Excited ions in intense femtosecond laser pulses: laser-induced recombination

Electron-ion recombination in a laser-induced electron recollision is of fundamental importance as the underlying mechanism responsible for the generation of high-harmonic radiation and hence for the production of attosecond pulse trains in the extreme ultraviolet and soft x-ray spectral regions. By using an ion beam target, remotely prepared to be partially in long-lived excited states, the recombination process has for the first time been directly observed and studied.     

Nonsequential double ionization of diatomic molecules by elliptically polarized laser pulses

Using the classical ensemble method, we investigate nonsequential double ionization （NSDI） of diatomic molecules by elliptically polarized laser pulses. The results show that the ellipticity of the laser field has a strong suppression effect on NSDI probabilities both in parallel and perpendicular alignments. The double ionization （DI） channel is commonly dominated by NSDI, and the NSDI channel changes with ellipticity. As ellipticity increases, more and more NSDIs occur through recollision excitation with subsequent field ionization （RESI）. Moreover, like the case of linear polarization, the two electrons involved in NSDI for perpendicularly aligned molecules are more likely to emit into the opposite hemispheres as compared to the case of parallel alignment. Additionally, this alignment effect increases as ellipticity increases.     

Above threshold Coulomb explosion of molecules in intense laser pulses

We have measured and explained a new mechanism of molecular ionization near the appearance intensity that produces a sequence of peaks in the nuclear kinetic energy spectrum separated by the photon energy. Our interpretation is based on an internally consistent model for the nuclear motion during an intense laser pulse. Within this model, the same concepts and language can be used for both dissociation and ionization, leading to a more unified understanding of the dynamics.     

Effects of multi orbital contributions in the angular-dependent ionization of molecules in intense few-cycle laser pulses

We outline the details of our new method to calculate angular-dependent ionization probabilities based on electronic structure theory for diatomic and larger systems. To demonstrate its abilities, we compare our calculations to measured ionization probabilities of the four molecules D₂, N₂, O₂ and CO in the strong-field regime. The calculated angular distributions yield better agreement with the experimental data than those obtained from the widely used MO-ADK theory. For CO the measured angular distributions of ionic fragments indicate contributions to the ionization from both the HOMO and the HOMO-1 orbital, an effect that is addressed by the theory.     

Symmetric fragmentation of polyatomic molecular ions in ultrashort intense laser pulses

The intense field-ionization behavior of linear homonuclear polyatomic molecular ions has been numerically investigated for different molecular configurations. The enhanced ionization effect of molecular ions is found as a general feature in a critical bond length range, but the enhancement degree of ionization is bond-selective. The more symmetric the molecular ion stretches, the more enhanced the ionization of the molecular ion. It implies that the symmetric fragmentation of linear molecular ions is most probable at moderate laser intensity. These results are explained in terms of the field-induced over-barrier ionization mechanism.     

Internal collision double ionization of molecules driven by co-rotating two-color circularly polarized laser pulses

The double ionization process of molecules driven by co-rotating two-color circularly polarized fields is investigated with a three-dimensional classical ensemble model. Numerical results indicate that a considerable part of the sequential double ionization(DI) events of molecules occur through internal collision double ionization(ICD), and the ICD recollision mechanism is significantly different from that in non-sequential double ionization(NSDI). By analyzing the results of internuclear distan...     

Suppression of multiple ionization of atomic ions in intense ultrafast laser pulses

The interaction of an intense laser field with a beam of atomic ions has been investigated experimentally for the first time. The ionization dynamics of Ar+ ions and Ar neutrals in a 60 fs, 790 nm laser pulse have been compared and contrasted at intensities up to 10(16) W cm-2. Our results show that nonsequential ionization from an Ar+ target is strongly suppressed compared with that from the corresponding neutral target. We have also observed for the first time the strong field ionization of high lying target metastable levels in the Ar+ beam.     

Electron vortices in photoionization by a pair of elliptically polarized attosecond pulses

The photoionization by two elliptically polarized, time delayed attosecond pulses is investigated to display a momentum distribution having the helical vortex or ring structures. The results are obtained by the strong field approximation method and analyzed by the pulse decomposition. The ellipticities and time delay of the two attosecond pulses are found to determine the rotational symmetry and the number of vortex arms. For observing these vortex patterns, the energy bandwidth and temporal duration of the attosecond pulses are ideal.     

Strong-field photoionization of intense laser fields by controlling optical singularities

The spatial features of a light field, such as in the form of the optical singularities, provide a new degree of freedom for the application of light fields in different areas of science and technology. However, although the exploration of structured light is growing rapidly, the investigation of strong-field photoionization using such light fields is noticeably lagging behind. Here, we present an experimental study that reveals the signatures of intense, structured light fields with controlled optical singularities in strong-field photoionization. The different types of optical singularities can be identified through photoionization observables,i.e., photoelectron momentum distributions(PMDs). By concurrently shifting the locations of the phase and polarization singularities, the focal electric field features can be designated, and subsequently, the photoionization appearances can be manipulated. In this process, the behaviors of the different intense optical singularities are clearly visualized by the PMDs. This work will advance both the strong-field science and singularity optics.     

