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When energy resolved photoelectron measurements have become possible in the end of the es, it was observed that in the photoionization process the ejected electrons could absorb photons in excess of the minimum number required for ionisation to occur. The electron energies appear at the values. This process was shown for the first time in and it was called above threshold ionization ATI.

A typical ATI spectrum shows a number of electron peaks separated by the laser photon energy. Thus ATI spectra with many decades in number of counts could be recorded. It was found that angular distributions exhibit a much more complex off-axis structure at the edge of the plateau, called "scattering rings". The large plateau and the complex angular structure originate from the rescattering of the electron wavepacket on the parent ion.

Not only one, but many electrons can be emitted from atoms subject to strong laser fields. Double ionisation of alkaline earth atoms was observed as early as in and the first evidence for non-sequential ionisation of rare gas atoms was first demonstrated in They can be emitted one at a time, in a sequential process, or simultaneously, a mechanism called direct, or non-sequential. The simplest multiple ionisation mechanism for atoms in strong laser fields is the so-called sequential stripping mechanism, i.

It turned out however, that sequential ionisation is not the only mechanism responsible for multiple ionisation. Progress in experimental techniques with, for example, recoil-ion momentum spectroscopy and electron-ion coincidence measurements allows now scientists to record the energies and angular distributions of the electrons emitted during a multiple ionisation process, thus providing better experimental insight.

Another effect also observed at sufficiently high intensities was high harmonic generation HHG. Atoms interacting with a strong laser field can emit radiation at frequencies that are high order multiples of the angular frequency of the pump laser. A high-order harmonic spectrum consists of a sequence of peaks at circular frequencies which are odd multiples of the driving, or fundamental circular frequency:. Only odd orders can be observed, owing to inversion symmetry in an atomic gas.

In the time domain, this means that the process is periodic with a periodicity twice the laser period. A HHG spectrum has a characteristic behaviour: it shows a fast decrease for the first few harmonics, followed by a long plateau of harmonics with approximately constant intensity. The plateau ends up by a sharp cut-off. Most of the early work on harmonic generation concentrated on the extension of the plateau, i.

Today, harmonic spectra produced with short and intense laser pulses extend to more than eV, down to the water window below the carbon K-edge at 4. The mechanism of the generation is shown in figure A large effort has been devoted to optimize and characterize the properties of this new source of XUV radiation.

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This result was interpreted in terms of a simplified theory, called a three step model shown in figure In the first step a tunnelling process takes place. In the second step the model neglects the Coulomb force of the core, and assumes that the electron moves only under the effect of the laser field, which first pulls away the electron from the core, but when during its oscillation it changes its sign the field may return the electron back to the core. Thirdly: when slowing down, the electron radiates in the form of harmonics whose energy comes from the energy lost by the returning electron.

A realistic description of HHG must involve the calculation of the single atom response, and also the solution of propagation Maxwell equations for the emitted radiation. There is a clear analogy here with mode-locked lasers, where axial modes oscillating in a laser cavity are locked in phase, leading to the production of trains of short pulses.

From a microscopic point of view, at each half-laser cycle, there is a short attosecond burst of light, as an electron recombines back to the ground state. Isolated attosecond pulses can be produced if one limits these returns to single events. The simplest idea is to use a very short 7 fs and intense laser pulse. Such a laser source should allow one to generate single attosecond pulses, because harmonic generation occurs during a limited time interval before the onset of ionisation.

Attosecond pulses have remained, however, essentially a theoretical prediction, until Under the menu "Light and Matter" a lot of interesting material can be found on attosecond physics, light matter interaction etc.. The harmonic radiation, with attosecond pulse duration, high brightness and good spatial and temporal coherence, could be used in a growing number of applications ranging from atomic and molecular spectroscopy to solid-state and plasma physics. It has also been proposed as an alternative source for nanolithography , in particular for metrological purposes.

9. Atoms V and Atoms in External Fields I

It opens up two new fields of research: multiphoton processes in the XUV range, and attosecond physics, where processes in atoms and molecules can be studied at an unprecedented time scale. Citation 0. Chinese Journal of Chemical Physics, , 30 6 : Dated: Received on November 14, ; Accepted on December 20, Hang Lv, E-mail: lvhang jlu. Comparing to atoms, molecules exhibit peculiar behaviors in strong-field ionization because of their diverse geometric structures, molecular electronic orbitals as well as extra nuclear degrees of freedom.

By comparing the ionization yields with that of the companion atom krypton Kr , which has similar ionization potential to the molecules, we investigate the effect of molecular electronic orbitals on the strong-field ionization. Based on our results and previous studies on homonuclear diatomic molecules N 2 and O 2 , the mechanism of different suppression effect is discussed. It is indicated that the different structure of the highest occupied molecular orbitals of CO and CO 2 leads to distinct behaviors in two-center interference by the electronic wave-packet and angular distributions of the ionized electrons, resulting in different suppression effect in strong-field ionization.

Figure options. The HOMOs of the molecules are also shown. Table options. All the three curves are normalized to its maximum respectively. Protopapas, C. Keitel, and P. Knight, Rep. Milosevic, G. Paulus, D. Bauer, and W. Becker, J. B 39 , Becker, X. Liu, P. Ho, and J. Eberly, Rev. Wang, X. Li, P. Fu, J. Chen, and J. Liu, Chin. Nubbemeyer, K. Gorling, A. Saenz, U. Eichmann, and A. Sandner, Phys. Zhao, J. Dong, H. Lv, T. Yang, Y. Lian, M. Jin, H. Xu, D. Ding, S. Hu, and J. Chen, Phys. A 94 , Muth, - Bohm, A. Becker, and F. Faisal, Phys. Saenz, J. B 33 , Fang, and G. Gibson, Phys. Peters, T.

Nguyen, - Dang, E.

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