Ionization Rate Ratio Research Articles

New O Partial Photoionization Cross Sections Resolve Ionospheric EUV Remote Sensing Issues

AbstractThe ionospheric O+ number density can be measured remotely during the day by observing its optically thick 83.4 nm radiance. Some ambiguity is present in the process of retrieving the density due to uncertainties in the initial excitation rate. This can be removed by observing a companion optically thin emission at 61.7 nm originating from the O+(3s 2P) state, providing that the ratio of the initial excitation rates is known. Analyses of ICON EUV data using an 83.4/61.7 emission ratio of order 10 result in O+ densities lower by ∼2 than other measurements. Key to relating the two emissions is accurate knowledge of the partial photoionization cross sections and the spectroscopy of O+—the topic of this paper. Up to now, no independent evaluation of the ratio of the 83.4/61.6 emission ratio exists. The recent availability of state‐of‐the‐art calculations of O partial photoionization cross sections into a variety of O+ states presents an opportunity to evaluate the O+(2p4 4P)/O+(3s 2P) ionization rate ratio. We calculate excitation of these parent states of the emissions including both direct and cascade excitation from higher lying O+ energy states. The resulting theoretical prediction gives ratios that range from 13.5 to 12 from solar minimum to maximum, larger than the value of 10 used by the ICON 83.4 and 61.7 nm algorithm. The higher theoretical values for the ratio reconcile the ∼2 discrepancy between simultaneous ICON and other electron density measurements.

Mass spectroscopic measurement of time-varying ion composition in a pulse-modulated Ar/C4F8/O2 dual-frequency capacitively coupled plasma

The time dependence of the ion composition in pulse-modulated dual-frequency capacitively coupled plasma with Ar/C4F8/O2 was measured using a quadrupole mass spectrometer with an electrostatic energy analyzer. After turning on the pulse, Ar+ ions were preferentially generated, and then, the composition of C x F y + ions, such as C2F4 + and C3F5 + ions, increased. This phenomenon was discussed on the basis of the time variation of electron temperature and the resultant change in the ratio of the C4F8 ionization rate to that of Ar atoms.

H i-to-H2 Transitions in Dust-free Interstellar Gas

Abstract We present numerical computations and analysis of atomic-to-molecular (H i-to-H2) transitions in cool (∼100 K), low-metallicity, dust-free (primordial) gas in which molecule formation occurs via cosmic-ray-driven negative ion chemistry and removal is by a combination of far-UV photodissociation and cosmic-ray ionization and dissociation. For any gas temperature, the behavior depends on the ratio of the Lyman–Werner (LW) band FUV intensity to gas density, I LW/n, and the ratio of the cosmic-ray ionization rate to the gas density, ζ/n. We present sets of H i-to-H2 abundance profiles for a wide range of ζ/n and I LW/n for dust-free gas. We determine the conditions for which H2 absorption-line self-shielding in optically thick clouds enables a transition from atomic to molecular form for ionization-driven chemistry. We also examine the effects of cosmic-ray energy losses on the atomic and molecular density profiles and transition points. For a unit Galactic interstellar FUV field intensity (I LW = 1) with LW flux 2.07 × 107 photons cm−2 s−1 and a uniform cosmic-ray ionization rate ζ = 10−16 s−1, an H i-to-H2 transition occurs at a total hydrogen gas column density of 4 × 1021 cm−2, within 3 × 107 yr, for a gas volume density of n = 106 cm−3 at 100 K. For these parameters, the dust-free limit is reached for a dust-to-gas ratio Z d ′ ≲ 10 − 5 , which may be reached for overall metallicities Z ′ ≲ 0.01 relative to Galactic solar values.

A Hybrid Approach Toward Simulating Reionization: Coupling Ray Tracing with Excursion Sets

Abstract This paper introduces a new method of generating 21 cm maps that is based on ideas from ray tracing and excursion sets. In this method, photons generated in each grid cell are computed using the excursion set ideas while their propagation is accounted for by ray tracing. The method requires the overdensity field over a grid as a starting point. Then the usual reionization parameters, minimum mass of collapsed halos (M min), number of ionizing photons deposited in the intergalactic medium per collapsed baryon (n ion), and ratio of ionization rate to recombination rate (represented through n rec) are used. Thus, this is a hybrid method that utilizes the results of theoretically motivated excursion sets and combines them with the computationally intensive procedure of ray tracing. As the method integrates simple principles of both the approaches, it is expected to yield precise and fast estimates of the power spectrum on the scales of interest (0.1 Mpc−1 ≲ k ≲ 1.0 Mpc−1).

