Exploration of magnetic field generation of H32+ by direc ionization and coherent resonant excitation

Coherent electronic dynamics are of great significance in photo-induced processes and molecular magnetism. We theoretically investigate electronic dynamics of triatomic molecule H₃²⁺ by circularly polarized pulses, including electron density distributions, induced electronic currents, and ultrafast magnetic field generation. By comparing the results of the coherent resonant excitation and direct ionization, we found that for the coherent resonant excitation, the electron is localized and the coherent electron wave packet moves periodically between three protons, which can be attributed to the coherent superposition of the ground A' state and excited E⁺ state. Whereas, for the direct single-photon ionization, the induced electronic currents mainly come from the free electron in the continuum state. It is found that there are differences in the intensity, phase, and frequency of the induced current and the generated magnetic field. The scheme allows one to control the induced electronic current and the ultrafast magnetic field generation.     

Tunnel ionization of molecules and orbital imaging

We study whether tunnel ionization of aligned molecules can be used to map out the electronic structure of the ionizing orbitals. We show that the common view, which associates tunnel ionization rates with the electronic density profile of the ionizing orbital, is not always correct. Using the example of tunnel ionization from the CO(2) molecule, we show how and why the angular structure of the alignment-dependent ionization rate moves with increasing the strength of the electric field. These modifications reflect a general trend for molecules.     

啁啾激光场中氢分子离子电离过程的理论研究

本文在非玻恩-奥本海默(Non Born-Oppenheimer)近似条件下,通过数值求解一维含时薛定谔方程,探究了氢分子离子体系在超短啁啾激光条件下的电离过程.数值结果表明:有效选取啁啾参数可显著增强氢分子离子的电离过程,并且激光强度越大,电离过程对啁啾参数越敏感.结合不同强度下平均核间距及电子波包密度随时间的变化,分别从核与电子运动的角度,有效的解释了啁啾脉冲对氢分子离子电离过程的影响.     

Mikrophysikalische Bestimmung elektronenkinetischer Kenngrößen im binären Molekül-Mischplasma von Stickstoff und Wasserstoff. II. Elektronenstoßfrequenzen für die Anregung und Ionisierung langlebiger elektronischer Molekülzustände und die Energieverlustraten der Elektronenkomponente des Mischplasmas

In this second part of the in [l] started paper the results of microphysical determined collision frequencies of the electrons for the excitation and ionization of metastable molecules, excited in electronic states, and of the energy loss rates of the electrons due to elastic collisions, vibrational excitation, electronic excitation, dissociation and ionization of the molecules in a N₂ - H₂ -plasma are represented and discussed. These investigations are related to a low ionized, homogeneous and stationary plasma in a mixture and are performed for the range 6- 100 V/(cm Torr) of the reduced electric field strength and for any composition of the N₂ - H₂-mixture.     

Dynamics of strong-field double ionization of H_2 in co-rotating two-color circularly polarized laser fields

Double ionization of H_2 in a co-rotating two-color circularly polarized(TCCP) laser field is theoretically investigated. By changing the ratio of electric field peak amplitudes of the TCCP laser pulses, the double ionization probability as a function of the laser intensity shows a clear knee structure, which is suppressed significantly in the case of the atom. Due to the large spatial range of the electronic initial distribution, with the analysis of classical trajectories of ionized electrons, it is found that the ionization of the electron in the farther distance increases the probability of recollision. Furthermore, the yield of nonsequential double ionization created by the recollision can be enhanced by controlling the amplitude ratio of the TCCP laser field.     

Photoelectron angular distributions of molecules in bichromatic laser fields of circular polarization

Photoelectron angular distributions (PADs) from above-threshold ionization of O₂ and N₂ molecules irradiated by a bichromatic laser field of circular polarization are studied. The bichromatic laser field is specially modulated such that it can be used to mimic a sequence of one-cycle laser pulses. The PADs are greatly affected by the molecular alignment, the symmetry of the initial electronic distribution, and the carrier-envelope phase of the laser pulses. Generally, the PADs do not show any symmetry, and become symmetric about an axis only when the symmetric axis of laser field coincides with the symmetric axis of molecules. This study shows that the few-cycle laser pulses can be used to steer the photoelectrons and perform the selective ionization of molecules.     

