高阶阈值上电离的量子电动力学理论

用非微扰量子电动力学理论研究一个由再散射产生的高阶阈值上电离（ATI）.利用形式散射理论给出了高阶ATI谱的解析形式，并且讨论了在高阶ATI中不同ATI通道的干涉效应和光电子以不同角度入射时的高阶ATI谱. 关键词： 阈值上电离 再散射 跃迁概率     

长脉冲激光场中H原子的阈上电离的理论研究

利用基于密度泛函理论下的窗算符技术求解含时薛定谔方法计算了H原子在长脉冲强激光场中能量谱, 并与文献[8]中的动量空间的投影技术方法的计算结果进行了比较。在文献[8]给出的最大光电子能量范围内,我们的计算结果与文献[8]中的计算结果符合得很好,但是我们的计算结果能够给出比文献[8]中能量更高的光电子能谱,光电子能量谱的变化趋势也与实际的物理过程相符。同时我们也给出了最大光电子能量与激光强度关系的经验公式,利用我们的经验公式计算的最大光电子能量与经典计算预测的最大光电子能量符合得很好,这表明们的计算方法可靠,能够给出高精度的计算结果。因此利用基于密度泛函理论下的窗算符方法能很好地用来研究H原子在长脉冲强激光场中的阈上电离的性质。     

激光场的空间非均匀性对阈上离化的影响

本文在基本态模型中计及激光场的空间非均匀性,得到了与实验相吻合的结果.     

Probing time delay of strong-field resonant above-threshold ionization

The high-resolution three-dimensional photoelectron momentum distributions via above-threshold ionization(ATI)of Xe atoms are measured in an intense near circularly polarized laser field using velocity map imaging and tomography reconstruction. Compared to the linearly polarized laser field, the employed near circularly polarized laser field imposes a more strict selection rule for the transition via resonant excitation, and therefore we can selectively enhance the resonant ATI through certain atomic Rydberg states. Our results show the self-reference ionization delay, which is determined from the difference between the measured streaking angles for nonadiabatic ATI via the 4 f and 5 f Rydberg states, is 45.6 as. Our method provides an accessible route to highlight the role of resonant transition between selected states, which will pave the way for fully understanding the ionization dynamics toward manipulating electron motion as well as reaction in an ultrafast time scale.     

Molecular imaging using high-order harmonic generation and above-threshold ionization

Accurate molecular imaging via high-order harmonic generation relies on comparing harmonic emission from a laser-irradiated molecule and an adequate reference system. However, an ideal reference atom with the same ionization properties as the molecule is not always available. We show that for suitably designed, very short laser pulses, a one-to-one mapping from high-order harmonic frequencies to electron momenta in above-threshold ionization exists. Comparing molecular and atomic momentum distributions then provides the electron recollision amplitude in the molecule for enhanced molecular imaging. The method retrieves the molecular recombination transition moments highly accurately, even with suboptimal reference atoms.     

Energy- and angle-resolved photoelectron spectra of above-threshold ionization and detachment

We analyze strong-laser-field induced above-threshold ionization and above-threshold detachment processes within the strong-field approximation. Energy and angular distribution of photoelectrons are presented for two cases: (1) high-order above-threshold ionization of electrons from neutral atoms by a linearly polarized laser field and (2) above-threshold detachment of electrons from negative ions by a bicircular laser field. In particular, for the case (1) we compare the results obtained using a long pulse with the flat-top spatial intensity distribution in the laser focus with the results obtained using the Gaussian distribution. We also analyze the intensity-dependent resonant-like enhancements. We show that the position and shape of these enhancements strongly depend on the atomic ground state parity. The bicircular field of the case (2) consists of two counter-rotating circularly polarized fields having the frequencies rω and sω with integer r and s. The properties of the photoelectron spectra generated by such bicircular field are illustrated using an ω–4ω example. The spectra obtained are explained in terms of the interference of partial detachment amplitudes which correspond to different complex solutions of the saddle-point equations.     

Quantum-orbit theory of low-energy above-threshold ionization on and off axis

Intense-laser-induced above-threshold ionization of a bound electron into continuum states with low energy is investigated in the context of the strong-field approximation that allows for one act of rescattering of the revisiting electron. The quantum orbits for forward and backward scattering are evaluated and generalized to arbitrary scattering angles. The velocity map of the liberated electron exhibits the well-known low-energy structure as well as other features off the polarization axis.     

Simulation of above-threshold ionization experiments using the strong-field approximation

Recently performed above-threshold ionization (ATI) experiments with noble gases allow the determination of the complete energy and angular distribution of the emitted photoelectrons. In order to simulate such experiments, we have generalized our numerical code for the calculation of ATI spectra in order to achieve a realistic simulation of ATI of the noble gases He, Ne, Ar, Kr, and Xe. Our method is based on an improved version of the strong-field approximation and includes focal averaging. A semiclassical analysis of the dependence of the cutoff law on the photoelectron emission angle is presented. The effects of channel closings on the high-energy spectra are analyzed.     

Influence of multi-photon excitation on the atomic above-threshold ionization

Using the time-dependent pseudo-spectral scheme, we solve the time-dependent Schr ¨odinger equation of a hydrogenlike atom in a strong laser field in momentum space. The intensity-resolved photoelectron energy spectrum in abovethreshold ionization is obtained and further analyzed. We find that with the increase of the laser intensity, the abovethreshold ionization emission spectrum exhibits periodic resonance structure. By analyzing the population of atomic bound states, we find that it is the multi-photon excitation of bound state that leads to the occurrence of this phenomenon, which is in fairly good agreement with the experimental results.     