Ion energy distribution in extremely high electric field discharges

Singly charged ion energy distribution functions (IEDFs) have been investigated by the particle-in-cell/Monte Carlo method in practical discharges at extremely high reduced electric field E/n, i.e. the ratio of electric field and gas density, up to 700 kTd in helium. It is found in high E/n regime that IEDFs deviate from Maxwellian form due to complex processes: (a) ions undergo a transition to runaway regime, (b) the copious energetic fast neutrals generated by charge exchange produce slow ions via ionization collisions, which load in the low-energy part of IEDF, as well as commonly assumed energy-dependent charge exchange cross section. For E/n > 100 kTd, the ratio of the volumetric ionization rate and charge exchange rate exceeds 10% due to heavy particles impact ionization. The ion mean energy close to the cathode is approximately reduced by in comparison with the value obtained by the reduced model with only the charge exchange collision considered. Charge production attributed to fast-neutral impact ionization not only causes the decrease of ion mean energy, but also enhances ion non-equilibrium.

Ionization and excitation of the excited hydrogen atom in strong circularly polarized laser fields

In the recent work of Herath et al. [T. Herath, L. Yan, S. K. Lee, and W. Li, Phys. Rev. Lett. 109, 043004 (2012)] the first experimental observation of a dependence of strong-field ionization rate on the sign of the magnetic quantum number $m$ [of the initial bound state $(n,l,m)]$ was reported. The experiment with nearly circularly polarized light could not distinguish which sign of $m$ favors faster ionization. We perform ab initio calculations for the hydrogen atom initially in one of the four bound substates with the principal quantum number $n=2$, and irradiated by a short circularly polarized laser pulse of $800\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$. In the intensity range of ${10}^{12}\ensuremath{-}{10}^{13}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$ excited bound states play a very important role, but also up to some ${10}^{15}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$ they cannot be neglected in a full description of the laser-atom interaction. We explore the region that with increasing intensity switches from multiphoton to over-the-barrier ionization and we find, unlike in tunneling-type theories, that the ratio of ionization rates for electrons initially counter-rotating and corotating (with respect to the laser field) may be higher or lower than 1.

Monolithic Germanium/Silicon Photodetectors With Decoupled Structures: Resonant APDs and UTC Photodiodes

The Ge/Si system is useful to realize avalanche photodetectors (APDs) operating at 1310~1550 nm because of the intrinsic advantages of complementary metal-oxide-semiconductor (CMOS) compatibility, high light-absorption of Ge, low ionization rate ratio of silicon, and high thermal conductivities of Si and Ge. With the Ge/Si system, it is convenient to realize photodetectors with decoupled structures including resonant Ge/Si APDs as well as uni-traveling carrier (UTC) photodiodes. The resonant Ge/Si APD with a separated absorption-charge-multiplication (SACM) structure, which decouples the light absorption and avalanche process, has high speed, high gain, and high gain-bandwidth product. The UTC photodiode, which decouples the light absorption and the carrier collection, is useful for high-power applications. This paper first reviews the structure and model of decoupled Ge/Si (A)PDs, particularly, the equivalent circuit models for explaining the peak enhancement of the frequency response in resonant SACM APDs. This model is also applied to UTC Ge/Si PDs developed recently for the high-power applications.

Alignment-dependent terahertz radiation in two-color photoionization of molecules

The generation of terahertz radiation from an ensemble of aligned air molecules in intense two-color laser fields has been investigated. Our experiments show that terahertz radiation yields are strongly modulated by the alignment of nitrogen and oxygen molecules. We explain this phenomenon in the context of the plasma current model combined with alignment-dependent ionization, in which nitrogen molecules aligned parallel to the laser field yield more ionization and plasma currents, resulting in enhanced terahertz radiation. Our measurements also provide the ratio of the ensemble-averaged ionization rates of nitrogen molecules aligned parallel versus orthogonal to the laser field.

