Momentum distributions of ne(n+) ions created by an intense ultrashort laser pulse

Vector momentum distributions of Ne(n+) (n = 1,2,3) ions created by 30 fs, approximately 1 PW/cm(2) laser pulses at 795 nm have been measured using recoil-ion momentum spectroscopy. Distinct maxima along the light polarization axis are observed at 4.0 and 7.5 a.u. for Ne2+ and Ne3+ production, respectively. Hence, mechanisms based on an instantaneous release of two (or more) electrons can be ruled out as a dominant contribution to nonsequential strong-field multiple ionization. The positions of the maxima are in accord with kinematical constraints set by the classical "rescattering model."     

Probing angular correlations in sequential double ionization

We study electron correlation in sequential double ionization of noble gas atoms and HCl in intense, femtosecond laser pulses. We measure the photoelectron angular distributions of Ne+ relative to the first electron in a pump-probe experiment with 8 fs, 800 nm, circularly polarized laser pulses at a peak intensity of a few 10(15) W/cm2. Using a linear-linear pump-probe setup, we further study He, Ar, and HCl. We find a clear angular correlation between the two ionization steps in the sequential double ionization intensity regime.     

Exploring interatomic Coulombic decay by free electron lasers

To exploit the high intensity of laser radiation, we propose to select frequencies at which single-photon absorption is of too low energy and two or more photons are needed to produce states of an atom that can undergo interatomic Coulombic decay (ICD) with its neighbors. For Ne(2) it is explicitly demonstrated that the proposed multiphoton absorption scheme is much more efficient than schemes used until now, which rely on single-photon absorption. Extensive calculations on Ne(2) show how the low-energy ICD electrons and Ne(+) pairs are produced for different laser intensities and pulse durations. At higher intensities the production of Ne(+) pairs by successive ionization of the two atoms becomes competitive and the respective emitted electrons interfere with the ICD electrons. It is also shown that a measurement after a time delay can be used to determine the contribution of ICD even at high laser intensity.     

锰原子的二步多光子与三步三光子共振电离研究

激光共振电离技术是痕量分析中的重要手段之一。文章以速率方程理论为基础,对锰原子的激光共振电离过程进行了分析,讨论了电离过程中各级激发光功率密度及激光作用时间对电离效率的影响;提出了根据所要求的电离效率和激光作用时间计算所需要的各激发光或电离激光的功率密度的方法;得到了饱和激发或饱和电离的规律及阈值条件。研究发现,在激光作用时间为10 ns时,锰原子饱和电离的激光强度阈值基本都在108 W·cm-2的量级,只有“1+1”双色双共振低三个量级;而“1+1”和“1+1+1”饱和激发的激光强度阈值则在102～103 W·cm-2量级;并且随着激光作用时间的增加,各过程的饱和激发和饱和电离的激光强度阈值将单调减少。     

Rescattering of ultralow-energy electrons for single ionization of Ne in the tunneling regime

Electron emission for single ionization of Ne by 25 fs, 1.0 PW/cm(2) laser pulses at 800 nm has been investigated in a kinematically complete experiment using a "reaction microscope." Mapping the complete final state momentum space with high resolution, a distinct local minimum is observed at P(e parallel )=0, where P(e parallel ) is the electron momentum parallel to the laser polarization. Whereas tunneling theory predicts a maximum at zero momentum, our findings are in good agreement with recent semiclassical predictions which were interpreted to be due to "recollision."     

Few cycle dynamics of multiphoton double ionization

In intense field ionization, an electron removed from the atomic core oscillates in the combined fields of the laser and the parent ion. This oscillation forces repeated revivals of its spatial correlation with the bound electrons. The total probability of double ionization depends on the number of returns and therefore on the number of optical periods in the laser pulse. We observed the yield of Ne(2+) relative to Ne(+) with 12 fs pulses to be clearly less compared to 50 fs pulses in qualitative agreement with our theoretical model.     

Properties of resonant interatomic Coulombic decay in Ne dimers

Properties of the interatomic Coulombic decay (ICD) process in Ne dimers have been obtained by tracking the formation of energetic Ne+ ions. The double photoionization cross section, deduced from the Ne+/Ne+ coincidence signal, is dominated by the ICD process and presents a threshold 280 meV below the atomic Ne+2s(-1) threshold. Rydberg excitation of a 2s electron in the dimer creates molecular Rydberg states whose Sigma and Pi symmetries have been resolved. These excited states decay by a resonant ICD process releasing an energetic Ne+ ion and a neutral excited Ne* fragment. Subsequent autoionization of the Ne* fragment explains a double photoionization threshold below the dimer 2s ionization threshold.     

