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.     

Double Ionization Dynamics of Molecular Hydrogen in Ultrashort Intense Laser Fields

We experimentally investigate the double ionization of molecular hydrogen subjected to ultrashort intense laser pulses.The total kinetic energy release of the two coincident H~+ ions,which provides a diagnosis of different processes to double ionization of H_2,is measured for two different pulse durations,i.e.,25 and 5fs,and various laser intensities.It is found that,for the long pulse duration(i.e.,25 fs),the double ionization occurs mainly via two processes,i.e.,the charge resonance enhanced ionization and recollision-induced double ionization.Moreover,the contributions from these two processes can be significantly modulated by changing the laser intensity.In contrast,for a few-cycle pulse of 5fs,only the recollsion-induced double ionization survives,and in particular,this process could be solely induced by the first-return recollision at appropriate laser intensities,providing an efficient way to probe the sub-laser-cycle molecular dynamics.     

Photofragmentation of Na2+ in intense femtosecond laser fields: from photodissociation on light-induced potentials to field ionization.

Photofragmentation of Na2 + molecules in well prepared vibrational levels has been studied employing intense ( 10(11)-10(14) W/cm2) and ultrashort (80 fs) 790 nm laser fields. Four fragmentation channels with different released kinetic energies are observed. Depending on the applied laser intensity, the fragmentation of Na2 + is governed by photodissociation on light-induced potentials and field ionization followed by Coulomb explosion. Below 1x10(12) W/cm2, only photodissociation on light-induced potentials is seen. For intermediate laser intensities, field ionization at large internuclear distances competes with photodissociation, thus preventing the observation of above threshold dissociation. Field ionization at small internuclear distances dominates for the highest laser intensities used.     

Study of surface ionization and LASER ionization processes using the SOMEIL ion source: application to the Spiral 2 laser ion source development

SPIRAL2 is the new project under construction at GANIL to provide radioactive ion beams to the Nuclear Physics Community and in particular neutron rich ion beams. For the production of condensable radioactive elements, a resonant ionization laser ion source is under development at GANIL. In order to generate the ions of interest with a good selectivity and purity, our group is studying the way to minimize surface ionization process by using refractory materials with low work function as ionizer tube. To do those investigations a dedicated ion source, called SOMEIL (Source Optimisée pour les Mesures d‘Efficacité d‘Ionisation Laser) is used. Numerous types of ionizer tubes made in various materials and geometry are tested. Surface ionization and laser ionization efficiencies can be measured for each of them.     

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."     

Cluster-assisted generation of multi-charged ions in nanosecond laser ionization of pulsed hydrogen sulfide beam at 1064 and 532nm

The multi-charged sulfur ions of S^q= (q\le 6) have been generated when hydrogen sulfide cluster beams are irradiated by a nanosecond laser of 1064 and 532,nm with an intensity of 10¹⁰\sim 10¹²W1\cdotcm^-2. S⁶⁺ is the dominant multi-charged species at 1064nm, while S⁴⁺, S³⁺ and S²⁺ ions are the main multi-charged species at 532nm. A three-step model (i.e., multiphoton ionization triggering, inverse bremsstrahlung heating, electron collision ionizing) is proposed to explain the generation of these multi-charged ions at the laser intensity stated above. The high ionization level of the clusters and the increasing charge state of the ion products with increasing laser wavelength are supposed mainly due to the rate-limiting step, i.e., electron heating by absorption energy from the laser field via inverse bremsstrahlung, which is proportional to \lambda ², \lambda being the laser wavelength.     

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。     

Influence of laser intensity on the double-resonance multiphoton ionization process of NO molecule

The analytic formula of the ionization efficiency in the process of double resonance enhanced multi-photon ionization （DREMPI） is derived from the dynamic rate equation about the interaction of photon and material. Based on this formula, the ionization efficiency and the laser power index versus laser intensity in the DREMPI process of NO molecule, via A2E and S2E intermediate resonant states, is numerically simulated. It is shown that the ionization efficiency of NO molecule increases with the laser intensity until getting saturation, while the laser power index decreases with the enhancement of the laser intensity and changes to zero at last. The variation of the laser power index with the laser intensity indicates that the ionization efficiency reaches saturation in the one, two, and three excitation steps respectively. It is also found that the narrower the laser pulse duration is, the higher becomes the laser intensity for saturation.     