异核双原子和三原子分子在红外强激光场中的电离抑制

Ionization is the fundamental process in interaction of atoms/molecules with femtosecond strong laser fields. Comparing to atoms, molecules exhibit peculiar behaviors in strong-field ionization because of their diverse geometric structures, molecular electronic orbitals as well as extra nuclear degrees of freedom. In this study, we investigate strong field single and double ionization of carbon monoxide (CO) and carbon dioxide (CO₂) in linearly polarized 50-fs, 800-nm laser fields with peak intensity in the range of 2×10¹³ W/cm² to 2×10¹⁴ W/cm² using time-of-flight mass spectrometer. By comparing the ionization yields with that of the companion atom krypton (Kr), which has similar ionization potential to the molecules, we investigate the effect of molecular electronic orbitals on the strong-field ionization. The results show that comparing to Kr, no significant suppression is observed in single ionization of both molecules and in non-sequential double ionization (NSDI) of CO, while the NSDI probability of CO₂ is strongly suppressed. Based on our results and previous studies on homonuclear diatomic molecules (N₂ and O₂), the mechanism of different suppression effect is discussed. It is indicated that the different structure of the highest occupied molecular orbitals of CO and CO₂ leads to distinct behaviors in two-center interference by the electronic wave-packet and angular distributions of the ionized electrons, resulting in different suppression effect in strong-field ionization.     

Heating of deuterium clusters by a superatomic ultra-short laser pulse

The mechanisms of heating of the electronic component of large deuterium clusters by a super-atomic ultra-short laser pulse field are considered. During pulse rise, the so-called “vacuum heating” plays the determining role. Electrons escaping from a cluster into the vacuum with a low energy return back in a time equal to the period of the laser under laser field action. The returning electrons have a higher energy (on the order of the vibrational energy in the laser radiation field), which causes cluster heating. As the laser field increases, the electronic temperature largely grows at the expense of decreasing the Coulomb potential energy of electron repulsion because of a decrease in the number of electrons. The dynamics of above-barrier cluster ionization at the leading edge of a superatomic laser pulse is calculated. The results are discussed in the light of recent experiments aimed at creating desktop sources of monoenergetic neutrons formed as a result of the fusion of deuterium nuclei in a cluster plasma.     

Selective steering of molecular multiple dissociative channels with strong few-cycle laser pulses

We report that multiple dissociative channels of carbon monoxide (CO) molecules are selectively controlled using intense phase-stabilized few-cycle laser fields (4.2 fs, 740 nm, 6×10(14) W/cm(2)). The controllable emission direction of C(2+) from charge asymmetrical dissociation and ionization of CO dications is out of phase in a linear polarized laser field. The strong coupling between the channels is explained as the competition of recollision excitation and recollision ionization in a recollision process, leading to the opposite asymmetrical property. The experimental result provides insight into high degree controlling molecular multiple dissociative processes in the time scale of electronic motion.     

铑原子基态的理论研究

在有心力场近似和组态相互作用理论框架下,通过对电子波函数、各个壳层上电子的束缚能、平均轨道半径、总束缚能、基组态时的激发能、电离能的分析和计算,研究了Lr原子的可能基组态及其基组态时的原子态.研究结果表明:基组态为5f147s27p,基态时的原子态为2P1/2.     

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.     

Double ionization of helium studied with the multi-configuration time-dependent Hartree–Fock method

The intensity dependence of double ionization probability of helium in a strong laser field is investigated by using the multi-configuration time-dependent Hartree–Fock (MCTDHF) method. The results agree with the previous theoretical report by directly solving the time-dependent Schrödinger equation. The time evolution of electronic population on the ground state, electronic energy and dipole moment are also presented and discussed.     

Excitation and emission of metal electrons in atom-surface collisions

Electron-hole pair excitation and ionization probabilities are calculated for atomic collisions with metal surfaces at high incident energies. The method adopted is based on a Sudden Collision Approximation, and a realistic model is employed for the bound and continuum electronic states involved. The parameters used in the calculations are for Ar, He, H atoms impinging on a Li surface at 300 eV. The main results are: (1) Only single electron-hole pair excitations are important; multiple pair contributions are small. (2) The transitions are dominated by the behavior of the electronic wavefunctions in the tunneling region and may serve as a probe of this regime. (3) The excitation efficiency is in the order H ? Ar ? He, the effectiveness of hydrogen being due to its stronger, longer-range coupling. (4) The maximum excitation probabilities are for electrons ejected with relatively low excess energies. (5) Total transition probabilities are about 0.5 per collision for H, and about 0.1 for Ar, indicating that these are important, easily detectable processes. Experiments in this field should provide important information on electronic wavefunctions at the metal-gas interface, and on gas-metal interactions at high energies.     