Multi-Gain-Stage InGaAs Avalanche Photodiode With Enhanced Gain and Reduced Excess Noise

We report the design, fabrication, and test of an InGaAs avalanche photodiode (APD) for 950-1650 nm wavelength sensing applications. The APD is grown by molecular beam epitaxy on InP substrates from lattice-matched InGaAs and InAlAs alloys. Avalanche multiplication inside the APD occurs in a series of asymmetric gain stages whose layer ordering acts to enhance the rate of electron-initiated impact ionization and to suppress the rate of hole-initiated ionization when operated at low gain. The multiplication stages are cascaded in series, interposed with carrier relaxation layers in which the electric field is low, preventing avalanche feedback between stages. These measures result in much lower excess multiplication noise and stable linear-mode operation at much higher avalanche gain than is characteristic of APDs fabricated from the same semiconductor alloys in bulk. The noise suppression mechanism is analyzed by simulations of impact ionization spatial distribution and gain statistics, and measurements on APDs implementing the design are presented. The devices employing this design are demonstrated to operate at linear-mode gain in excess of 6000 without avalanche breakdown. Excess noise characterized by an effective impact ionization rate ratio below 0.04 were measured at gains over 1000.

Alignment-dependent ionization of H2+: From multiphoton ionization to tunneling ionization

We investigate the strong-field ionization for the ground state of H${{}_{2}}^{+}$ by numerically solving the three-dimensional (3D) time-dependent Schr\odinger equation (TDSE), and comparisons have been made among the TDSE, the different versions of molecular strong-field approximation (MO-SFA) and the molecular Ammosov-Delone-Krainov (MO-ADK) approximation. The study shows that, for the TDSE results, the ratio of ionization rates between perpendicular and parallel alignments displays a step-like structure against the Keldysh parameter $\ensuremath{\gamma}$. For small internuclear distances, the transition between the steps are found to be around $\ensuremath{\gamma}\ensuremath{\approx}1$ and is recognized as the competition between the multiphoton ionization (MPI) and tunneling ionization (TI). The ionization is more isotropic in the MPI regime. For large internuclear distances, the transition position shifts to larger $\ensuremath{\gamma}$ due to the charge-resonance-enhanced ionization (CREI). Different versions of strong-field ionization theories are compared against the TDSE results.

Dynamic recovery in silicate-apatite structures under irradiation and implications for long-term immobilization of actinides

The irradiation responses of Ca2La8(SiO4)6O2 and Sr2Nd8(SiO4)6O2 with the apatite structure are investigated to predict their long-term behaviour as host phases for immobilization of actinide elements from the nuclear fuel cycle. Different ions and energies are used to study the effects of dose, temperature, atomic displacement rate and ionization rate on irradiation-induced amorphization and recrystallization. The dose for amorphization increases with temperature in two stages, below and above 150 K. In the high temperature stage relevant to actinide immobilization, the increase of amorphization dose with temperature exhibits a strong dependence on the ratio of ionization rate to displacement rate for the different ions. Data analysis using a dynamic model for amorphization reveals that ionization-induced processes, with activation energy of 0.15 ± 0.02 eV, dominate dynamic recovery for ions from Ne through Xe. For heavier Au ions or for alpha-recoil nuclei emitted in alpha decay of actinides, ionization becomes less dominant and dynamic recovery is controlled primarily by thermally-driven processes. In post-irradiation annealing studies of amorphous samples, epitaxial thermal recrystallization is observed at 1123 K, and irradiation-enhanced nucleation of nanocrystallites is observed under irradiation with heavier ions. The recrystallization temperature under irradiation decreases with increasing ion mass to a value of ∼823 K, which also defines the thermally-driven critical temperature for amorphization under irradiation with heavy ions. Some partial recovery due to alpha particle irradiation at 300 K is observed that suggests a self-healing mechanism in apatite phases containing actinides. Based on the results and dynamic model, the temperature and time dependences of amorphization in silicate-apatite host phases for actinide immobilization are predicted.

