Processes in condensed inert gases involving excess electrons

Drift of an excess electron in dense and condensed inert gases in external electric field and excitation of atoms by electron impact in these systems are analyzed. The effective potential energy surface for an excess electron at a given electric field strength consists of wells and hills, and the actions of neighboring atoms are therefore separated by saddles of the potential energy. At such atomic densities that the difference of interaction potentials for an excess electron between neighboring wells and hills of the potential energy surface becomes small, the electron mobility is large. This is realized for heavy inert gases (Ar, Kr, Xe) with a negative scattering length of an electron on individual atoms. In these cases, the average potential energy of the electron interaction with atoms corresponds to attraction at low atomic densities and to repulsion at high densities. The transition from attraction to repulsion at moderate atomic densities leads to a maximum of the electron mobility. A gas model for electron drift in condensed inert gases is constructed on the basis of this character of interaction. Due to high electron mobility, condensed inert gases provide high efficiency of transformation of the electric field energy into the energy of emitting photons through drifting electrons. It is shown that, although the role of formation of autodetaching states in the course of electron drift is more important for condensed inert gases than for rare gases, this effect acts weakly on exciton production at optimal atomic densities. The parameters of a self-maintained electric discharge in condensed inert gases as a source of ultraviolet radiation are discussed from the standpoint of electron drift processes.     

Far-infrared recombination radiation from impact-ionized donors in germanium

We studied the far-infrared emission spectrum resulting from recombination of an electron with an ionized impurity of As and Sb in germanium under impact ionization at liquid, helium temperatures. The emission peaks at the position corresponding to the transition from the 2p _± excited state to the ground state. This observation indicates that recombination occurs through the capture by the excited states of the donor impurity, which is consistent with the cascade trap model. The intensity of emission radiation is of the order of 10⁻⁷ watts for the excitation power of about one watt, which implies a dominant process of recombination to be accompanied by phonon emission.     

ZnSe在外电场下的基态性质和激发特性研究

以LANL2DZ为基组, 采用Hartree-Fock(HF)方法研究了不同外电场(-0.025–0.040 a.u.)对ZnSe分子的基态几何结构、电荷分布、能量、电偶极矩、最高占据轨道(HOMO)能级、最低空轨道(LUMO)能级、能隙、红外光谱特性的影响; 继而采用含时的TD-HF方法研究了ZnSe分子在外电场下前9 个激发态的吸收谱、激发能、振子强度等激发特性. 研究结果表明: 当电场从-0.025 a.u.变化到0.04 a.u. 时, 键长先减小后增加; 分子偶极矩先由正减小到0, 然后又反向增加; 体系总能量一直减小; 谐振频率先增加后减小, 红外光谱强度先减小后又增加. ZnSe分子的LUMO能级一直增加, HOMO能级先增加后又减小, 变化趋势较小, 而能隙一直增大. 外电场对ZnSe分子的激发特性影响较大, 当电场从-0.025 a.u.变化到0.04 a.u.时, 激发能增加, 相应的激发波长减小; 对应的振子强度也受到很大影响, 原来振子强度最强的激发态变得很弱, 而原来振子强度很弱的激发态变得最强. 因此, 可以通过改变电场来控制ZnSe的激发特性.     

氦气空心阴极放电动力学过程的模拟研究

利用流体模型模拟研究了氦气空心阴极放电的时空动力学过程,计算得到了不同放电时刻电子和亚稳态氦原子密度、电势、电场、基态电离速率和分步电离速率等的时空分布特性。特别是讨论了亚稳态原子和分步电离对于放电的影响。结果表明,随着电流的增长,放电处于五个不同的放电模式:第一阶段电流上升非常缓慢,为汤生放电模式,带电粒子密度、亚稳态原子密度和径向电场均很弱;第二阶段电流迅速上升,放电模式由汤生放电向空心阴极放电过渡,带电粒子密度、亚稳态原子密度和径向电场迅速增强;第三阶段达到准稳态阶段,放电电流增长速度变缓,形成了明显的阴极鞘层结构;第四阶段为空心阴极效应形成阶段,向稳态阶段过渡;第五阶段为稳态放电阶段。研究结果同时表明,亚稳态氦原子和分步电离在放电的初始阶段对于放电的发展作用较弱,在前三阶段中,电子的产生以基态电离为主。随着放电的发展,由亚稳态原子引起的分步电离对新的电子产生的作用逐渐接近并超过基态电离,对总电离的贡献率越来越高。     