Interaction of superintense laser pulses with relativistic ions

At high intensities, three-step recollision processes driven by low frequency laser pulses, such as high-order harmonic generation and high-order above-threshold ionization, are normally severely suppressed by the magnetic-field component of the laser field. It is shown that this suppression is not severe, even for ponderomotive energies well above 10 keV, for multicharged ions moving at a sufficiently high relativistic velocity against a counterpropagating infrared laser pulse. Numerical results are presented for high-order harmonic emission by a single Ne9+ ion moving with a Lorentz factor gamma=15 against a Nd:glass laser beam. The calculations are done within a Coulomb-corrected nondipole strong field approximation. The approximation is tested by comparing with accurate results.     

Frequency of lamb-dip-stabilized 1.52 μm He-Ne lasers

A ³He-²⁰Ne and a ³He-²²Ne 1.52 m laser were frequency stabilized to the Lamb-dip of their respective gain curve and provided a resettability of 2 MHz. Using these lasers, absolute frequencies for the ²⁰Ne and ²²Ne laser transitions were measured by interferometric frequency comparison with an I₂-stabilized He-Ne laser at 0.633 m. A least-square analysis which accounts for the linear frequency dependence of the laser gas pressure yielded two parameters which can reproduce the measured frequencies within an accuracy of ±1.0·10^–8 at 1.52 m.     

Photoelectron momentum distributions of Ne and Xe dimers in counter-rotating circularly polarized laser fields

The strong-field ionization of dimers is investigated theoretically in counter-rotating circularly polarized laser fields. By numerically solving the two-dimensional (2D) time-dependent Schrödinger equation (TDSE) with the single-electron approximation (SEA) frame, we present the photoelectron momentum distributions (PMDs) and photoelectron angular distribution (PADs) of aligned Ne and Xe dimers. It is found that the PMDs and PADs strongly depend on the time delays by counter-rotating circularly polarized laser pulses. The results can be explained by the ultrafast photoionization model and the evolution of electron wave packets for Ne and Xe dimers. Besides, We make a comparison of PMDs between Ne atom and Ne dimer.     

Correlated multielectron dynamics in ultrafast laser pulse interactions with atoms

We present the results of the detailed experimental study of multiple ionization of Ne and Ar by 25 and 7 fs laser pulses. Whereas in multiple ionization of Ar different mechanisms, involving field ionization steps and recollision-induced excitations, play a role, for Ne only one channel, where the highly correlated instantaneous emission of up to four electrons is triggered by a recollisional electron impact, is found to be important. Using few-cycle pulses we are able to suppress those processes that occur on time scales longer than one laser cycle.     

Recoil-ion momentum distributions for two-photon double ionization of He and Ne by 44 eV free-electron laser radiation

Recoil-ion momentum distributions for two-photon double ionization of He and Ne (variant Planck's over omega=44 eV) have been recorded with a reaction microscope at FLASH (the free-electron laser at Hamburg) at an intensity of approximately 1 x 10(14) W/cm2 exploring the dynamics of the two fundamental two-photon-two-electron reaction pathways, namely, sequential and direct (or nonsequential) absorption of the photons. We find strong differences in the recoil-ion momentum patterns for the two mechanisms pointing to the significantly different two-electron emission dynamics and thus provide serious constraints for theoretical models.     

Cold atomic collisions: coherent control of penning and associative ionization

Coherent control techniques are computationally applied to cold (1 mK相似文献   

Interference effects in the resonant photoemission channels to the Ne+ 2p(4)(1D2)3p 2P, 2D,and 2F states in the Ne 1s excitation region

We present measurements of total and partial photoionization cross sections of Ne in the 1s excitation region. The total cross section exhibits resonances with symmetric profiles, whereas the branching ratios to the Ne+ 2p(4)(1D2)3p 2P, 2D, and 2F states present strong oscillations in the interresonance regions. We prove that not only the interference between the direct and resonant ionization processes but also the interference among the ionization processes via different nonisolated resonances are important for this effect.     