Correlation between double and nonresonant single ionization

We performed simultaneous measurements of electrons and ions produced in Xe photoionization driven by an 800-nm, 100-fs laser pulse. The obtained energy and angular resolved electron spectra allow the identification of the electronic states populated during the ionization. Xe2+ ions appear at the same laser intensity as electrons emerging from a nonresonant 9-photon ionization process of Xe. Similar to optical tunneling ionization, the nonresonant ionization delivers low-energy electrons needed for the formation of Xe2+ by a backscattering process.     

磁场梯度对Hall推力器放电特性影响的实验研究

为进一步探索Hall推力器通道内磁场优化设计理论,通过实验分析了强场区磁场梯度对推进剂的电离与加速等放电过程的影响. 研究发现,在本实验设计的磁场梯度范围内,磁场梯度大小对推进剂的电离过程影响较小,但是对离子流的加速特性会产生较为明显的影响.随着磁场梯度的增加,离子束的能量分布会趋于集中,推力器效率提高. 最后,对磁场轴向梯度进一步变大可能会引起的一系列物理问题如有限Larmor半径效应、电子传导机理转变规律和梯度漂移效应等进行了分析和思考.     

The emission properties of organic field ion emitters: A combined FIM and FI-MS investigation

The emission properties of tungsten emitters activated with benzonitrile, such as are used in analytical organic field ion mass spectrometry, have been investigated using n-heptane as test gas. It was discovered that these emission properties are influenced principally by the morphological structure and electronic state of the surfaces of the microneedles; the bulk structure of these organic agglomerates has practically no effect on their ionization behaviour.Field ion microscope investigation of emitters activated at high temperature (HT) revealed a similar surface structure to that of pyrolytic graphite; emitters activated at room temperature (RT) showed less order. The emission regions of highest electric field strength on the tips of the needles are projecting points, ledges, and in the case of HT-emitters, structure planes.A decrease in the mean emission field strength results from the formation of solid deposits on the needle surfaces. A field corrosion of the needle tips occurs in the presence of high pressures of water and leads also to an irreversible decrease in the mean emission field strength. The effect of physically adsorbed layers on the emission properties of the emitter surface is discussed in terms of the influences of the electric field penetration on the electronic properties of the needles.     

Influence of the initial electronic state on minima of high-order harmonic spectrum radiated from hydrogen molecular ion

We theoretically investigate the high-order harmonic generation of the one-dimensional hydrogen molecular ion at fixed intermediate internuclear distance, driven by a multicycle laser field. Our results show that the initial electronic state of the hydrogen molecular ion affects the modulation of the high-order harmonic spectrum, especially the positions of the minima. Based on the two-state model, the underlying physical mechanism of the minimum is analyzed and discussed.Further analysis shows that the different positions of the minima in the different initial electronic states can be understood via the different interferences of the two phase-adiabatic states at the ionization times.     

超短超强激光与薄膜靶相互作用中不同价态碳离子的来源

研究了超短超强激光与不同厚度薄膜Al靶相互作用中靶背法线方向碳离子的最初来源. 通过对比分析碰撞电离率和场致电离率所起的作用, 发现C⁴⁺ 及更低价态的碳离子主要由场致电离产生, 而高价态的C⁵⁺和C⁶⁺ 离子主要来自于超热电子与靶表面的碰撞电离. 关键词： 超短超强激光与等离子体相互作用 离子加速 场致电离 碰撞电离     

非均匀激光场中氢分子离子高次谐波的增强

通过数值求解非波恩-奥本海默近似下的一维含时薛定谔方程,研究了蝴蝶结型纳米结构基元中氢分子离子高次谐波的产生.研究表明,在蝴蝶结型纳米结构基元内部产生的非均匀场的空间位置对高次谐波的发射有较大影响.当非均匀场的空间位置从30 a.u.平移到-30 a.u.时,高次谐波的截止位置被延展且形成光滑的超连续的谐波谱,并应用时频分析方法、经典三步模型以及电离概率等解释了高次谐波发射的物理机理.研究了高次谐波谱对非均匀场空间位置的依赖性与载波包络值的关系,发现随着载波包络值的变化,非均匀场在不同空间位置处的高次谐波谱变化趋势相同.     

Photoassociation in cold atoms via ladder excitation

We explore 2-color photoassociative ionization in cold Rb vapor and present experimental evidence that the molecular ions are produced from the stepwise excitation of a ladder of molecular states. We also explore a new process, dubbed photoassociative-dissociative ionization, by which atomic ions are created by excitation through a ladder of molecular states, finally autoionizing to a dissociative potential curve of Rb2+. We submit that these experiments could be the starting point for the same sort of high resolution spectroscopy that has already been done for lower electronic states of cold Rb2 at large internuclear separation.