量子统计权重对原子场电离效率的影响

原子能级的量子统计权重(G)是原子的重要光谱参数,但在研究原子的电离过程中通常却为了简化问题而被忽略.本文在锂原子的三步光激发(PE)+电场电离(EFI)过程中计入了其影响,并发现其对原子EFI效率的影响显著.本文精心设计了一套锂原子的PE+EFI方案:首先,采用三台不同波长的脉冲激光器分三步将原子从基态激发到某一Rydberg态上,经过一段时间延迟后再施加脉冲电场将其电离.针对原子所经历的PE、零场和EFI这三个物理过程,本文对其物理机制进行了分析并建立了服从粒子数守恒的物理模型进而导出了显含G参数的普适的速率方程组.其次,通过Matlab编程,分别针对精心选定的两条激发路径2S_(1/2)→2P_(1/2)→3S_(1/2)→25P_(1/2,3/2)和2S_(1/2)→2P_(3/2)→3D_(5/2)→25F_(5/2,7/2)开展了数值计算.研究发现:PE+EFI过程的总体效率的上限既与激光参数无关,也不依赖于G参数的绝对值,而是决定于Rydberg态的G参数的分支比.总之,通过精选激发路径可以调控PE过程各相关态的布居率,并能适当提高PE+EFI过程的总电离效率,但却因受到Rydberg态布居率的限制而很难进一步提高.     

Exciton states in asymmetric GaInNAs/GaAs coupled quantum wells in an applied electric field

The electronic and optical properties of exciton states in GaInNAs/GaAs coupled quantum well (CQW) structure have been theoretically investigated by solving the Schrödinger equation in real space. The effect of well width on the exciton states has been also studied by varying the well width from 5?nm to 10?nm in asymmetric structures. The electron, hole and exciton states are calculated in the presence of an applied electric field. It is found that there are two direct (bright) exciton states with the largest oscillator strengths. Their energies weakly depend on the electric field due to the compensation between the blue shift and red shift of the electron–hole pair states. In addition, these two states are overlap in the case of symmetric CQWs and one of them is then shifted to higher energy in asymmetric CQWs. The ground state exciton has the binding energy of approximately 7.3?meV and decrease to around 3.0?meV showing the direct to indirect transition of the ground state. The direct–indirect crossover is observed at different electric field for different structure. It happens at the electric field when the e1–e2 electron anticrossing or h1–h2 hole anticrossings is observed, so that the crossover can be controlled by the well width of CQWs structure.     

Impact of the Vibrationally Excited States on the Macroscopic Behaviour of the Band-like Electron Beam Discharge Plasma in H/H2-Mixture

The investigation of the impact of the vibrationally excited molecules in the electronic ground state was performed by simultaneously solving a balance equation system for the main charge carriers, the H atoms, the metastable H atoms, the H₂ molecules in the different vibrational states and for the power transfer of the electrons in the beam discharge mixture plasma. The balance equations for the vibrational states include in particular one-quantum step excitation and deexcitation, electronic excitation, dissociation and ionization from each vibrational level in electron collisions as well as the finite life time of these states because of the gas transfer through the band-like plasma. A main finding is that due to the additional impact of vibrationally excited molecules there is a marked enhancement of the resulting dissociation and ionization degree in the beam discharge plasma at medium power input from the turbulent electric field. For discharge parameters of practical interest the ionization and dissociation budget, the population of the vibrational states, the different energy dissipation processes and the energy pumping into the ladder of the vibrational states were calculated and discussed in detail.     

非相对论性和相对论性原子组态相互作用理论——激发能量和辐射跃迁几率

我们建立的非相对论性和相对论性原子组态相互作用理论计算方法可用来计算低度激发态的激发能量和辐射跃迁几率(包括允许跃迁和禁戒跃迁)。为了检查我们的计算程序,分别用两种方法计算了氦原子N=2激发态的激发能和相应的辐射跃迁几率。结果表明:非相对论性理论和相对论性理论计算结果基本一致。原子激发能量的精度为千分之几,振子强度的精度为百分之几。上述原子组态相互作用理论方法可用于计算任何原子或离化态原子。 关键词：     

同轴空心阴极氦等离子体的电子激发温度研究

利用同轴空心阴极放电装置,产生氦低温等离子体。通过对等离子体的发射光谱进行测量和计算,研究放电功率以及氦气压强对等离子体的电子激发温度的影响。结果表明:氦低温等离子体的发射光谱主要由连续谱和原子谱线构成,放电功率和压强对谱线的强度具有明显影响。压强的变化不仅影响电子从电场中获得的能量,还会影响电子与原子的碰撞频率,从而导致电子激发温度随着氦气压强的增大,出现先上升后下降的变化趋势。     

外电场作用下乙酸乙烯酯的分子结构和激发态

采用密度泛函B3LYP方法在6-31G(d)基组水平上,计算了乙酸乙烯酯分子在不同外电场(0.000 a.u.~0.030 a.u.)作用下的基态几何结构、电偶极距、电荷分布及分子总能量,然后利用杂化CIS-DFT方法在相同基组下研究了外电场对乙酸乙烯酯分子前8个激发态的激发能、波长和振子强度的影响.结果表明,分子的几何结构、电荷分布与外电场的大小呈现强烈的依赖关系.随着外电场的不断增加,分子的总能量逐渐减小,偶极距逐渐增大,激发能随电场的增加总体上呈减小的趋势,表明在外电场作用下,乙酸乙烯酯分子易于激发,激发态波长随电场的增加总体上呈增大的趋势,且电子跃迁光谱都集中在紫外区.     

