Comments on Gallon's method for determining the ionization cross-section and the backscattering factor

The energy dependence of the ionization cross-section and the backscattering factor resulting from the experimental method of Gallon are shown to be affected by simplifying the definition of the backscattering factor. In the case of Auger transitions following the ionization of the L₃ shell the errors in the ionization cross-section and the backscattering factor reach 10% and 4%, respectively. Larger deviations are probable for Auger transitions involving higher shells.     

Relativistic theory of tunnel and multiphoton ionization of atoms in a strong laser field

Relativistic generalization is developed for the semiclassical theory of tunnel and multiphoton ionization of atoms and ions in the field of an intense electromagnetic wave (Keldysh theory). The cases of linear, circular, and elliptic polarizations of radiation are considered. For arbitrary values of the adiabaticity parameter γ, the exponential factor in the ionization rate for a relativistic bound state is calculated. For low-frequency laser radiation , an asymptotically exact formula for the tunnel ionization rate for the atomic s level is obtained including the Coulomb, spin, and adiabatic corrections and the preexponential factor. The ionization rate for the ground level of a hydrogen-like atom (ion) with Z ? 100 is calculated as a function of the laser radiation intensity. The range of applicability is determined for nonrelativistic ionization theory. The imaginary time method is used in the calculations.     

The backscattering factor for the Au N67VV Auger transition

The Au N₆₇VV Auger transition may take place after direct ionization of the N₆₇ subshells or after ionization of the N₄₅ subshells followed by the Coster-Kronig transitions N₄₅N₆₇V. The subshells N₆₇ and N₄₅ have much different ionization energies, by a factor of four, and this creates a problem in quantification of the N₆₇VV signal. Calculations of the backscattering factor from the analytical expressions require knowledge of a single value of the ionization energy. Furthermore, a single value of the ionization energy is needed in calculations of the ionizations cross section. An attempt is made here to decide which ionization energy should be used in calculations by comparison of the experimental energy and emission angle dependence of the AES signal intensity with this dependence determined from theory using different ionization energies. It has been found that the N₆₇VV signal intensity is strongly dominated by ionizations of the N₅ subshell. Furthermore, the ionization cross section for the N₅ subshell is well described by the Casnati et al. formula.     

Calculation of cross sections of simultaneous ionization in ion-atom collisions by the generalized geometrical model

The geometrical model (GM) of ionization in ion—atom collisions [8, 9] was generalized to describe ionization of both colliding particles (simultaneous ionization) due to electron—electron interaction. The generalized GM (GGM) allows calculation of the cross sections for electron loss by an incident particle with simultaneous target ionization at collision velocities higher than characteristic electron velocities, accurate within a factor of two with respect to the Born or impulse approximation. An advantage of the GGM, except for its simplicity, is easy calculation of p(b) (p is the ionization probability and b is the impact parameter), which makes it possible to include the contribution of simultaneous ionization into more general approximate schemes for calculating cross sections of multielectron ionization of atoms or ions.     

Collisional-radiative ionization and recombination coefficients for quasi-stationary homogeneous hydrogen and hydrogenic ion plasmas

The recombination and ionization coefficients have been re-calculated on the basis of the collisional-radiative model. The numerical results obtained deviate from those published byBates, Kingston, andMcWhirter. The deviations reach a factor of about four for the recombination and a factor ten for the ionization coefficients. The smallest deviations occur in the recombination coefficients when the plasma is assumed to be optically thin in all transitions. The conditions for the applicability of the collisional-radiative model are discussed.     

钙、锶、钡和汞离子激光中的共振辐射俘获效应

根据Holstein的共振辐射俘获理论，讨论了气体温度和辐射俘获下能级粒子数密度对辐射俘获上能级有效辐射寿命的影响，显示辐射俘获下能级粒子数密度是辐射俘获效应中的主导因素.计算了钙，锶，钡和汞四种离子七条共振-亚稳能级跃迁激光在不同电离率下产生共振辐射俘获效应的阈值条件，发现当电离率为5%时这类激光的出光温度与共振辐射俘获的阈值温度相一致，这与实验得出的电离率为3%—5%相符合.表明达到共振辐射俘获阈值条件是该类激光实现粒子数反转的重要因素.     

Multiple ionization and the charge state evolution of ions exposed to electron impact

Charge state abundances of atoms exposed to an electron flux for a time t are calculated from experimental cross sections by considering either electron impact single ionization only or by including multiple ionization. When multiple ionization is neglected Xe^q+ ion abundances (q = 0,1,…,6) for an electron energy of 700 eV are off by a factor of up to 2 both in peak size and in time necessary to reach the peak value.     

