Interference between electric and magnetic amplitudes for K shell excitation of high-Z H-like projectiles

We report an investigation of projectile K shell excitation for high-Z ions in relativistic ion-atom collisions. For the case of H-like Bi, the excitation process is unambiguously identified by observing the radiative decay of the excited levels to the vacant 1s shell, in coincidence with ions that did not undergo any charge exchange in the reaction target. The results are compared with detailed relativistic calculations, showing that the magnetic interaction is of considerable importance for the K shell excitation process in high-Z ions. For excitation to the L shell sublevels, the experimental data confirm that the magnetic part of the Liénard-Wiechert interaction must be added coherently with the electric part leading to a significant reduction of the K shell excitation cross section.     

铜蒸气激光脉冲终止机理的数值研究

建立了一个反映纵向脉冲放电激励铜蒸气激光动力学过程的自洽物理模型并进行了数值求解. 根据模拟结果深入分析了铜蒸气激光脉冲的终止机理，表明受激辐射跃迁、激光下能级的电子碰撞激发和激光上能级的铜原子经电子碰撞被抽运到更高激发能级这三个过程，是导致激光脉冲终止的重要因素. 关键词： 铜蒸气激光 激光脉冲终止机制 电子碰撞 更高激发能级     

Classical study of and collisions in intense laser fields

In this paper, ion-atom and ion-ion collisions in the presence of intense laser fields are qualitatively studied by Classical Trajectory Monte Carlo (CTMC) simulations. It is found that in contrast to the field-free collisions, the colliding ion and the target nucleus could absorb energy from the applied laser fields when the electrons escape from the collision system. This result is explained in terms of Coulomb explosion induced by the enhanced ionization at the so-called critical internuclear distance. Also, the corresponding energy gain cross-sections are evaluated. Received: 7 October 1998 / Received in final form: 28 January 1999     

Investigation of some elementary processes in a vacuum-UV hydrogen laser discharge

A novel type of pulse generator is utilized for travelling wave excitation of hydrogen in a multiple electrode laser tube. With this arrangement a laser peak power in the Lyman bands of about 45 kW and a pulse width of 1 ns are achieved. The effect of electron impact deexcitation of upper laser levels on the pulse width was investigated. Some irregularities in the intensity distribution of laser lines could be explained by considering collisions of molecules in the upper laser levels with molecules in the electronic ground state.     

Ultrahigh harmonics from laser-assisted ion-atom collisions

We present a theoretical analysis of high-order harmonic generation from ion-atom collisions in the presence of linearly polarized intense laser pulses. Photons with frequencies significantly higher than in standard atomic high-harmonic generation are emitted. These harmonics are due to two different mechanisms: (i) collisional electron capture and subsequent laser-driven transfer of an electron between projectile and target atom; (ii) reflection of a laser-driven electron from the projectile leading to recombination at the parent atom.     

Radiation field and resonant two-center dielectronic transitions in relativistic ion-atom collisions

We consider projectile-electron excitation and loss in relativistic collisions of ionic projectiles with excited atoms. We show that under certain conditions electron transitions in the ion and atom can be resonantly coupled in the collision via the radiation field. The resonance becomes possible due to the Doppler effect, has a well-defined impact energy threshold, and clearly manifests itself in the cross sections. Since the range of the ion-atom interaction in the resonance case is very long, the presence of other atoms in the target medium as well as the size of the space occupied by the medium have to be taken into account. As a result, the cross section may become dependent on the density of the target atoms and/or the target size.     

Inner-Shell X-ray production in adiabatic iodine-Z 2 collisions,withZ 1+Z 2≧100

K-, L- andM-x ray production cross sections for the molecular excitation regime in heavy ion-atom collisions (Z ₁+Z ₂≧100) are reported for the projectile velocity range 1.3–3.7 a.u. Outer-shell vacancy configurations of the collision partners having experienced inner shell vacancy creating collisions are deduced from the x-ray spectra. The influence of outer shell vacancy occupation on fluorescence yields is discussed and vacancy production cross sections are derived from x-ray production cross sections.     

Atomic collisions involving C60 and collective excitation

Here we review and discuss some of our recent investigations on collective excitation in a free C₆₀ molecule and its influence on the atomic collisions. In particular, emphasis has been given for collisions with fast highly charged ions. It is demonstrated, from the charge-state-dependence studies of recoil-ion spectra, that the plasmon excitation plays a dominant role in the single and double ionization process. The observed linear charge-state-dependence is in contrast to the expected behavior predicted by ion-atom collisions models. This behavior was observed for different projectiles and at different energies. The time-of-flight recoil-ion mass spectroscopy experiments involve 1–5 MeV/u C, O, F and Si ion beams with different charge states, ranging between 4⁺ and 14⁺. In addition, the influence of the collective excitation on the electron capture process was also investigated. The wake-field induced Stark-mixing and splitting of sub-levels of projectile-ions following electron capture from C60 carries signature of the collective plasmon excitation. For the electron capture studies X-ray spectroscopic technique was used for collisions with bare and dressed S and Cl ion beams. The results on the TOF data on fullerene target obtained in last few years will be summarized.     

Comparison of three methods for calculation of electron transfer probability in H + Ne

We have developed a theoretical model of ion-atom collisions where we described electron dynamics by the time-dependent density-functional theory (TDDFT) and the ion dynamics by classical mechanics through the Ehrenfest method. We have compared three methods to calculate the probability of electron transfer during H⁺ + Ne collision. By discussing these issues we shall be able to understand how these methods work, what their limitations are and whether they admit of any improvements.     