六方氮化硼单层中一种(CN)3VB缺陷的第一性原理计算

二维六方氮化硼(hBN)的点缺陷最近被发现可以实现室温下的单光子发射,而成为近年的研究热点.尽管其具有重要的基础和应用研究意义,hBN中发光缺陷的原子结构起源仍然存在争议.本文采用基于密度泛函理论的第一性原理计算,研究hBN单层中一种B空位附近3个N原子被C替代的缺陷(CN)3VB.在hBN的B空位处,3个N原子各自带一个在平面内的悬挂键及相应的未配对电子,而通过C替换可以消除未配对的电子.系统研究了(CN)3VB缺陷的几何结构、电子结构以及光学性质,结果表明,缺陷可以由一个对称的亚稳态经过原子结构弛豫变成1个非对称的、3个C原子连在一起的基态结构.缺陷的形成在hBN中引入了一些由缺陷悬挂σ键及重构的π键贡献的局域缺陷态.这些缺陷态可以导致能量阈值在2.58 eV附近的可见光内部跃迁.本文的工作有助于进一步理解hBN中点缺陷的构成及光学性质,为实验上探讨发光点缺陷的原子结构起源及其性质提供理论依据.     

二氢化钚分子激发态结构的外场效应

采用密度泛函(DFT)方法B3LYP/Gen，在Pu为SDD基组、H为6-311++G^**基组水平上优化得到了分子轴方向不同电偶极场(-0.005—0.005a.u.)作用下，二氢化钚的基态电子状态、几何结构、电偶极矩和分子总能量.在优化构型下用同样的基组采用含时密度泛函(TDDFT)方法(TD-B3LYP)研究了同样外电场条件下对二氢化钚的激发能和振子强度的影响.计算结果表明，分子几何构型与电场大小和方向呈现较强的依赖，电场强度增加基态偶极矩随电场强度线性增加，H-Pu-H的角度线性减小，分子总能量线性减小；激发能随电场强度增加而减小，且对电场方向的依赖呈现近似对称性，满足Grozema关系.电场对振子强度的影响比较复杂，但仍满足跃迁选择定则. 关键词： 二氢化钚 激发态 电偶极场 TD-DFT     

外电场下CdSe的基态性质和光谱特性研究

采用密度泛函(DFT)B3PW91方法在Lanl2dz基组下优化得到CdSe分子的基态稳定构型, 并研究了外电场对CdSe基态分子的总能量、HOMO能级、LUMO能级、能隙、电偶极矩μ、电荷布居、红外光谱的影响. 在相同的基组下用TD-DFT 方法计算了外电场下CdSe分子的前9个激发态的激发能、激发波长和振子强度. 结果表明: 无电场时CdSe分子的激发波长与实验结果符合较好, 相应的激发能也很接近. 随着电场增加, CdSe基态分子键长、偶极矩、红外谱强度先减小后增大; HOMO能级、LUMO能级、能隙随电场增加而减小; 总能量、谐振频率则是先增大后减小. 此外, 外电场对CdSe分子的激发能, 激发波长和振子强度均有较大影响.     

外场下丙烯酸甲酯的激发特性研究

采用密度泛函B3LYP方法在6-311+ +G* *基组上优化了不同外电场作用下丙烯酸甲酯分子的基态几何结构、电偶极矩和分子的总能量,并且分析了分子的HOMO-3到LUMO+3轨道的能量变化,然后利用杂化CIS-DFT方法(CIS-B3LYP)在相同基组下探讨了无电场时丙烯酸甲酯分子前9个激发态的激发能、波长和振子强度和外电场对丙烯酸甲酯分子激发态的影响规律.结果表明,分子的几何构型与外电场大小有着较强的依赖关系.随着外电场的增大,分子总能量先增大后减小,电偶极矩μ先减小后增大,激发能随电场 关键词： 丙烯酸甲酯 外电场 振子强度 激发能     

Continuous wave field ionization laser spectroscopy

A new technique is described which allows Doppler-free, isotope-selective excitation of atoms by continuous wave laser radiation and continuous ionization of the atoms by an electric field. The atoms are excited to high Rydberg states in an electric-field-free region of a collimated atomic beam. Because the lifetimes of Rydberg atoms are long they can reach a spatially separated region of the atomic beam where they are ionized by a continuous electric field with a probability of unity. In the case of lithium we obtained a 10³ times larger ion signal by field ionization of Rydberg atoms than by direct photoionization from low excited states.     

