碰撞阈值下氩原子非次序双电离

利用三维经典系综模型研究了碰撞阈值下氩原子的非次序双电离.计算结果表明,关联电子末态纵向动量主要分布在二、四象限,且在原点附近几乎没有分布;Ar²⁺离子末态纵向动量谱在零动量附近呈单峰结构.上述结果与实验结果 定量一致.轨迹分析表明,在碰撞阈值下,氩原子非次序双电离的微观物理机理在不同激光强度下是不相同的.当激光强度I=0.7×10¹⁴ W/cm²时,一次碰撞主导重碰撞过程.而当I=0.4×10^{14关键词： 非次序双电离 库仑引力 碰撞阈值 电子关联}     

Multiple recollisions in nonsequential double ionization below the recollision-ionization threshold

By using the three-dimensional classical ensemble model, the recollision dynamics in nonsequential double ionization (NSDI) of Ar by 780-nm laser pulses at (6-1.2)×10¹⁴ W/cm² was extensively studied. We revealed the picture of multiple-recollision in the double ionization events at the laser intensity region below the recollision-ionization threshold. Via tracing the NSDI trajectories, it was found that the contribution of these multiple-recollision events increases as the laser intensity decreases. In this low intensity region, many multiple-recollision induced NSDI trajectories occur through the doubly excited states. The decay speed of the doubly excited state decreases with the decreasing laser intensity.     

Attosecond correlated dynamics of two electrons passing through a transition state

The strong-field induced decay of a doubly excited, transient Coulomb complex Ar**→Ar(2+)+2e(-) is explored by tracing correlated two-electron emission in nonsequential double ionization of Ar as a function of the carrier-envelope phase. Using <6 fs pulses, electron emission is essentially confined to one optical cycle. Classical model calculations support that the intermediate Coulomb complex has lost memory of its formation dynamics and allows for a consistent, though model-dependent definition of "emission time," empowering us to trace transition-state two-electron decay dynamics with sub-fs resolution. We find a most likely emission time difference of ～200±100 as.     

Energy Levels of Doubly Excited States in Argon

Term energies of doubly excited states in argon and argon ions have been determined using single-configuration Hartree-Fock calculation. The energies calculated for KL3s3p⁶nl and KL3s⁶3p⁴nln1 states of Ar show a fairly good agreement with the experimental results obtained by various techniques. However, for Ar+, Ar²+, Ar²+ and Ar³+ states there are no experimental data available in the literature for comparison.     

Transition from strong-field sequential to nonsequential double ionization at near-infrared wavelengths and low intensities

Within a quantum-mechanical model, we investigate strong-field double ionization of a model helium atom by near-infrared, linearly polarized laser pulses at intensities far below the recollision threshold. The quantum simulations show a clear mechanism change from sequential to nonsequential double ionization (NSDI) as the laser intensity increases. For NSDI, the two-electron correlated momentum distribution exhibits a strong final-state Coulomb repulsion effect for high-energy photoelectrons, but absent for low-energy photoelectrons. This repulsion effect is ascribed to field double ionization from doubly-excited states populated by recollision of the first ionized electron when it returns to the parent ion. Such recollision-induced excited states are absent at ultraviolet wavelengths due to the very low returning kinetic energies, resulting to the absence of final-state repulsion effect in NSDI.     

Nonsequential double ionization of nonaligned diatomic molecules N2 and O2

Nonsequential double ionization (NSDI) processes of nonaligned diatomic molecules N2 and O2 are studied using the S-matrix theory. Our results show that the NSDI process significantly depends on the molecular symmetry and structure. The ratio of NSDI rate to single ionization rate as a function of the field intensity is obtained. It is found that N2 behaves closely with its companion atom Ar in the ratios over the entire intensity range, while O2 exhibits an obvious suppression effect, which is qualitatively consistent with the experiment.     

Low field laser ionization of argon clusters: the remarkable fragmentation dynamics of doubly ionized clusters

We have investigated the fission following a Coulomb explosion in argon clusters (up to Ar800) irradiated by a femtosecond infrared laser with moderate intensity IL approximately 10(13) W cm(-2). We report the a priori surprising observation of well-defined velocity distributions of the ionized fragments Ar+n<50. This is interpreted by the formation of a valence shell excited charged ion, followed by relaxation, charge transfer by autoionizing collision at very short distance, and asymmetric fission.     