电子碰撞激发机制中自电离与双电子俘获

 以Ge为例，研究了双电子复合代替自电离与双电子俘获对离子布居的影响；通过解包括双激发态和自电离与双电子俘获过程的速率方程组，研究了类F离子与类Ne离子基态对19.6nm与23.6nm激光线上、下能级的布居贡献因子及类Na离子与类Ne离子的电离速率，并讨论了这两条激光线的反转与增益。     

(1+2+1)双共振多光子电离概率的理论研究

共振增强多光子电离光谱技术已成为研究原子、分子高激发态能级结构的重要方法。运用光和物质相互作用的速率方程理论,推导出四能级物质系统1+2+1双共振增强多光子电离概率的解析表达式,以此为基础,理论模拟了电离概率随激发光强、激光脉冲宽度和碰撞弛豫速率的变化,发现在1+2+1多光子电离机制中,电离概率随光强的增加而增大,继而出现单步、双步激发饱和的现象,直至饱和值1;继续增大光强,电离概率将围绕饱和值1窄幅振荡,振荡幅度随光强增加而增大。随激光脉冲宽度的增大,电离概率从零开始逐渐增大直至饱和值1。而随碰撞弛豫速率的增大电离概率以线性规律减小。     

CS2分子Penning电离的流动余辉光谱测量

用PIOS(Penning Ionization Opnieal Spectroseopy)方法首次观察到470～490nm间CS~+(A-X)13-2、14-3的发射带,发现此发射带偏重于在Ne/CS_2中产生.这些谱带的产生可用二次碰撞模型加以解释.     

Multiphoton and tunneling ionization of atoms in an intense laser field

We study the ionization probabilities of atoms by a short laser pulse with three different theoretical methods,i.e.,the numerical solution of the time-dependent Schro¨dinger equation(TDSE),the Perelomov-Popov-Terent’ev(PPT) theory,and the Ammosov-Delone-Krainov(ADK) theory.Our results show that laser intensity dependent ionization probabilities of several atoms(i.e.,H,He,and Ne) obtained from the PPT theory accord quite well with the TDSE results both in the multiphoton and tunneling ionization regimes,while the ADK results fit well to the TDSE data only in the tunneling ionization regime.Our calculations also show that laser intensity dependent ionization probabilities of a H atom at three different laser wavelengths of 600 nm,800 nm,and 1200 nm obtained from the PPT theory are also in good agreement with those from the TDSE,while the ADK theory fails to give the wavelength dependence of ionization probability.Only when the laser wavelength is long enough,will the results of ADK be close to those of TDSE.     

非全裸离子-原子碰撞中的多重电离研究

利用加速器提供的非全裸重离子束 (q=1 - 4)轰击氦、氖和氩原子 ,研究了由非全裸离子引起原子多重电离的有效电荷效应和电荷交换效应 .提出用一个有效电荷来等价描述非全裸入射离子对靶原子的作用 ,将描述全裸离子 -原子碰撞过程的物理模型推广到处理非全裸离子 -原子碰撞过程 ,采用轨道贯穿模型对有效电荷的内在物理机制进行了解释 ,提出一种新的理论方法将多重电离中俄歇过程的贡献予以扣除 .通过研究发现 ,扣除俄歇过程贡献后 ,多重电离截面随入射离子q/ v的变化与多步过程的预言相符.The partially stripped C q+ , O q+ and F q+ ions (q=1-4) delivered by a 2×1.7 MV tandem accelerator were employed to bombard He, Ne and Ar targets. The cross section ratios of multiple ionization to single ionization were measured for different collision systems by using time of flight technique. The effective charge effects and charge exchange processes were studied. By using an effective charge q eff , the multiple ionization processes induced by partially stripped ions could...     

Development of an optical time scale

A time standard based on the use of an optical oscillation period of a frequencystable He–Ne laser as a time scale is first described. We obtained highly frequency-stable oscillations in the SHF range that were locked to the oscillations of a He–Ne laser stabilized to an absorption resonance in methane at 3.39 m. A direct comparison of frequency stabilities of a rubidium standard and He–Ne/CH₄ laser has been made. The absolute measurement of the frequency of the He–Ne/CH₄ laser we performed gave a new value of frequency.This work was reported at the 3rd Frequency Standards and Metrology Symposium