Total Ionization Rate Research Articles

Numerical simulation of dynamics behavior of pre-ionized pulsed-DC helium plasma jets at atmospheric pressure

A two-dimensional axisymmetric fluid model was established to investigate the dynamic behavior of pre-ionized pulsed-direct-current helium plasma jets at atmospheric pressure. Our simulation results show that, at a relatively low pre-ionization level, the electron number density is reduced and the streamer propagation is decelerated before the plasma jet is ejected from the tube, which is attributed to the inhibitory effect of a recombination process between the positive ions in the streamer and the seed electrons near the anode. As the pre-ionization reaches a relatively high level, the electron number density is larger than that without pre-ionization before the plasma jet is ejected from the tube, which originates from the promotion effect of decreased breakdown voltage. These two competing mechanisms jointly dominate the dynamic behavior of gas discharge in the presence of pre-ionization. After the plasma jet is ejected from the tube, the enhanced discharge power is responsible for the strengthened electric field in the streamer head, augmented total ionization rate, accelerated streamer propagation, and increased number density of electrons and active species, whatever the pre-ionization density is. With the increase in pre-ionization density, the plasma jet length, streamer propagation speed, discharge power, and discharge energy exhibit the initial increase and subsequent decrease variation trend. The optimal enhancement effect is obtained at the pre-ionization density of 6 × 1012 m−3, with the plasma jet lengthened by 28.4% and the energy deposition efficiency enhanced by 28.1%.

The Astrochemistry Low-energy Electron Cross-Section (ALeCS) database

Context. Electron–molecule interaction is a fundamental process in radiation-driven chemistry in space, from the interstellar medium to comets. Therefore, knowledge of interaction cross sections is key. There have been a plethora of both theoretical and experimental studies of total ionization cross sections spanning from diatomics to complex organics. However, the data are often spread over many sources or are not public or readily available. Aims. We introduce the Astrochemistry Low-energy Electron cross-section (ALeCS) database. This is a public database for electron interaction cross sections and ionization rates for molecules of astrochemical interest. In particular, we present here the first data release, comprising total ionization cross sections and ionization rates for over 200 neutral molecules. Methods. We include optimized geometries and molecular orbital energies at various levels of quantum chemistry theory. Furthermore, for a subset of the molecules, we have calculated ionization potentials. We computed the total ionization cross sections using the binary-encounter Bethe model and screening-corrected additivity rule, and we computed ionization rates and reaction network coefficients for molecular cloud environments. Results. We present the cross sections and reaction rates for >200 neutral molecules ranging from diatomics to complex organics, with the largest being C14H10. We find that the screening-corrected additivity rule cross sections generally significantly overestimate experimental total ionization cross sections. We demonstrate that our binary-encounter Bethe cross sections agree well with experimental data. We show that the ionization rates scale roughly linearly with the number of constituent atoms in the molecule. Conclusions. We introduce and describe the public ALeCS database. For the initial release, we include total ionization cross sections for >200 neutral molecules and several cations and anions calculated with different levels of quantum chemistry theory, the chemical reaction rates for the ionization, and network files in the formats of the two most popular astrochemical networks: the Kinetic Database for Astrochemistry, and UMIST. The database will be continuously updated for more molecules and interactions.

Numerical study on dust particle charging and dynamics in continuous and pulsed radio frequency argon discharges

AbstractCharging and dynamics of a spherical dust grain injected into a continuous and pulsed radio frequency (RF) discharge have been studied using a one‐dimensional fluid model. First, the plasma characteristics of the two types of discharges are computed and compared. In the pulsed discharge, it is found that the central electron density exhibits a periodic variation while the averaged electron density is lower compared to that in the continuous discharge due to the decrease in the total ionization rate. Further, the dust charge is computed using the plasma characteristics. It is found that the dust charge negatively increases as the duty cycle ratio increases. Also, the charge in the pulsed discharge is lower in comparison to the continuous discharge due to the shorter duration of the pulsed RF discharge limiting the amount of energy transferred to electrons. On the other hand, the dust particle remains in the powered sheath region exhibiting a damped oscillation in the two discharges with higher oscillation frequency in the continuous discharge.