原子激发态在高频强激光作用下的光电离研究

本文通过数值求解动量空间的三维含时薛定谔方程, 研究了原子高激发态在高频激光脉冲作用下, 在电离阈值附近的光电子能谱和两维动量角分布. 研究结果表明: 在该能量范围内, 单光子电离过程的贡献是最主要的. 体系初态的主量子数可以由光电子能谱峰值的位置来确定; 体系初态的角量子数可以通过光电子的两维动量角度分布确定. 在比较宽泛的参数范围内, 这一规律不随入射激光的强度和脉冲时间宽度的改变而改变, 因此原则上可以利用它对原子的初态进行识别. 此外, 还研究了体系的初态为相干叠加态, 光电子动量谱随着叠加态相对相位的变化规律. 关键词： 阈上电离 激发态 高频激光脉冲 两维动量角度分布     

Spectral energetic properties of the X-ray-boosted photoionization by an intense few-cycle laser

We report a discovery that an intense few-cycle laser pulse passing through gas leaves a fingerprint of its field en- velope on the photoelectron energy spectrum, which involves continuous X-ray radiations. The spectrum resulting from the photoionization processes includes significant quantum enhancement and interference and exhibits interesting energetic properties. The spectral cut-off energies reflect the strength, time, and interference of the laser field modulation on the photoelectron energy. These energetic properties suggest a new method for precise intense-laser-pulse measurement in situ. The method has the advantages of accuracy, simplicity, speed, and large dynamic ranges （up to many orders of intensity）.     

Alignment of molecules by strong laser pulses

We review how moderately intense laser fields offer an approach to alignment of molecules [1]. In particular, molecules can be aligned along a given space fixed axis, forced to a plane, or their rotations about all three possible axes can be eliminated by choosing a linearly polarized, a circularly polarized, or an elliptically polarized alignment field, respectively. We show how molecules in the gas phase can be aligned by turning on the laser field either slowly (a few nanoseconds) or fast (a few picoseconds) with respect to the rotational period of the molecules. The role of the intensity of the laser field and the rotational temperature of the molecules is discussed. Before concluding we describe how aligned molecules enables control and selectivity in the interaction between polarized light and molecules.Received: 15 November 2002, Published online: 18 March 2003PACS: 33.15.Bh General molecular conformation and symmetry; stereochemistry - 32.80.Lg Mechanical effects of light on atoms, molecules, and ions - 33.80.Gj Diffuse spectra; predissociation, photodissociation - 33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g. Rydberg states) - 34.50.Lf Chemical reactions, energy disposal, and angular distribution, as studied by atomic and molecular beams     

Ionization of atoms and ions by intense laser radiation

The ionization of negative ions by ultrashort electromagnetic pulses, the multiphoton ionization of atoms beyond the perturbation theory taking into account the Coulomb interaction, and the relativistic theory of tunneling in application to the ionization problems have been analyzed. The main results have been obtained using the imaginary time method.     

X-ray-boosted photoionization for the measurement of an intense laser pulse

Investigations show that X-ray-boosted photoionization(XBP) has the following advantages for in-situ measurements of ultrahigh laser intensity I and field envelope F(t)(time t,pulse duration τL,carrier-envelope-phase Φ):accuracy,dynamic range,and rapidness.The calculated XBP spectra resemble inversely proportional functions of the photoelectron momentum shift.The maximum momentum p and the observable value Q(defined as a double integration of a normalized photoelectron energy spectrum,PES) linearly depend on I1/2 and τL,respectively.Φ and F(t) can be determined from the PES cut-off energy and peak positions.The measurable laser intensity can be up to and over 1018 W/cm2 by using high energy X-rays and highly charged inert gases.     

Ultrafast quenching of ferromagnetism in InMnAs induced by intense laser irradiation

Time-resolved magneto-optical Kerr spectroscopy of ferromagnetic InMnAs reveals two distinct demagnetization processes--fast (<1 ps) and slow (approximately 100 ps). Both components diminish with increasing temperature and are absent above the Curie temperature. The fast component rapidly grows with pump power and saturates at high fluences (>10 mJ/cm(2)); the saturation value indicates a complete quenching of ferromagnetism on a subpicosecond time scale. We attribute this fast dynamics to spin heating through p-d exchange interaction between photocarriers and Mn ions, while the approximately 100 ps component is interpreted as spin-lattice relaxation.