铀原子多色三光子共振电离谱研究

利用Ｎｄ：ＹＡＧ二倍频激光同步泵浦的脉冲染料激光系统、原子束装置、飞行时间质谱和Ｂｏｘｃａｒ技术测量了＾２３８Ｕ的多色三光子共振电离谱。研究了电场对铀原子第一激发态的影响。解决了如何确定三角三光子共振电离谱图中所含的单色,双色和三色共振电离峰的问题。     

NO通过C~2Π←X~2Π跃迁的共振多光子离化谱

以皮秒Nd∶YAG激光器抽运光学参变发生 /放大器做激发源 ,得到了NO分子在 4 90～ 5 80nm波长范围内通过C2 Π态共振增强的多光子离化谱 ,离化谱由有规则的谱线序列组成。将理论计算的峰值位置与实验结果进行比较 ,确定了离化通道为 :NO(X2 Π) 3hνNO(C2 Π) 2hν(orhν) NO+ +e ,离化信号强度随激光强度的近五次方变化关系进一步验证了此结论。分析讨论了谱线强度的分布不符合夫兰克康登原理的可能原因。根据谱线峰值位置 ,利用最小二乘法拟合获得NO分子C2 Π态振动常数′ωe=(2 35 4 .9± 6 .4 )cm-1,′ωe ′χe=(14 .7± 2 .5 )cm-1及平衡位置的力常数k=(2 .4 4± 0 .0 8)× 10 3 N·m-1。结果可为用激光离化光谱技术探测大气污染物NO分子提供参考。     

Differential cross sections for ionization of laser-aligned atoms by electron impact

The first experimental data are given for (e,2e) ionization from laser-aligned atoms. A linearly polarized laser excited Mg atoms to the 31P? state prior to ionization by low energy electrons. The scattered and ejected electrons were detected in coincidence and the differential cross section determined for a range of alignment angles. An asymmetric coplanar geometry was used, with one electron fixed and the other detected at different angles. The data are compared to that from the spherically symmetric 31S? state. Significant differences are found, in both magnitude and angular distribution.     

CS_2分子激发态的紫外多光子电离——双色激光单共振离化的功率密度关系

本文报道气相CS_2紫外双色激光单共振离化的功率密度关系和对高激发态(1~Ⅱg),共振离化特性的研究结果,实验指出通过中间共振态(1~Ⅱ_g)的双光子共振激发三光子离化具有比通过中间共振态((?)A_2)的单光子共振激发三光子离子为高的总离化率,在10~7W/cm~2的功率密度下,观察到饱和效应.实验显示了双色激光单共振离化功率密度关系的研究提供了一种新的光谱学分析方法.这对研究分子高激发态的结构是很重要的.     

分子对飞秒激光场响应的含时密度泛函理论研究

采用含时密度泛函理论方法研究线性分子碳化锂（Li2C2）对飞秒激光场响应的电子-离子动力学行为。在典型的近共振和非共振的激光频率作用下,分别对比分析了分子的共振和非共振电离过程。研究发现：分子在共振频率激光场的作用下发生更强的电离过程,并倾向于发生库伦爆炸,键长的振荡断裂与电离相互促进影响,而分子在较弱的激光场作用下发生单光子电离过程;随着双脉冲时间间隔的增加,离化电子数在一定范围内呈振荡上升趋势,随后趋于常数。     

分子对飞秒激光场响应的含时密度泛函理论研究

采用含时密度泛函理论方法研究线性分子碳化锂(Li2C2)对飞秒激光场响应的电子-离子动力学行为.在典型的近共振和非共振的激光频率作用下,分别对比分析了分子的共振和非共振电离过程.研究发现:分子在共振频率激光场的作用下发生更强的电离过程,并倾向于发生库伦爆炸,键长的振荡断裂与电离相互促进影响,而分子在较弱的激光场作用下发生单光子电离过程;随着双脉冲时间间隔的增加,离化电子数在一定范围内呈振荡上升趋势,随后趋于常数.     