Interaction effect of adjacent pores on plasma generation inside pores of porous catalysts

A study on the influence of pore characteristics on the microdischarge plasma inside pores of catalyst is important for understanding plasma catalysis mechanism. This article focuses on the effect of pore interaction on plasma production in pores including electron density, electron temperature, and total ion density, etc. These parameters are calculated by solving the electron energy density conservation equation, electron convection diffusion equation, heavy species conservation equation and Poisson equation. The numerical simulation results show that the distribution of electron density, electron temperature and electric field in and near a pore is apparently affected by the adjacent pore, namely, the time-averaged electron density, total ion density and ionization rate in a pore increase with the pore spacing in the range of 8–150 μm because the loss of electrons to the wall due to the adjacent pore decreases with the increasing pore spacing, but there is an opposite law for the time-averaged electron temperature and electric field in a pore; moreover, the pore spacing presents more apparent effect on the plasma generation in a smaller pore and at higher applied voltage, indicating stronger interactions between pores at these conditions. The simulation results show that the pore characteristics of porous catalysts including pore spacing and pore diameter may affect the degradation of pollutants in plasma catalysis processes. The study is important for understanding the plasma catalysis mechanisms, especially the influence mechanism of pore interaction on microdischarge in porous catalysts.

High-Order Above-Threshold Ionization Using a Bi-Elliptic Orthogonal Two-Color Laser Field with Optimal Field Parameters

In the present paper, we study the high-order above-threshold ionization of noble-gas atoms using a bi-elliptic orthogonal two-color (BEOTC) field. We give an overview of the SFA theory and calculate the differential ionization rate for various values of the laser field parameters. We show that the ionization rate strongly depends on the ellipticity and the relative phase between two field components. Using numerical optimization, we find the values of ellipticity and relative phase that maximize the ionization rate at energies close to the cutoff energy. To explain the obtained results, we present, to the best of our knowledge, for the first time the quantum-orbit analysis in the BEOTC field. We find and classify the saddle-point (SP) solutions and study their contributions to the total ionization rate. We analyze quantum orbits and corresponding velocities to explain the contribution of relevant SP solutions.

Relativistic strong-field ionization of hydrogenlike atomic systems in constant crossed electromagnetic fields

Relativistic strong-field ionization of hydrogen-like atoms or ions in a constant crossed electromagnetic field is studied. The transition amplitude is formulated within the strong-field approximation in G\"oppert-Mayer gauge, with initial and final electron states being described by the corresponding Dirac-Coulomb and Dirac-Volkov wave functions, respectively. Coulomb corrections to the electron motion during tunneling are taken into account by adjusting an established method to the present situation. Total and energy-differential ionization rates are calculated and compared with predictions from other theories in a wide range of atomic numbers and applied field strengths.

Polarization effects in the total rate of biharmonic ω+3ω ionization of atoms

The total ionization rate of biharmonic ($\ensuremath{\omega}+3\ensuremath{\omega}$) ionization is studied within the independent particle approximation and the third-order perturbation theory. Particular attention is paid to how the polarization of the biharmonic light field affects the total rate. The ratios of the biharmonic ionization rates for linearly and circularly polarized beams as well as for corotating and counter-rotating elliptically polarized beams are analyzed, and how they depend on the beam parameters, such as photon frequency or phase between $\ensuremath{\omega}$ and $3\ensuremath{\omega}$ light beams. We show that the interference of the biharmonic ionization amplitudes determines the dominance of a particular beam polarization over another and that it can be controlled by an appropriate choice of beam parameters. Furthermore, we demonstrate our findings for the ionization of neon $L$ shell electrons.

