controlled discharges in helium–argon gas mixtures, the study presents a collisional-radiative model of the discharge. In order to show variations of excited-state population densities and line-intensity ratios related to changes of plasma density and electron temperature, the model is applied to surface-wave-sustained discharges, since the axial structure of these discharges displays self-consistent variations of the discharge characteristics. The comparison with the corona model shows the necessity of developing a collisional-radiative model. Possibilities for simultaneous determination—based on the collisional-radiative model—of electron temperature and plasma density from measurements of line-intensity ratios are outlined.

Phase-resolved emission spectroscopy of a hydrogen rf discharge for the determination of quenching coefficients

Collisional effects can have strong influences on the population densities of excited states in gas discharges at elevated pressure. The knowledge of the pertinent collisional coefficient describing the depopulation of a specific level (quenching coefficient) is, therefore, important for plasma diagnostics and simulations. Phase-resolved optical emission spectroscopy (PROES) applied to a capacitively coupled rf discharge excited with a frequency of 13.56 MHz in hydrogen allows the measurement of quenching coefficients for emitting states of various species, particularly of noble gases, with molecular hydrogen as a collision partner. Quenching coefficients can be determined subsequent to electron-impact excitation during the short field reversal phase within the sheath region from the time behavior of the fluorescence. The PROES technique based on electron-impact excitation is not limited---in contrast to laser techniques---by optical selection rules and the energy gap between the ground state and the upper level of the observed transition. Measurements of quenching coefficients and natural fluorescence lifetimes are presented for several helium $(3{}^{1}S,4{}^{1}S,3{}^{3}S,3{}^{3}P,4{}^{3}S),$ neon ${(2p}_{1}{,2p}_{2}{,2p}_{4}{,2p}_{6}),$ argon ${(3d}_{2}{,3d}_{4}{,3d}_{1}^{\ensuremath{'}}$ and ${3d}_{3}),$ and krypton ${(2p}_{1}{,2p}_{5})$ states as well as for some states of the triplet system of molecular hydrogen.

Nonequilibrium excited-state distribution of atoms in dense mercury plasmas

The nonequilibrium excited-state distribution of mercury atoms up to the ionization limit in high-pressure discharges is deduced through comparison of the actual level-population densities with those calculated from the Saha equation at the plasma-electron temperature. Emission data obtained from two ac (50 Hz) mercury discharges at 2 and 5 bar are used to deduce the absolute excited-state population densities and the electron temperature. It is found that at the maximum emission phase (5 ms), the excited states deviate from the Saha equilibrium, the levels are overpopulated, the overpopulation decreases with the level-excitation energy, the deviation is larger for the higher-pressure discharge, and the two discharges are closer to the equilibrium at the minimum emission phase (0.5 ms). Possible causes of the observed deviations from the Saha equilibrium are suggested. The plasma temperature and densities are also found assuming equilibrium conditions and compared with the results obtained initially.

Non-equilibrium and equilibrium problems in laser-induced plasmas

Several problems which can determine non-equilibrium effects in laser-induced breakdown plasmas are analysed in this study. In particular, we focus our attention on problems associated with the fluid dynamics of the expanding plume, with time-dependent collisional-radiative models for describing the population densities of excited states, with the time-dependent Boltzmann equation for characterizing the electron energy distribution function in laser-induced breakdown spectroscopy (LIBS) plasmas. The results show that these problems should be carefully taken into account for developing a calibration-free LIBS methodology. Finally, problems associated with equilibrium plasmas, and in particular the dependence of the partition function on the particular environment surrounding the different components of a plasma are discussed.

X-ray spectrum of Be-like ions in recombining plasmas

X-ray spectra of Be-like Fe and Ne ions are studied theoretically. We have constructed a collisional-radiative model to calculate population densities of excited states of Be-like Fe and Ne ions, taking into account the recombination processes. The dielectronic recombination rate coefficient of each excited state is calculated and included in the model. Dielectronic recombination and radiative cascades play important roles for Be-like ions in recombining plasmas. Using the model we have calculated the line intensities as functions of electron temperature and density. The results can be applied to diagnose various plasmas.

Spectroscopic Techniques for the Characterisation of Spectrochemical Plasma Sources

In spectrochemistry, various plasma sources are used for the analysis of samples. The simplest way to characterise these plasma sources is to determine the absolute intensities of several atomic lines of the main plasma gas (argon or helium). From these measurements absolute population densities of excited states can be deduced and, via a so-called Boltzmann plot, the electron temperature Te and density ne. This method works quite well as long as the plasma is relatively close to equilibrium. The reason why spectrochemical plasmas can show strong deviations from equilibrium will be discussed. An other limitation of emission spectroscopy is the lack of accurate transition probabilities for transitions from highly excited states. For argon 15 of these probabilities have been determined with an uncertainty of ±25%.

Study of the discrete-to-continuum transition in a Balmer spectrum from Alcator C-Mod divertor plasmas

Under detached plasma conditions in the Alcator C-Mod tokamak, the measured spectra show pronounced merging of the Balmer series lines and a photo-recombination continuum edge which is not a sharp step. This phenomenon, known as a smooth discrete-to-continuum (D-C) transition, is typical only for high-density , low-temperature (Te≈1 eV), and recombining plasma. As we will discuss, this type of transition emphasizes the degree of plasma non-ideality. A theoretical model capable of treating spectra from a detached divertor plasma, and those with a smooth D-C transition in particular, has been developed. It is comprised of three parts: (i) a collisional-radiative model for the population densities of the excited states, (ii) atomic structure and collision rates for an atom affected by statistical plasma microfields, and (iii) a model for calculating the line profiles and the extended photo-recombination continuum. The effects of statistical plasma microfields on the population densities of excited states, on the profiles of Balmer series lines, and on the photo-recombination continuum edge are discussed. The changes in spectrum characteristics with plasma parameter variation, leading to the smooth D-C transition, are analysed. The relevance of volumetric plasma recombination to the spectra observed from a detached divertor plasma is discussed. A comparison of the calculated and measured spectra is used to determine the plasma parameters in the recombining plasma region. Along with other properties of measured spectrum, the smooth D-C transition provides evidence in support of the recombining state of the plasma attained under detached divertor conditions.

Effect of excited-state population density on nonradiative multiphonon relaxation rates of rare-earth ions

Multiphonon relaxation rate saturation is observed for rare-earth-doped glasses for high excited-state densities. This behavior is analyzed in a statistical approach; a theoretical model for the microscopic process is proposed. A phonon bottleneck effect on radiationless relaxation related to an accepting-modes saturation is suggested. At high excited-state density, ions in a phonon diffusion volume simultaneously fill the accepting modes. The critical distance below which excited ions share a common phonon bath is related to the phonon diffusion length in the host and deduced from our model. The results are in good agreement with the phonon mean free path independently deduced from sound velocity, thermal conductivity, and heat capacity.