In-source laser spectroscopy developments at TRILIS—towards spectroscopy on actinium and scandium

Resonance Ionization Laser Ion Sources (RILIS) have become a versatile tool for production and study of exotic nuclides at Isotope Separator On-Line (ISOL) facilities such as ISAC at TRIUMF. The recent development and addition of a grating tuned spectroscopy laser to the TRIUMF RILIS solid state laser system allows for wide range spectral scans to investigate atomic structures on short lived isotopes, e.g., those from the element actinium, produced in uranium targets at ISAC. In addition, development of new and improved laser ionization schemes for rare isotope production at ISAC is ongoing. Here spectroscopic studies on bound states, Rydberg states and autoionizing (AI) resonances on scandium using the existing off-line capabilities are reported. These results allowed to identify a suitable ionization scheme for scandium via excitation into an autoionizing state at 58,104 cm^???1 which has subsequently been used for ionization of on-line produced exotic scandium isotopes.     

Multiphoton double ionization of Ar in intense extreme ultraviolet laser fields studied by shot-by-shot photoelectron spectroscopy

Photoelectron spectroscopy has been performed to study the multiphoton double ionization of Ar in an intense extreme ultraviolet laser field (hν ～ 21 eV, ～ 5 TW/cm2), by using a free electron laser (FEL). Three distinct peaks identified in the observed photoelectron spectra clearly show that the double ionization proceeds sequentially via the formation of Ar(+): Ar+hν→Ar (+) + e? and Ar2(+) + 2hν→Ar(+) + e?. Shot-by-shot recording of the photoelectron spectra allows simultaneous monitoring of FEL spectrum and the multiphoton process for each FEL pulse, revealing that the two-photon ionization from Ar(+) is significantly enhanced by intermediate resonances in Ar(+).     

Ratiometric comparison of intense field ionization of atoms and diatomic molecules

Intense-laser ionization rates for rare gas atoms and diatomic molecules have been precisely compared by making simultaneous measurements of ionization yield vs laser intensity for mixed atomic and molecular targets. At a given laser intensity, the N (2) and F (2) ionization yields are slightly greater than that of Ar. Conversely, comparison of O (2) and S (2) with Xe indicates significant ionization suppression in these molecules. Recent molecular ionization models that successfully describe ionization suppression in O (2) and its absence in N (2) fail to explain our observations in F (2) and S (2).     

Muonic molecules in superintense laser fields

We study theoretically the ionization and dissociation of muonic molecular ions (e.g., dd mu) in superintense laser fields. We predict that the bond breaks by tunneling of the lightest ion through a bond-softened barrier at intensity I > or =10(21) W/cm(2). Ionization of the muonic atomic fragment occurs at much higher intensity I > or =6 x 10(22) W/cm(2). Since the field controls the ion trajectory after dissociation, it forces recollision of a approximately 10(5)-10(6) eV ion with the muonic atom. Recollision can trigger a nuclear reaction with sub-laser-cycle precision. In general, molecules can serve as precursors for laser control of nuclear processes.     

Experimental preparation of the initial state of Xe+-N laser-induced collisional charge transfer system: Multi-photon resonant ionization of xenon

A novel one-color Xe⁺-N laser induced collisional charge transfer system is proposed, and preparation of the initial state of the system, i.e., Xe⁺ is experimentally implemented through resonance enhanced multi-photon ionization (REMPI) by ～440 nm dye laser. The REMPI of Xe is experimentally investigated through time-of-flight (TOF) mass spectrometry and the intensity dependence of Xe⁺ is obtained, aiming at the preparation of Xe⁺. The resonant ionization spectra of Xe at ～440 nm under several different conditions are measured, showing the impacts of mode purification and source pressure on the resonant ionization spectrum. The results indicate the feasibility of preparing the initial state of the Xe⁺-N system by ～440 nm multi-photon resonant ionization, which prepares for a further experiment of laser-induced collisional charge transfer.