Многолетний тренд реакции Е-слоя ионосферы на солнечные вспышки

Using data from ground-based vertical sounding of the ionosphere (VS) at the station Moscow and five Japanese stations for the period from 1969 to 2015, we have examined the long-term response of the E layer to solar X-ray flares. The analysis relies on the previously developed method for estimating the ratio between rates of ionization by X-rays qx and ultraviolet radiation qu during flares. We confirmed the existence of a long-term (at least covering the 45-year observation period) increase in the proportion of X-rays in the total ionization rate of the ionospheric E layer, characterized by the ratio qx/q, where q=qx+q. The qx/q ratio is shown to increase throughout the period of interest at a rate independent of the solar activity cycle. There is also no dependence of the qx/q trend rate on the season, latitude (26°–56° N), and longitude (37°–28° E).

Long-term trend of the ionospheric E-layer response to solar flares

Using data from ground-based vertical sounding of the ionosphere (VS) at the station Moscow and five Japanese stations for the period from 1969 to 2015, we have examined the long-term response of the E layer to solar X-ray flares. The analysis relies on the previously developed method for estimating the ratio between rates of ionization by X-rays qx and ultraviolet radiation qu during flares. We confirmed the existence of a long-term (at least covering the 45-year observation period) increase in the proportion of X-rays in the total ionization rate of the ionospheric E layer, characterized by the ratio qx/q, where q=qx+q. The qx/q ratio is shown to increase throughout the period of interest at a rate independent of the solar activity cycle. There is also no dependence of the qx/q trend rate on the season, latitude (26°–56° N), and longitude (37°–28° E).

Atomic Siegert states in a rotating electric field

We show that the time-dependent Schr\"odinger equation describing in the dipole approximation the interaction of a one-electron atom with a monochromatic circularly polarized electromagnetic field can be reduced to a stationary Schr\"odinger equation in the form which allows to separate variables in the asymptotic region and explicitly formulate outgoing-wave boundary conditions. The solutions to this equation are called atomic Siegert states (SSs) in a rotating electric field. We develop the theory of such states and propose an efficient method to construct them using powerful techniques of stationary scattering theory. The method yields not only the SS eigenvalue defining the Stark-shifted energy and total ionization rate of the state, but also the SS eigenfunction defining partial ionization amplitudes and the photoelectron momentum distribution. The theory is illustrated by calculations for a model potential.

Strong chiral dichroism and enantiopurification in above-threshold ionization with locally chiral light

We derive here a new highly selective photoelectron-based chirality-sensing technique that utilizes 'locally-chiral' laser pulses. We show that this approach results in strong chiral discrimination, where the standard forwards/backwards asymmetry of photoelectron circular dichroism (PECD) is lifted. The resulting dichroism is much larger and more robust than conventional PECD, is found in all hemispheres, and is not symmetric or antisymmetric with respect to any symmetry operator. Remarkably, a CD of up to 10% survives in the angularly-integrated above-threshold ionization (ATI) spectra, and of up to 5% in the total ionization rates. We demonstrate these results through ab-initio calculations in the chiral molecules Bromochlorofluoromethane, Limonene, Fenchone, and Camphor. We also explore the parameter-space of the locally-chiral field and show that the observed CD is strongly correlated to the degree of chirality of the light, validating it as a measure for chiral-interaction strengths. Our results pave the way for highly selective probing of ultrafast chirality in ATI, can potentially lead to all-optical enantio-separation, and motivate the use of locally-chiral light for enhancing ultrafast spectroscopies.

