Gd原子的共振多光子电离研究

本文首次报道了用共振多光子电离(RMPI)技术探测到四条在可见光范围内的Gd原子强双光子跃迁线.观察到双色三光子Gd原子激光同位素分离.研究了高分辨率Gd原子电离谱,并解释了电离谱的线型.     

Observation of two-photon above-threshold ionization of rare gases by xuv harmonic photons

We have successfully observed two-photon above-threshold ionization in rare gas atoms (Ar, Xe, and He) by the fifth harmonic (25 eV photon energy) of a KrF laser. Use of the energy-resolved photoelectron counting system together with our laser, providing strong 25 eV radiation at 40-100 Hz, enabled us to detect the very weak single-color two-photon above-threshold ionization signals. Experimental data are in good agreement with our theoretical calculations newly developed along the line of multichannel quantum defect theory.     

Few-photon multiple ionization of ne and ar by strong free-electron-laser pulses

Few-photon multiple ionization of Ne and Ar atoms by strong vacuum ultraviolet laser pulses from the free-electron laser at Hamburg was investigated differentially with the Heidelberg reaction microscope. The light-intensity dependence of Ne2+ production reveals the dominance of nonsequential two-photon double ionization at intensities of I<6x10(12) W/cm2 and significant contributions of three-photon ionization as I increases. Ne2+ recoil-ion-momentum distributions suggest that two electrons absorbing "instantaneously" two photons are ejected most likely into opposite hemispheres with similar energies.     

钡准分子共振电离光谱及离化机制研究

本文报道用激光共振电离法获得一个新的钡准分子结构.在染料激光波长扫描范围内观察到钡原子双光子共振跃迁、双光子混合跃迁和钡准分子谱带.我们提出一种用热离子二极管信号特性分析原子和分子里德伯态的离化机制的方法,并以此对钡原子和钡分子的离化通道进行讨论.     

Time-resolved photoelectron angular distributions from strong-field ionization of rotating naphthalene molecules

A nanosecond laser pulse confines the spatial orientation of naphthalene in 1D or 3D while a femtosecond kick pulse initiates rotation of the molecular plane around the fixed long axis. Time-dependent photoelectron angular distributions (PADs), resulting from ionization by an intense femtosecond probe pulse, exhibit pronounced changes as the molecular plane rotates. Enhanced 3D alignment, occurring shortly after the kick pulse, provides strongly improved contrast in molecular-frame PADs. Calculations in the strong-field approximation show that the striking structures observed in the PADs originate from nodal planes in occupied valence orbitals.     

The effects of self-induced resonance in multiphoton ionization

The ionization of atoms in the presence of an intermediate two-photon resonance is considered. The ionization probability and spectral distribution of the ionized electrons are obtained for the case when the resonant is turned on adiabatically. The limiting cases of small and large ionization widths are investigated. At sufficiently large intensities of the resonant field an essential asymmetry in the ionization probability depending on the sign of the two-photon detuning is obtained.     

Laser resonant ionization of plutonium

Resonance ionization mass spectrometry (RIMS) has been tested for the isotope-selective determination of trace amounts of plutonium. An atomic beam is formed by evaporating plutonium atoms from a rhenium-filament heated to 1800 °C. The radiation of a pulsed dye laser excites the atoms in a two-photon process (=595.2 nm) followed by photoionization of the excited atoms. Mass selectivity is obtained by use of a time-of-flight spectrometer. A resonant signal of²³⁹Pu was measured with 10¹³ atoms deposited on the filament.     