电荷载流子倍增寄存器中的倍增噪声

信号电荷在电荷载流子倍增寄存器中的强场下,吸收电场能量激发碰撞电离过程。电子碰撞电离过程激发的电子-空穴对具有独立性和随机性,其激发过程产生的倍增噪声主要是散粒噪声。借助于马尔可夫链定理,得到了CCM单元的倍增因子的计算方法,建立了电子碰撞电离的数学模型。在此基础上,推导了CCM单元倍增噪声的功率谱密度,表明其与倍增因子相关。     

On the relativistic generalization of Keldysh ionization theory

Relativistic generalization of the semiclassical theory of tunnel and multiphoton ionization of atoms and ions in the field of a high-intensity electromagnetic wave (Keldysh theory) with linear and elliptic polarizations is developed. The exponential factor in the ionization rate of relativistic bound state is calculated for arbitrary values of the adiabaticity parameter γ. In the case of low-frequency laser radiation, $\gamma \ll 1$, the asymptotically exact formula is derived for the ionization rate of atomic s-level, including the Coulomb, spin and adiabatic corrections and the pre-exponential factor. The ionization rate of the ground state of a hydrogen-like atoms (ions) with $Z\lesssim 100$ is calculated, depending on laser intensity and ellipticity of radiation.     

Field ionization of free helium atoms: Correlation between the kinetic energy of ionized atoms and probability of their field ionization

In this paper the correlation between the kinetic energy of helium atoms and the probability of field ionization is investigated by exploiting the narrow velocity distribution of supersonic molecular beams. Field ionization measurements were carried out on supersonic helium beams at 298 K and 95 K corresponding to energies of about 65 meV and 20 meV, respectively, for the individual atoms. The field ionization was performed with a tungsten tip, radius of curvature 12 nm, kept at room temperature. The ionization probability was found to increase by about a factor 10 when the beam was cooled from 298 K to 95 K. The results presented in this paper are of importance for improving the understanding of field ionization and for the development of a new detector for helium and other molecular beams.     

Combination mechanism of ionization and low-temperature conductivity

A combination electrothermal (ET) mechanism of ionization of neutral centers is demonstrated to exist. The corresponding ionization coefficient is calculated and conditions at which the ET mechanism becomes dominant in the process of ionization of neutral centers are elucidated. Investigations show that the ET ionization mechanism leads to an increase in the free-charge carrier density in the pre-breakdown state and for a low compensation factor reduces the breakdown electric field intensity, thereby leading to better agreement with the experimental data. I. Dzhavakhishvili Tbilisi State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 3–6, November, 1999.     

Design and preliminary test of a free-air ionization chamber for low-energy X-ray

A free-air ionization chamber in low-energy X-ray has been designed and manufactured at theNational Institute of Metrology (NIM, China) according to the defination of alr-kerma. The results of a preliminary test show that the leakage current of ionization chamber is around 2×10<'-15>A, and the correction factor of ion recombination for the ionization chamber is also obtained. The free-air ionization chamber is suitable for the primary standard in low-energy X-rays.     

Monte Carlo simulation of exposure factor

In this paper, we simulate the exposure factor by a simple model of a free-air ionization chamber with the Monte Carlo programme Geant4. Special emphasis is placed on the discussion of the exposure factor related to parameters of the chamber model. The reason for the variation in exposure factor with incident ray energy is also analysed in terms of reaction cross section for different types of reactions. The obtained results indicate that our simulation is accurate in the calculation of the exposure factor and can serve as a reference in designing air ionization chambers.     

Enhancement of the molecular nitrogen dissociation and ionization levels by argon mixture in flue nitrogen plasma

In this work, the nitrogen molecular dissociation and ionization levels in Ar/N₂ flue plasma are evaluated as functions of plasma parameters such as Ar mixture quantity and N₂ flux in order to obtain the best condition for various applications such as thin film deposition and material surface modification. This plasma is operated at 10 kV and the nitrogen dissociation rate is determined by analyzing the optical emission of the nitrogen band. For different operating conditions, the dissociation rate [N] of N₂ molecules was enhanced, as the mixture quantity of Ar increased from 0.06 m³/h to 0.9 m³/h and the max of enhancement factor is 4.3. This factor becomes bigger when the N₂ flux becomes bigger. Moreover, the molecular nitrogen ionization density is calculated from the current intensity of the plasma. The ionization density was also enhanced, as the mixture quantity of Ar increased from 0.1 m³/h to 1.5 m³/h, under three different voltages. The max of enhancement factor of 1.96 is much smaller than the factor of the dissociation rate. These results are discussed in terms of the kinetics of the electrons, nitrogen ions, atoms and molecules.     

Electron lucky-drift impact ionization coefficients of ZnS : Mn

Fit of the experimental data of ZnS : Mn by a modified lucky-drift formula has been performed using the least square algorithm. The fit agrees well with the experimental data only at high field. The best fitting parameters at high field are the mean free path of order 102.74 Å and Keldysh factor,p ₀ = 0.0138. A generalized Keldysh formula has been used, due to introduction of a soft threshold factor. The soft lucky-drift theory can also be used to calculate the impact ionization coefficients of high electron energy of ZnS : Mn without losing its physical significance compared to full band-structure Monte Carlo calculation with a remarkably reduced amount of computer resources. The curvature on semi-log plot of experimental impact ionization coefficient against the inverse of electric field is different from what is observed for other materials at low electric fields due to impact ionization of deep level impurities.     