Tunneling and multiphoton ionization of atoms in two-color linear and circular laser fields and possibility of coherent polarization control of terahertz waves

The tunneling and multiphoton ionization of atoms in an intense two-color linearly and circularly polarized laser fields are discussed in the Keldysh theory framework. We use the “imaginary-time” method, where tunneling of the photoelectron is described by the classical equations of motion but with purely imaginary “time.” Together with using of the saddle-point method, this allows to obtain the dependence of the total ionization rate and the net photoelectron current, generated due to the interaction of an intense two-color laser field with an atom, on the ratio of the second and fundamental harmonic amplitudes, their relative phase and an angle between harmonics. Application of the “imaginary-time” method also allows us to specify the parameters maximizing the net photocurrent and to determine the Coulomb correction to the ionization rate. We investigate the properties of polarization and spectral intensity of terahertz (THz) radiation and also the possibility of the coherent control of THz waves polarization in two-color scheme, through the relative phase between harmonics. We theoretically demonstrate that the amplification of THz radiation in the case of parallel co-rotating circular laser pulses is greater than for a combination of circularly and linearly polarized harmonics and provides the most promising conditions for increasing the efficiency of THz emission and coherent control of the THz beam polarization.

Influence of metastable atoms in low pressure magnetized radio-frequency argon discharges

One-dimensional particle-in-cell simulations with Monte Carlo collisions are used to investigate the influence of metastable atoms in low pressure radio-frequency argon discharges with magnetic fields ranging from 0 G to 60 G. Two metastable levels of argon species are included and tracked as particles, enabling multistep ionization and metastable pooling. At low magnetic fields, the metastable argon atoms have little influence on the discharge. At higher magnetic fields, the electron density increases and the electron temperature decreases at the center of the discharge with the inclusion of metastable atoms. The reduction in electron temperature is attributed to the depletion of energetic electrons due to low-energy-threshold reactions related to the metastable atoms. The reduced electron temperature leads to a reduction in the total ionization rate, albeit the contribution of multistep ionization increases with the magnetic field. The suppression of plasma diffusion at low electron temperatures plays a greater role than the reduction in ionization rate, resulting in a higher electron density. A transformation in metastable density profiles from parabolic to saddle type is observed with the increase in the magnetic field. Metastable atoms may play an important role in modulating the electron temperature in low pressure magnetized discharges.

Sun–Heliosphere Observation-based Ionization Rates Model

Abstract The solar wind (SW) and the extreme ultraviolet (EUV) radiation modulate fluxes of interstellar and heliospheric particles inside the heliosphere both in time and in space. Understanding this modulation is necessary to correctly interpret measurements of particles of interstellar origin inside the heliosphere. We present a revision of heliospheric ionization rates and provide the Sun–Heliosphere Observation-based Ionization Rates model based on the currently available data. We calculate the total ionization rates using revised SW and solar EUV data. We study the in-ecliptic variation of the SW parameters, the latitudinal structure of the SW speed and density, and the reconstruction of the photoionization rates. The revision most affects the SW out of the ecliptic plane during solar maximum and the estimation of the photoionization rates, the latter due to a change of the reference data. The revised polar SW is slower and denser during the solar maximum of solar cycle (SC) 24. The current estimated total ionization rates are higher than the previous ones for H, O, and Ne, and lower for He. The changes for the in-ecliptic total ionization rates are less than 10% for H and He, up to 20% for O, and up to 35% for Ne. Additionally, the changes are not constant in time and vary as a function of time and latitude.

Angular Dependence of Strong Field Ionization of 2-Phenylethyl-N,N-dimethylamine (PENNA) Using Time-Dependent Configuration Interaction with an Absorbing Potential.

Ionization of 2-phenylethyl-N,N-dimethylamine (PENNA) may lead to charge migration between the amine group and the phenyl group. The angular dependence of strong field ionization of PENNA has been modeled by time-dependent configuration interaction with an absorbing potential. The total ionization rate can be partitioned into contributions from the amine group and the phenyl group, and these components have very distinct shapes. Ionization from the amine is primarily from the side opposite to the lone pair and is dominated by the CH2 and CH3 groups. Similarly, trimethylamine (N(CH3)3), dimethyl ether (CH3OCH3), and methyl fluoride (CH3F) are also found to ionize primarily from the methyl groups. The predominance of ionization from the methyl groups can be attributed to the fact that the orbital energies of individual lone pairs of N, O, and F are lower than the CH3 groups. Because the angular dependence of ionization of the two groups is quite different, alignment of PENNA could be used to control the ratio of the amine and phenyl cations and potentially probe charge migration in PENNA cation.