Multiphoton ionization of iodine atoms and CF<Subscript> 3</Subscript>I molecules by XeCl laser radiation

We report about effective ionization of iodine atoms and CF₃I molecules under the action of intense XeCl laser radiation (308 nm). The only ion fragment resulting from the irradiation of the CF₃I molecules is the I⁺ ion. We have studied the influence of the intensity, spectral composition, and polarization of the laser radiation used on the intensity of the ion signal and the shape of its time-of-flight peak. Based on the analysis of the results obtained, we have suggested the mechanism of this effect. The conclusion drawn is that the ionization of the iodine atoms by the ordinary XeCl laser with a nonselective cavity results from a three- (2 + 1)-photon REMPI process. This process is in turn due to the presence of accidental two-photon resonances between various spectral components of the laser radiation and the corresponding intermediate excited states of the iodine atom. The probability of ionization of the atoms from their ground state I(²P_3/2) by the radiation of the ordinary XeCl laser is more than two orders of magnitude higher than the probability of their ionization from the metastable state I^*(²P_1/2). The ionization of the CF₃I molecules by the XeCl laser radiation occurs as a result of a four-photon process involving the preliminary one-photon dissociation of these molecules and the subsequent (2 + 1)-photon REMPI of the resultant neutral iodine atoms.     

Comparative analysis of the strong-field ionization of a quantum system with the Coulomb and short-range potentials

The dynamics of a hydrogen atom and a 3D model quantum system with a short-range potential is investigated using the direct numerical integration of the nonstationary Schrödinger equation in a wide range of laser intensities and frequencies. The simulation data are compared with the predictions of variants of the Keldysh-type theories. It is demonstrated that, in the low-frequency (tunnel) limit, the ionization rates of the systems with the Coulomb and short-range potentials and the same values of the ionization potential significantly differ from each other whereas, in the high-frequency (single-photon) limit, we do not observe a substantial difference between the ionization rates. Specific features of the angular distribution of the photoelectron emission and the photoelectron energy spectra are investigated in detail. In addition, the ionization suppression is studied for both Coulomb- and short-range-potential atoms. The stabilization is due to the dramatic reconstruction of the atom in the presence of a strong laser field and the formation of a new system (Kramers-Henneberger atom) that exhibits an increasing resistance to the ionization upon an increase in the laser intensity. In the two-photon ionization regime, the stabilization phenomenon is substantially more pronounced for the system with the Coulomb potential. This results from the effective excitation of the Rydberg states of the dressed atom in the strong-field limit.     

Doubly charged ions resulting from the multiphoton atomic ionization of alkaline-earth metals

The study of the creation of doubly charged ions upon the laser-induced multiphoton ionization of alkaline-earth metals is reviewed. The results show that the features of the creation of the doubly charged ions depend on the spectral range of the ionizing laser radiation. In the ionization of the alkaline-earth atoms by the visible laser radiation, the cascade creation of the doubly charged ions is predominantly implemented when the ions result from the multiphoton ionization of singly charged ions created in the presence of the same laser pulse. In the multiphoton ionization of the alkaline-earth atoms by the IR laser radiation, the two-electron process occurs when the doubly charged ions are directly created from neutral atoms owing to the ionization via induced resonances with strongly perturbed states of these atoms.     

Influence of multi-photon ionization of Xe on a XeF discharge induced by KrF laser irradiation

Excimer laser preionization induced by an external laser was investigated for XeF excimer laser discharge induced by a KrF laser, paying attention to the ionized species in the gas. Enhancement of amplified spontaneous emission (ASE) was observed for XeF gas discharge preionized by a KrF laser with and without its irradiation. In contrast, no ASE enhancement was observed for KrF gas discharge with KrF laser preionization. ASE enhancement was larger for the longer wavelength, which was close to the two-photon resonance of the Xe atom, when tunable KrF laser irradiation was used. These results indicate that multi-photon ionization of Xe atoms supplied the initial electrons to the laser-induced preionization.     