Theoretical description of ultrafast phenomena in clusters

A theoretical study of different ultrafast nonequilibrium processes taking place during and after ultrashort excitation of clusters is presented. We discuss similarities and differences for several processes involving nonequilibrium ultrafast motion of atoms and electrons. We study ultrashort relaxation of clusters in response to excitations produced by femtosecond laser pulses of different intensities. We show how different relaxation processes, such as bond breaking, melting, fragmentation, emission of atoms, or Coulomb explosion, can be induced, depending on the laser intensity and laser pulse duration. We also discuss processes involving nonequilibrium electron dynamics, such as intraband Auger decay in clusters and ultrafast electronic motion during collisions between clusters and surfaces. We show that this electron dynamics leads to Stückelberg-like oscillations of measurable quantities, such as the electron emission yield. Received: 4 April 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Influence of collisions and pulse intensity on multiple photon absorption in SF6

Using a pyroelectric detector, the multiple photon absorption (MPA) of the SF₆ molecule in a wide range of pressures (10^-3 -1 torr) has been studied. The significant role of collisions in MPA has been shown. The fraction of molecules excited under essentially collisionless conditions has been defined. It is shown that under collisionless excitation of SF₆ (p < 10^-2 torr) at energy fluences E < 10^-1 J/cm² the intensity of the laser pulse plays the essential role, while in presence of collisions MPA is determined mainly by the energy fluence in the pulse.     

Experimental observation of revival structures in picosecond laser-induced alignment of I2

We report the experimental observation of revival structures in the alignment of a ground-state rotational wave packet following nonresonant excitation of I2 molecules by an intense picosecond laser pulse. The revivals appear at characteristic time delays following the excitation by the pump laser pulse, and show a significant narrowing of the angular distribution during a few picoseconds. The interaction with the pump laser also leads to a steady-state alignment of the molecule, due to rotational pumping.     

Interaction of picosecond laser pulses with a thin target

Interaction between a picosecond laser pulse of nonrelativistic intensity with a thin target is studied in terms of the kinetic theory of laser plasma, which is based on constructing propagators for the plasma particle distribution functions. Allowance is made for both the self-consistent plasma field and plasma particle correlations (collisions). The interaction causes charge separation in the target and strong heating of electrons.     

基于旋转相干光谱的整形飞秒激光转动动力学调控

我们实验上发展了基于飞秒激光旋转相干光谱的整形飞秒激光转动动力学调控方法,通过脉冲整形技术调控激发脉冲的光谱相位,从而实现对飞秒激光作用下转动态相干激发过程中复原信号及转动布居数的调控. 研究了飞秒激光旋转相干光谱对激光频谱相位的响应机制,突出了飞秒激光频谱相位在气相分子转动态相干激发中的重要作用. 为飞秒激光作用下生物大分子和团簇鉴别及结构探测研究提供了新的参考.     

Cross section for single-electron capture by fast He+ ions

A method for calculating the ion-material interaction in which the amplitude of the process of single-electron capture is expressed in terms of an elementary function is proposed. A monopole approximation for heavy particle interaction is used without considering the effect of passive electrons of the target atom. For the cross section of the single-electron capture in He⁺ ion-atom collisions, the proposed method makes it possible to improve the agreement of the calculated results with the experimental data by one order of magnitude in comparison with the OBK approximation.     

On the accuracy of the BGK model for ion drift in own gas

The model of collisions of ions with gas atoms, considering resonant charge exchange of ions, polarization and elastic (gas-kinetic) interactions is constructed. Ion drift characteristics in the dc electric field are calculated. The results are compared to calculations based on the Bhatnagar-Gross-Krook model collision integral (BGK integral). It is shown that the use of the BGK collision integral leads to significant errors due to the specificity of ion-atom collisions.     

Multiple electromagnetic excitation with relativistic heavy ions

The multiple electromagnetic excitation with fast projectiles (heavy ions) is studied theoretically in the sudden approximation. Of special interest is the excitation of rotational states coupled to giant (dipole) vibrations. Closed form expressions are obtained for the excitation of a rigid rotor. The strong pulse of high energy equivalent photons in relativistic heavy ion collisions opens up new possibilities for nuclear structure studies, not possible e.g. with electron scattering or nuclear Raman scattering. It is also pointed out that the “Brink-hypothesis” can be investigated in a new way by means of multiple electromagnetic excitation with relativistic heavy ions of low lying states coupled to the giant dipole mode.     

Influence of the carrier-envelope phase of few-cycle pulses on ponderomotive surface-plasmon electron acceleration

Control over basic processes through the electric field of a light wave can lead to new knowledge of fundamental light-matter interaction phenomena. We demonstrate, for the first time, that surface-plasmon (SP) electron acceleration can be coherently controlled through the carrier-envelope phase (CEP) of an excitation optical pulse. Analysis indicates that the physical origin of the CEP sensitivity arises from the electron's ponderomotive interaction with the oscillating electromagnetic field of the SP wave. The ponderomotive electron acceleration mechanism provides sensitive (nJ energies), high-contrast, single-shot CEP measurement capability of few-cycle laser pulses.     

Generation of hard x rays by femtosecond laser pulse interaction with solid targets in atmosphere

X ray radiation as high as 50 keV, including K(α) of Ba and Mo, have been observed from a solid target during the interaction of low energy ~0.65 mJ, 1 kHz 40 femtosecond laser pulses focused in air at atmospheric pressure. Energetic electrons generating such x rays are possibly produced when the field strength in laser pulse wake exceeds the runaway threshold in air. Two dimensional particle-in-cell simulations that include optical field ionization of air and elastic collisions support this mechanism.