Filling gap of combination of gauge and analytical method in KFR-like theory**Project supported by the National Natural Science Foundation of China (Grant Nos. 11274149 and 11304185) and the Program of Shenyang Key Laboratory of Optoelectronic Materials and Technology, China (Grant No. F12-254-1-00).

Bauer recently presented a formula for the ionization rate of a hydrogen atom in a strong linearly polarized laser field [J. Phys. B 49 145601 (2016)]. He started from the Keldysh probability amplitude in the length gauge and utilized Reiss’s method in the velocity gauge. Instead, according to the Reiss probability amplitude in the velocity gauge and Keldysh’s derivation for the length gauge, we derive a formula for the ionization rate of a ground-state hydrogen atom or a hydrogen-like atom in a strong linearly polarized laser field. We compare the numerical results of the total ionization rate and the photoelectron energy distribution calculated from our formula with the results from Keldysh, Reiss, and Bauer. We find that the apparent discrepancies in the ionization rate are caused not only by different gauges, but also by different analytical methods used to derive the ionization rate.

Relativistic electron impact ionization cross sections of carbon ions and application to an optically thin plasma

Context.Ionization through electron impact is a fundamental process associated with the evolution of the ionic structure and emissivity of astrophysical plasmas. Over several decades substantial efforts have been made to measure and calculate the ionization cross sections of ionization through electron impact of different ions shell by shell, in particular, of carbon ions. Spectral emission codes use electron-impact ionization cross sections and/or rates taken from different experimental and theoretical sources. The theoretical cross sections are determined numerically and include a diversity of quantum mechanical methods. The electron-impact ionization database therefore is not uniform in the methods, which makes it hard to determine the reason for the deviations with regard to experimental data. In many cases only total ionization rates for Maxwell–Boltzmann plasmas are available, which makes calculating inner-shell ionization in collisional-radiative models using thermal and nonthermal electron distribution functions difficult. A solution of this problem is the capability of generating the cross sections with an analytical method using the minimum number of atomic parameters. In this way, uniformity in the database is guaranteed, and thus deviations from experiments are easily identified and traced to the root of the method.Aims.The modified relativistic binary encounter Bethe (MRBEB) method is such a simple analytical scheme based on one atomic parameter that allows determining electron-impact ionization cross sections. This work aims the determination of K- and L-shell cross sections of the carbon atom and ions using the MRBEB method and show their quality by: (i) comparing them with those obtained with the general ionization processes in the presence of electrons and radiation (GIPPER) code and the flexible atomic code (FAC), and (ii) determining their effects on the ionic structure and cooling of an optically thin plasma.Methods.The MRBEB method was used to calculate the inner-shells cross sections, while the plasma calculations were carried out with the collisional+photo ionization plasma emission software (CPIPES). The mathematical methods used in this work comprise a modified version of the double-exponential over a semi-finite interval method for numerical integrations, Gauss-elimination method with scaled partial pivoting for the solution of systems of linear equations, and an iterative least-squares method to determine the fits of ionization cross sections.Results.The three sets of cross sections show deviations among each other in different energy regions. The largest deviations occur near and in the peak maximum. Ion fractions and plasma emissivities of an optically thin plasma that evolves under collisional ionization equilibrium, derived using each set of cross sections, show deviations that decrease with increase in temperature and ionization degree. In spite of these differences, the calculations using the three sets of cross sections agree overall.Conclusions.A simple model like the MRBEB is capable of providing cross sections similar to those calculated with more sophisticated quantum mechanical methods in the GIPPER and FAC codes.