空心阴极类火花放电初始电离过程的PIC/MCC模拟

采用粒子模拟与蒙特卡罗相结合(PIC/MCC)的方法,应用静电模型,编写了准三维的模拟程序.该程序能够较好地描述空心阴极类火花放电初始电离过程的演化步骤.通过研究电离过程的细节,可以认为该阶段电离过程是空心阴极效应和局部强电场共同作用的结果.从起始电离到空心阴极初始阶段,局部强电场在电离过程中起到了支配作用;随后空心阴极效应占据主导地位. 关键词： 粒子模拟 蒙特卡罗 空心阴极 类火花放电     

Sputtering and secondary ion yields of metals subjected to oxygen ion bombardment

The yields of sputtering and secondary ion emission of metals under oxygen ion bombardment were simultaneously measured for multi-layer targets with known layer thickness. The sputtering yield of each layered metal was determined by the time to sputter away the thin film. The experimental results revealed that the sputtering yields of metals were reversely proportional to the energy transfer factor in the classical head-on collision model and a linear relationship was found between the ionization potential and a modified degree of ionization which can be expressed by the ionization potential and the secondary O⁺₂ current.     

Temperature dependent characteristics of submicron GaAs avalanche photodiodes obtained by a nonlocal analysis

In this paper, using a nonlocal analysis we have extracted the temperature dependent ionization coefficients and threshold energies of submicron GaAs avalanche photodiodes (APDs) with multiplication region thicknesses as narrow as 49 nm, from electron and hole injection photo-multiplication processes. These extracted parameters have been used to predict the temperature dependence of APDs characteristics, such as mean gain, 3 dB-bandwidth, gain-bandwidth product, excess noise factor, performance factor, and breakdown field, over a temperature range of 20 K to 290 K. In the nonlocal analysis we have taken the effects of nonuniform electric filed within the multiplication region and its surrounding depletion regions, injected carrier’s initial ionization energy, carrier’s spatial ionization rate as well as the carrier’s dead space and its previous ionization history into account. We have shown that our predicted gain values are in excellent agreement with existing experimental data measured by others.     

Multielectron ionization of atoms by fast ions: An approximation by normalized exponentials

Multielectron ionization of neutral atoms by fast positive ions is considered in terms of the independent particle model. A relatively simple technique for calculating the multielectron ionization probabilities and cross sections through the impact parameter is suggested in which one-electron ionization probabilities are represented as normalized exponentials p _nl(b) = p _nl(b) = p _nl(0)exp(?α_nl b), where b is the impact parameter and n and l are quantum numbers of the target atomic shell. Exponent α_nl is determined from the Born one-electron ionization cross section for target atoms, and preexponential p _nl(0) (the ionization probability at a zero impact parameter) is found from a geometrical model. This technique provides the normalization condition p _nl(b) ≤ 1 irrespective of the velocity and charge of striking ions and makes it possible to calculate the one-, two-, and three-electron ionization cross sections, which, when added up, make a major contribution to the total cross section, up to a factor of 2. The results of our computations are compared with experimental data and analytical results of other authors.     

The fate of the OH radical in molecular beam sampling experiments

The collisional history of ionized molecules in a molecular beam mass spectrometric flame experiment is target of our present investigation. Measurements in a double imaging photoelectron photoion coincidence spectroscopy (i²PEPICO) were performed at the Swiss Light Source (SLS) of the Paul Scherrer Institute to use the ion imaging device for separating the molecular beam ions from rethermalized ions. This enables the precise composition study of the individual types of ions. Results show clearly for the OH radical that the complete signal is obtained from the molecular beam, while the signal from other combustion compounds features additional rethermalized molecules. As for OH radicals, the mole fraction is reduced by sampling effects and contact with the ionization vessel walls significantly. Consequently, this leads to signal loss and lower mole fractions, when using ionization cross sections for the quantification. To improve on this, a beam fraction (BF) factor is presented. The factor describes the ratio of the separated beam signal without rethermalized ions with the total ion signal, consisting of the mass to charge ratio from the molecular beam and additional rethermalized ions. Since the detected OH radicals are solely from the molecular beam, a new method of comparing two molecular beam alignments using the OH to H₂O signal ratio is presented. This method has a decent potential for the optimization of the quality of molecular beams. Finally, the separated beam signal (without the rethermalized ions) was used to determine mole fraction profiles for the OH radical using ionization cross sections. These profiles are in good agreement with model predictions of the USC-II and the Aramco Mech 2.0 mechanisms, while the total signal leads to factor of 12 smaller OH mole fractions.