Na离子和Cl离子引起He原子直接多重电离过程的研究

利用2～8 MeV的Naq+、Clq+(q=2,3,4,5)轰击氦原子,对碰撞的直接多重电离过程进行研究.实验采用反冲离子-散射离子飞行时间符合技术,通过反冲离子飞行时间谱区分不同价态反冲离子;利用静电偏转和位置灵敏探测技术区分不同电荷态散射离子;结合CAMAC-PC多参数获取系统得到一定价态散射离子所对应的反冲离子电荷态分布谱;经分析该谱得到直接多重电离截面与直接单电离截面之比R21.讨论了R21随入射离子速度和电荷态的变化关系.     

Waves in plasma formed by above-threshold ionization of gas atoms

In the regime of above-threshold ionization of gas atom in the field of laser radiation, plasma with photoelectron distribution consisting of peaks at discrete energy values is formed. It is shown that the number of longitudinal waves in such plasma coincides with the number of peaks in the distribution function. When peaks practically don't overlap, the dispersion law of each wave in the region of short waves is determined by electrons from the corresponding peak. In this case the phase and group velocities of the waves are close to the electron velocity, which corresponds to the peak maximum. It is possible to talk about such waves as an electronic sound, since the perturbations of the electron density mainly arise due to pressure perturbations. When the peaks are narrow, but having a finite width, the Cherenkov damping of waves is exponentially small. Numerical calculations the dispersion laws for of the two and four waves in photoionized xenon plasma, in which the electron distribution function consists of two or four narrow peaks are given.     

Ellipticity dependence of high-order above-threshold ionization from aligned diatomic molecules

We investigate high-order above-threshold ionization of the Ar₂ molecule by an elliptically polarized laser field. In order to describe this process we use the modified molecular strong-field approximation. We have found that the differential ionization rate is a very sensitive function of the ellipticity of the laser field and of the molecular orientation. For parallel orientation the rate has the maximum value for linear polarization, independently of the laser intensity. On the other hand, for perpendicular orientation the rate is maximal for a particular value of the ellipticity which is different from zero. Such a behavior can be related to the molecular symmetry. We consider the differential ionization rate of high-energy electrons as a function of the ellipticity for different internuclear distances. We also present, in the electron energy-emission angle plane, our new results for the focal-averaged spectra of electrons ionized by an elliptically polarized laser field. We have found that, in this case, the two-source two-rescattering-centers interference minima are blurred.     

Multiple-plateau structure and scaling relation in photoelectron spectra of high-order above-threshold ionization

By developing a full quantum scattering theory of high-order above-threshold ionization,we study the energy spectra and the angular distributions of photoelectrons from atoms with intense laser fields shining on them.We find that real rescattering can occur many times,and even infinite times.The photoelectrons from the rescattering process form a broad plateau in the kinetic-energy spectrum.We further disclose a multiple-plateau structure formed by the high-energy photoelectrons,which absorb many photons during the rescattering process.Moreover,we find that both the angular distributions and the kinetic-energy spectra of photoelectrons obey the same scaling law as that for directly emitted photoelectrons.     

Emergence of classical orbits in few-cycle above-threshold ionization of atomic hydrogen

The time-dependent Schr?dinger equation for atomic hydrogen in few-cycle laser pulses is solved numerically. Introducing a positive definite quantum distribution function in energy-position space, a straightforward comparison of the numerical ab initio results with classical orbit theory is facilitated. Integration over position space yields directly the photoelectron spectra so that the various pathways contributing to a certain energy in the photoelectron spectra can be established in an unprecedented direct and transparent way.     

Dependence of above-threshold ionization spectrum on polarization directions of two-color laser fields

Using the frequency-domain theory, we investigate the above-threshold ionization(ATI) process of an atom in twocolor laser fields. When both photon energies of the two-color laser fields are much smaller than the atomic ionization threshold, the ATI spectrum depends on the angle between the two lasers' polarization directions. While when the photon energy of one laser is comparable with or larger than the atomic ionization threshold, the ATI spectrum is independent of the angle, and only several dips appear at certain angles. By analyzing the contributions of different quantum channels, we find that, for the case that both frequencies of the two color lasers are low, the quantum interferences between the channels are strong, and hence the spectrum changes with the angle between the two lasers' polarization directions. While for the case that the frequency of one of the two color lasers is high, the contributions of the channels to the ATI spectrum decrease dramatically with increasing channel order, hence the interferences between the channels disappear, and the ATI spectrum has a step-like structure, which is independent of the angle between the two lasers' polarizations. These results can shed light on the study of the corresponding relation between classical and quantum mechanisms of the matter–laser interaction in high-frequency laser fields.     

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.     

Predicting intense-field photoionization of atoms and molecules from their linear photoabsorption spectra in the ionization continuum

We report a new approach to intense-field photoionization that is based on the ad hoc assumption that m photons of energy arriving within a typical electronic response time are effectively equivalent to a single photon of energy . The heuristic model contains no adjustable parameters and unifies apparent multiphoton and field aspects. Moreover, nonsequential, suppressed and above-threshold ionization phenomena become readily understandable. Predicted ionization intensities are in satisfactory agreement with available experimental data ranging from C₆H₆ to Ne^{3 +} , from femtosecond to nanosecond laser pulses, and from ultraviolet to infrared laser radiation.Received: 20 January 2004, Published online: 17 August 2004PACS: 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 32.80.Wr Other multiphoton processes - 42.50.Hz Strong-field excitation of optical transitions in quantum systems; multiphoton processes; dynamic Stark shift