Angular dependence of strong field ionization of N2 by time-dependent configuration interaction using density functional theory and the Tamm-Dancoff approximation

The ionization of N2 serves as an important test case for computational methods for strong field ionization. Because Koopmans’s theorem fails for Hartree-Fock calculations of N2, corrections for electron correlation are needed to obtain the proper ordering of ionization energies of N2. Lopata and co-workers found that real-time integration of time-dependent Hartree-Fock (rt-TD-HF) gave a ratio for strong field ionization parallel and perpendicular to the molecular axis that was too small compared to experiment, but real-time integration of time-dependent density functional theory (rt-TD-DFT) with an appropriately tuned long-range corrected functional, lc-ωPBE*, was in good agreement with experiment. The present study finds that time-dependent configuration interaction (TDCI) with single excitations based on a Hartree-Fock reference determinant (TD-CIS) has the same problems as rt-TD-HF. These problems can be overcome within the TDCI framework by calculating the excitation energies and transition dipole moments with density functional theory using linear response TD-DFT in the Tamm-Dancoff approximation (TDA) with suitably tuned long-range corrected functionals (TD-TDA). The correct angular dependence of the total ionization rate is obtained with TD-TDA using tuned lc-ωPBE*, lc-BLYP*, and ωB97XD* functionals. Partitioning of the total ionization rate into orbital components confirms that the larger ionization rate perpendicular to the molecular axis found for TD-CIS is due to greater π orbital contributions than those seen in TD-TDA. The use of density functional theory corrects this problem. At higher fields, both the TD-CIS and TD-TDA simulations show an increased ionization rate perpendicular to the molecular axis because of increased ionization from the π orbitals.

Global Distribution of the Solar Wind Flux and Velocity From SOHO/SWAN During SC‐23 and SC‐24

AbstractWe analyze SOHO (SOlar Heliospheric Observatory)/SWAN (Solar Wind ANisotropy) hydrogen Lyman‐α data collected between 1996 and 2018 to derive the solar wind latitudinal distribution over time. Full‐sky interplanetary Lyman‐α maps are inverted to derive the total hydrogen ionization rate latitude profiles, normalized to proton charge‐exchange and photoionization. Using Interplanetary Scintillation velocities to calculate the velocity‐dependent charge‐exchange cross‐sections, we produce the solar wind flux latitudinal profiles. Finally, we compute solar wind velocity latitude profiles, based on the dynamic pressure and energy flux conservation (calculated from OMNI data) over latitude. SWAN reproduces the Interplanetary Scintillation velocity profiles up to at least ±60°, and also agrees with Ulysses in situ measurements for solar minimum periods in 1996–1997 and 2007. During solar maximum, discrepancies are more frequent because in situ data reflect local solar wind conditions, while SWAN data reflect global conditions in the heliosphere.

Interstellar Neutral Gas Species and Their Pickup Ions inside the Heliospheric Termination Shock. Ionization Rates for H, O, Ne, and He

Abstract Solar ionizing factors are responsible for modulation of interstellar neutral gas and its derivative populations inside the heliosphere. We provide an overview of the current state of knowledge about them for heliospheric particles inside the termination shock. We discuss charge exchange with solar wind particles, photoionization, and electron impact ionization for hydrogen, oxygen, neon, and helium from 1985 to 2018 both in the ecliptic plane and in the polar regions. We discuss ionization rates as a function of time, distance to the Sun, and latitude. We compare the total ionization rates among the species within a consistent and homogeneous system of calculation of the ionization rates. The highest total ionization rates at 1 au in the ecliptic plane are for hydrogen and oxygen, and the lowest are for helium. In the polar regions, the strongest ionization losses are for oxygen, regardless of the solar activity. Photoionization is the dominant ionization reaction for helium and neon, and a reaction of high significance for oxygen. Charge exchange with solar wind particles is the dominant ionization reaction for hydrogen and the second important ionization reaction for oxygen. Electron impact ionization is an important ionization reaction for Ne and He, with the contribution to the total ionization rates stronger within 1 au and smaller outside. The total ionization rates for He and Ne vary in time with the solar activity, whereas the total ionization rates for H and O follow the cyclic solar wind variations out of the ecliptic plane and aperiodic variations in the ecliptic plane.