Analytical Model for Obtaining the Ionization Rate Ratio of Mesa InAlAs Avalanche Photodiodes

We proposed a new analytical model to extract the effective ionization rate ratio k from the measured multiplication noise of Avalanche photodiodes (APDs). In this model, k was expressed as a function of the multiplication factor M. We extracted the effective k of the mesa-structured InAlAs APDs using this model. A measurement system that consisted of a low-noise differential preamplifier was developed for this purpose, leading to the measurements of the noise data, particularly in the region where it was small. APD noise data could be fitted to the curve based on this new analytical model better than with other analytical models over the wide range of M. The effective k in the region where M was small became appropriate using this model. The quantum efficiency of the APDs was also found to be reasonable compared with the results obtained using the other analytical models.

40 Gbit/s waveguide avalanche photodiode with p-type absorption layer and thin InAlAs multiplication layer

Waveguide avalanche photodiodes exhibiting both wide bandwidth and high gain bandwidth product have been developed. An absorption layer includes a p-type quasi-field-formed layer and a multiplication layer consists of InAlAs with a low ionisation rate ratio. Optimisation of the design yielded superior performance such as a wide bandwidth of 36.5 GHz, a gain band width of 170 GHz and a high quantum efficiency of 0.75 A/W.

On Modeling the Evolution of Radiation Damage in Silicon Carbide

AbstractExperimental charged-particle irradiations and multi-scale computer simulations have been used to investigate the primary damage state and evolution of damage in silicon carbide as functions of temperature and charged-particle mass and energy. Atomistic simulations of energetic C and Si collision cascades, similar to those created by reactor neutrons, indicate that single interstitials, vacancies, antisite defects, and small defect clusters are produced. The point defects are dominated by close Frenkel pairs, and atomistic simulations indicate that the activation energies for recombination of most close pairs range from 0.24 to 0.38 eV, which suggest significant reduction in defect survivability at room temperature. Atomistic simulations have also determined that the activation energies for long-range diffusion of C and Si interstitials are 0.7 and 1.5 eV, respectively. Using these activation energies and ab initio results as input parameters, a kinetic Monte Carlo (MC) simulation model has been developed to study isochronal annealing of defects in SiC between cascade events. The defects are produced by a 10 keV Si cascades in a molecular dynamics (MD) simulation cell, and these defects are then accurately transferred to defect lattice sites in the Monte Carlo model to investigate defect recovery. By transferring defects states back and forth between the MD and MC environments, damage accumulation can be investigated as a function of temperature. Charged particle irradiations are often used to simulate radiation damage from neutrons and radioactive decay; however, at extreme charged-particle fluxes used in irradiation studies to simulate radiation damage in nuclear materials, the ratio of ionization rate to displacement rate can have a significant impact on observed temperature-dependent processes, which can affect both interpretation and model development. At high charged-particle fluxes, the defect recovery rates in SiC increase nearly linearly with the ratio of ionization rate to displacement rate. A fundamental understanding of these ionization effects is needed if charged particle irradiation results are to be used to develop predictive models of damage evolution in nuclear materials, such as SiC, as functions of time, temperature and dose rates.

Chemical sensitivity to the ratio of the cosmic-ray ionization rates of He and H2in dense clouds

Aim: To determine whether or not gas-phase chemical models with homogeneous and time-independent physical conditions explain the many observed molecular abundances in astrophysical sources, it is crucial to estimate the uncertainties in the calculated abundances and compare them with the observed abundances and their uncertainties. Non linear amplification of the error and bifurcation may limit the applicability of chemical models. Here we study such effects on dense cloud chemistry. Method: Using a previously studied approach to uncertainties based on the representation of rate coefficient errors as log normal distributions, we attempted to apply our approach using as input a variety of different elemental abundances from those studied previously. In this approach, all rate coefficients are varied randomly within their log normal (Gaussian) distribution, and the time-dependent chemistry calculated anew many times so as to obtain good statistics for the uncertainties in the calculated abundances. Results: Starting with so-called ``high-metal'' elemental abundances, we found bimodal rather than Gaussian like distributions for the abundances of many species and traced these strange distributions to an extreme sensitivity of the system to changes in the ratio of the cosmic ray ionization rate zeta\_He for He and that for molecular hydrogen zeta\_H2. The sensitivity can be so extreme as to cause a region of bistability, which was subsequently found to be more extensive for another choice of elemental abundances. To the best of our knowledge, the bistable solutions found in this way are the same as found previously by other authors, but it is best to think of the ratio zeta\_He/zeta\_H2 as a control parameter perpendicular to the ''standard'' control parameter zeta/n\_H.