Spectroscopic study of the production of magnesium ions from the atomic metastable states 3s3p 3 P 0, 2 by electron impact

Spectroscopic studies of the production of MgII ions from atomic metastable states by electron impact in crossed atomic and electron beams were made. Cross sections for excitation of the strongest spectral transitions and their energy dependences were determined. The maximum value of the cross section for ionization with excitation of the 3² S _1/2–3² P _3/2 transition reached 2.4×10^?16 cm². Possible mechanisms of production of excited ions from metastable states and their contribution to the total ionization process are discussed.     

外电场下2,2’,5,5’-四氯联苯的分子结构与电子光谱

多氯联苯(PCBs)是难降解有机污染物, 2, 2’, 5, 5’-四氯联苯(PCB52)是PCBs的一种, 研究通过对PCB52分子加外电场的方法来降解该物质. 采用密度泛函B3LYP方法在6-311+g(d)基组水平上优化并计算了不同外电场(-0.04–0.04 a.u.)作用下PCB52的基态分子结构参数、分子总能量、电偶极矩和电荷分布. 然后利用含时密度泛函方法研究了PCB52分子在外电场下的前六个激发态的波长、激发能量和振子强度的影响. 结果表明: 随着外电场的增加, 1C–21Cl和14C–20Cl键的键长增大; PCB52的两个苯环在外加电场下, 二面角增大, 分子毒性减弱; PCB52分子能隙减小, 导致分子更容易受激发而跃迁到激发态发生还原脱氯反应. 外电场的增大, 激发态的激发能在迅速减小, 吸收波长也迅速红移, 振子强度不再为零. 表明电场作用下, 分子易于激发和解离. 关键词： 2,2’,5,5’-四氯联苯 电场 基态 激发特性     

Influence of helium metastable states on the E-H mode transition in an inductively coupled plasma

A helium (He) inductively coupled plasma (ICP) source combined with two diode laser systems with the wavelengths of 396.5 nm (2¹S–4¹P) and 1082.9 nm (2³S–2³P) was configured to investigate the influence of He metastable states on the E-H mode transition. The population densities of 2¹S and 2³S states were measured by laser absorption spectroscopy as the functions of RF power, He pressure, and the distance from RF antenna. Correlations between the E-H mode transition and the population density ratio of the 2³S metastable state to the 2¹S metastable state as well as the line intensity ratio (LIR) of the 3³D-2³S transition to the 3¹P–2¹S transition were investigated. The E-H mode transition of He ICP was analyzed by the electron impact ionization rate and it was confirmed that the 2³S metastable state plays a key role in the E-H mode transition.     

The photodetachment spectrum of OHF−: the influence of vibration at a transition state

New ab initio potential energy functions for collinear OHF^? and OHF have been used with time-dependent wavepacket studies in a simulation of the photodetachment spectrum of the OHF^? anion. Franck—Condon excitation of OHF^? lies within the transition state region for the bimolecular reaction F + OH ? O + HF on the lowest triplet surface. The branching ratios for fragmentation of the excited OHF are such that the peaks in this spectrum may each be correlated with dissociation to a single vibrational state of the O + HF(v) or OH(v) + F products. The possible influence of vibrational excitation of the anion has been explored also. This is predicted to lead to marked changes in the spectra. Experimental observations of such spectra could be used in refinement of the potential as could an experiment in which the electron kinetic energy is detected in coincidence with the vibrational states of the molecular products or with the kinetic energy of the atomic products. The calculations revealed a number of metastable resonances on the OHF surface, although these are all at higher energies than are relevant to the photodetachment spectrum from the ground state anion.     

Metastable states of hydrogen: their geometric phases and flux densities

We discuss the geometric phases and flux densities for the metastable states of hydrogen with principal quantum number n = 2 being subjected to adiabatically varying external electric and magnetic fields. Convenient representations of the flux densities as complex integrals are derived. Both, parity conserving (PC) and parity violating (PV) flux densities and phases are identified. General expressions for the flux densities following from rotational invariance are derived. Specific cases of external fields are discussed. In a pure magnetic field the phases are given by the geometry of the path in magnetic field space. But for electric fields in presence of a constant magnetic field and for electric plus magnetic fields the geometric phases carry information on the atomic parameters, in particular, on the PV atomic interaction. We show that for our metastable states also the decay rates can be influenced by the geometric phases and we give a concrete example for this effect. Finally we emphasise that the general relations derived here for geometric phases and flux densities are also valid for other atomic systems having stable or metastable states, for instance, for He with n = 2. Thus, a measurement of geometric phases may give important experimental information on the mass matrix and the electric and magnetic dipole matrices for such systems. This could be used as a check of corresponding theoretical calculations of wave functions and matrix elements.