用光电子成像技术研究Cu-的飞秒双光子光脱附过程

用光电子成像技术研究了Cu^-的飞秒双光子光脱附过程．光脱附产生的光电子能量分布和角度分布通过光电子成像直接获得．实验发现,在光强为6.0×10¹⁰ W/cm²下,双光子光脱附通道的光电子角度分布随激光波长出现剧烈的变化．通过低阶微扰理论和实验结果对比分析,Cu^-的双光子脱附产生的光电子角度分布特征可以归结于脱附过程中Cu^-的s和d轨道波函数之间的干涉效应．通过分析光电子角度分布特征,获得了不同波长下s和d轨道波函数对双光子脱附过程的贡献,该结果符合Wigner阈值理论的预期．     

脉冲激光诱导金属薄膜电离的分子动力学模拟

采用分子动力学方法模拟研究了激光诱导金属薄膜的电离过程,对激光等离子体形成早期原子的运动轨迹、薄膜表面的温度变化以及原子的电离特性进行了详细分析,并探究了脉冲激光参数对原子电离过程的影响.结果表明,在激光照射过程中,薄膜表面先熔化而后又气化,气化的原子继续吸收激光能量继而电离.激光的峰值功率密度越大,原子电离速率越快,电离数目越多,薄膜表面的温度越高.脉冲宽度越小,原子电离速率越快,薄膜表面的温度越高,但原子的电离数目先增加后减小.     

Resonant two-photon single ionization of two identical atoms

Resonant two-photon single ionization in a system consisting of two spatially well-separated identical atoms is studied. Because of two-center electron-electron correlations, the ionization may also proceed through photoexcitation of both atoms with subsequent interatomic Coulombic decay. We show that this channel may qualitatively change the dependence of the photoionization on the field intensity as well as the spectra of emitted electrons.     

Resonant two-photon ionization with high-order continuum states

In this paper,the resonant two-photon ionization of atoms with high-order con-tinuum state is studied.It's found that the C-C coupling among the continuum states enhancesthe two-level atomic Rabi oscillation,and the direct transition from the intermidiate excitedstate to the continuum weakens the Rabi oscillation.Therefore the photoelectron energy spec-tra and the population are changed.     

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.     

Nonsequential two-photon double ionization of atoms: identifying the mechanism

We develop an approximate model for the process of direct (nonsequential) two-photon double ionization of atoms. Employing the model, we calculate (generalized) total cross sections as well as energy-resolved differential cross sections of helium for photon energies ranging from 39 to 54 eV. A comparison with results of ab initio calculations reveals that the agreement is at a quantitative level. We thus demonstrate that this complex ionization process can be described by the simple model, providing insight into the underlying physical mechanism. Finally, we use the model to calculate generalized cross sections for the two-photon double ionization of neon in the nonsequential regime.     

Two photon ionization of rubidium

Using the implicit summation technique, the parameters of two-photon ionization of rubidium by the second harmonic of ruby laser are reported. The Hartree-Fock and Hartree-Fock-Slater wavefunctions have been used and the spin-orbit perturbation has been included.     

Wavelength dependent Stark compensation in isotope selective Doppler-free two photon ionization

A novel wavelength dependent Stark shift compensation method for isotope selective ionization in Doppler free two-photon spectroscopy is theoretically investigated. In the present paper, certain wavelengths for the ionization step termed as “magic wavelengths” are identified for compensating the Stark shift induced by the excitation laser. The numerical calculations indicate that, ionization efficiencies can be maximized by the judicious choice of the intensities of the excitation and the ionization lasers without much degradation in the optical selectivity. The effects of Stark shift induced asymmetry and its reversal by selecting the appropriate magic wavelength for the ionization step for various excitation and ionization laser intensities have been investigated. The wavelength dependent Stark compensation method can be adapted for the isotope selective Doppler free two-photon excitation followed by non-resonant ionization process for any atomic system. In the present work, the theoretical results pertaining to the specific example of ⁴¹Ca has been presented.     

Resonant two photon ionization of H and D atoms

H and D atoms have been ionized with tunable VUV + UV laser light by two-photon absorption, resonantly through the 2² P_{$\text{1}\text{2}$}, 2²P_{$\text{3}\text{2}$} fine structure doublet, with total energies very close to the ionization potential. Experiments have been carried out in the ionization chamber of a quadrupole massfilter, in a crossed atomic-laser beam configuration with sub- Doppler resolution of ～ 10 GHz. The sensitivity is ? 2.5 x 10⁵ atoms per cm³.