Penning ionization of NO and O2(1Δg) by argon in the 3P1 state

Electronically excited argon in the ³ P ₁ state was generated in a flow system by resonance absorption of the λ=106·7 nm emission line. This photochemical source is particularly convenient for kinetic studies since Ar(³ P ₁) alone is produced in the primary exciting step. Energy transfer from excited argon may lead to Penning ionization:

This process has been studied for M=NO and M=O₂(¹Δ _g ). Relative concentrations of Ar* were estimated from the Penning ion current, and it is shown that Ar* is probably in the initially excited ³ P ₁ state and not in the metastable ³ P ₂ state. Kinetic studies showed that Penning ionization of NO occurs for about one in every five gas kinetic collisions with Ar(³ P ₁). The Penning process can only compete effectively with radiative loss of Ar(³ P ₁) as a result of extensive imprisonment of resonance fluorescence. Penning ionization of O₂(¹Δ _g ) is shown to proceed at nearly the same rate as ionization of NO. The decrease in Penning ion current on addition of a quenching gas, such as N₂ or O₂, is used to derive rate constants for the quenching process relative to the rate of quenching by NO. Estimated absolute rate constants are presented in a table.     

Towards Stronger Coulomb Coupling in an Ultracold Neutral Plasma

Ultracold neutral plasmas are strongly coupled Coulomb systems that are generated by photoionizing lasercooled atoms close to the ionization threshold. The strong coupling parameter Γ is limited at times later than ～100 ns by disorder‐induced heating. A recent simulation predicted that higher values of Γ can be realized in ultracold neutral plasmas if the plasma ions are excited to higher ionization states. In this paper we present recent results from an experiment that increases the strong coupling of an ultracold neutral plasma by promoting the plasma ions to the second ionization state. Using laser‐induced fluorescence we map out the ion velocity distribution of the Ca⁺ ions in a partially doubly ionized plasma and show that the heating due to the second ionization depends on the timing of the second ionization laser pulses. We compare our results to MD simulations, which estimate that Γ increases from approximately 2.5 to 3.6. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

低于再碰撞阈值下强激光场原子非序列双电离

在最近的实验和理论研究中,我们探讨了氩原子和氖原子在红外强激光场中低于再碰撞阈值的非序列双电离问题。在研究中,我们发现在非序列双电离过程中,氖原子的电子关联表现为在激光偏振面内肩并肩出射,而对于氩原子的电子关联行为表现为在激光偏振面内的背对背出射,我们采用三维半经典模型（考虑电子隧道电离）很好地解释了实验结果。在阈值附近,我们发现电子在激光场中的多次散射以及电子再碰撞激发后电子隧道电离是氩原子反关联行为的主要原因,而电子在激光场作用下的单次碰撞是电子关联行为的主要原因。通过测量双电离过程中产生电子的横向电子动量分布,观察到了库伦聚焦效应,我们认为这是非经典的关联行为。最后,我们给出了氩原子和氖原子在激光场中阈值的解析模型,并给出了原子的关联和反关联激光强度区域.     

Outers-shell ionization of neon,argon, and krypton by H 2 + and He+ impact (l00 keVto 1000 keV)

The outers-shell ionization of Ne, Ar, and Kr by H ₂ ⁺ and He⁺ impact has been studied in the energy range of 100 keV to 1000keV. The observed structures in the ionization functions are explainable by molecular processes and by direct Coulomb ionization.     

<Emphasis Type="Italic">D</Emphasis>-Wave Resonances in Three-Body System Ps<Superscript>−</Superscript> with Pure Coulomb and Screened Coulomb (Yukawa) Potentials

We have investigated the doubly excited ¹ D ^e resonance states of Ps⁻ interacting with pure Coulomb and screened Coulomb (Yukawa) potentials employing highly correlated wavefunctions. For pure Coulomb interaction, in the framework of stabilization method and complex coordinate rotation method we have obtained two resonances below the n = 2 threshold of the Ps atom. For screened Coulomb interaction, we employ the stabilization method to extract resonance parameters. Resonance energies and widths for the ¹ D ^e resonance states of Ps⁻ for different screening parameter ranging from infinity (pure Coulomb case) to a small value are also reported.     