Monte Carlo simulations of Hg0.7Cd0.3Te avalanche photodiodes and resonance phenomenon in the multiplication noise

Monte Carlo simulations of Hg0.7Cd0.3Te avalanche photodiodes are presented. The simulated very low excess noise and exponential gain curve are consistent with those that have been experimentally observed and are consistent with the speculated large ratio of electron and hole impact ionization rates. The simulations suggest that there is a large difference between the scattering rates of electrons and holes, a direct consequence of the band structure. A resonance behavior in the excess noise factor at gain values near 2, 4, 8, and 16 is also revealed in the simulations. This effect is explained by comparing to the gain and noise of a photomultiplier tube.

Comparison of intense-field ionization of diatomic molecules and rare-gas atoms

Strong, short laser pulses have been used to ionize diatomic molecules and rare-gas atoms. Using mixed species targets, intensity-dependent ionization yield ratios have been measured directly for pairs of molecules and atoms with similar ionization potentials. Specifically, ionization rate ratios for homonuclear $({\mathrm{N}}_{2}:\mathrm{Ar},{\mathrm{F}}_{2}:\mathrm{Ar},{\mathrm{D}}_{2}:\mathrm{Ar},{\mathrm{O}}_{2}:\mathrm{Xe},{\mathrm{S}}_{2}:\mathrm{Xe})$ and heteronuclear (CO:Kr, NO:Xe, SO:Xe) molecules have been obtained. Our experimental results are compared to the predictions of several approximate theoretical models. In general, these models fail to accurately describe the detailed differences in the intense laser ionization rates of atomic and diatomic targets.

Noise Analysis of InP/InGaAs Superlattice Avalanche Photodiode

A theoretical study of the noise characteristics of an InP/In0.53 Ga0.47 As Superlattice Avalanche photodiode (SL-APD) has been presented. It has been found that an improvement in the value of the effective hole-to-electron ionisation rate ratio (βeff/αeff) for the SL-APD structure results in a low-noise behaviour of the device. The device has a very high gain-bandwidth product (≈450 GHz) and a low excess noise factor at moderate gain. It is expected to find application as a highly sensitive low-noise photodetector in optical receiver unit for long-haul fiber optic communication system in 1.3 to 1.6 μm.

Variations of the thermospheric nitric oxide mass mixing ratio as a function of Kp, altitude, and magnetic local time

In this letter, data from the Halogen Occultation Experiment (HALOE) and the Particle Environment Monitor on the Upper Atmospheric Research Satellite are presented. Five years of data from each instrument have been used to create high‐latitude maps of Nitric Oxide (NO) mass mixing ratio and electron impact and bremsstrahlung ionization rates as a function of altitude and Kp. We show that enhancements in NO at 100 km are coupled to the auroral oval and to increases in the local ionization rate, with both the peak in the ionization rate and NO mass mixing ratio occuring in the dawn magnetic local time sector between 60° and 70° magnetic latitude. Below this altitude, the NO is shown to be enhanced at lower latitudes and sunward of the dawn‐dusk meridian, most likely due to solar illumination. Above 100 km, the NO density enhancement stays in approximately the same MLT sector, while the peak ionization rate expands in MLT towards the night side. This is attributed to a build up of NO created by particle precipitation during the night, which is corotated towards the dawn region.

A 2.0 μm cutoff wavelength separate absorption, charge, and multiplication layer avalanche photodiode using strain-compensated InGaAs quantum wells

We report an avalanche photodiode structure for use at wavelengths as long as 2.1 μm. Light is absorbed in a 100 period structure consisting of In0.83Ga0.17As quantum wells strain compensated by In0.83Ga0.17P barrier layers. Photogenerated electrons are injected into a high field In0.52Al0.48As (ionization rate ratio=0.2) gain region initiating low noise avalanche multiplication. Primary dark currents of ∼5 nA and responsivities of 45 A/W at a wavelength of 1.9 μm are observed.