Electron Transitions during the Capture of an Electron by a He<Superscript>2+</Superscript> Ion at the Argon Atom

We have measured the absolute values of the total cross section of the one-electron capture by He²⁺ ions in the kinetic energy range 2–30 keV at the Ar atoms. The absolute values of the differential scattering cross sections of He⁺ ions formed during the one-electron capture and the electron capture with ionization at energies of 2.2, 5.4, and 30 keV have been determined. The electronic states of the formed ions have been determined using collision spectroscopy based on analysis of the change in the kinetic energy of He⁺ after the interaction. We have measured doubly differential (with respect to the kinetic energy and the scattering angle) cross sections of the formation of free electrons. The free electron formation channels (direct ionization and electron capture with ionization) have been analyzed by calculating the electron terms of the (HeAr)²⁺ system. The calculated cross section of capture with ionization is in conformity with the cross section measured using collision spectroscopy.     

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.     

Pulsed UV laser spectroscopy of thermal Eu2+ ions

An investigation of the radiative lifetimes of several excited Eu²⁺ levels has been performed using a novel ion source for the production of thermal, doubly charged positive ions in a continuous gas discharge. This ionization device, first applied to the generation of Eu²⁺, is suitable also for other elements with similar ionization potential and vapour pressure.     

Cold Ar 3 + generated by ionization of argon clusters in large helium clusters: Temperature dependence of the photofragmentation

Ionized argon clusters were generated by electron impact ionization of neutral argon clusters embedded in large neutral helium clusters. Photofragmentation spectroscopy of Ar ₃ ⁺ and Ar ₃ ⁺ He produced in this way demonstrates the strong influence of vibrational excitation on the photodissociation dynamics, and indicates the low internal energy of the latter cluster.     

低于再碰撞阈值下强激光场原子非序列双电离(英文)

在最近的实验和理论研究中,我们探讨了氩原子和氖原子在红外强激光场中低于再碰撞阈值的非序列双电离问题。在研究中,我们发现在非序列双电离过程中,氖原子的电子关联表现为在激光偏振面内肩并肩出射,而对于氩原子的电子关联行为表现为在激光偏振面内的背对背出射,我们采用三维半经典模型(考虑电子隧道电离)很好地解释了实验结果。在阈值附近,我们发现电子在激光场中的多次散射以及电子再碰撞激发后电子隧道电离是氩原子反关联行为的主要原因,而电子在激光场作用下的单次碰撞是电子关联行为的主要原因。通过测量双电离过程中产生电子的横向电子动量分布,观察到了库伦聚焦效应,我们认为这是非经典的关联行为。最后,我们给出了氩原子和氖原子在激光场中阈值的解析模型,并给出了原子的关联和反关联激光强度区域。     

Observation of intense low energy autoionization lines in the wings of the forward peak from fast ion-atom collisions

In the ejected electron spectra from collisions of C²⁺, O⁴⁺ and Si^10,11,12+ with argon toms at energies of 2–4 MeV/amu, a series of intense lines has been observed, which originates from doubly excited autoionizing states of Belike projectiles. These lines correspond to electron energies between 2 and 20 eV in the projectile frame. For kinematic reasons these lines can only be detected near the forward direction.     

Complex-Scaling Calculations for Doubly Excited Resonances in Ps? Interacting with Screened Coulomb (Yukawa) Potentials

We have investigated the effect of screened Coulomb potentials on the high-lying doubly excited resonance states of the positronium negative ion in the framework of complex-scaling method. Highly correlated wave functions in Hylleraas coordinates are used. The resonance parameters below the Ps (2s²S) and Ps (3s²S) thresholds, for various screening parameters, are reported.     

Ultrafast Coulomb explosion imaging of molecules

Multiple ionization of small molecules leads to multifragmentation called Coulomb explosion. The atomic positions prior to the fragmentation event are determined from the multicharged fragments trajectories. Following the pioneering contribution of Prof. N.B. Delone on atomic ionization using strong laser fields, intense femtosecond laser pulses in the 10¹⁵ W cm^?2 range permit an efficient removal of valence electrons. Pulse durations have to be reduced to a few cycles in order to avoid any significant molecular stretching during multiple ionization. Pumpprobe excitation schemes offer promising perspectives for straightforward ultrafast Coulomb explosion imaging of excited and ionized molecular species.