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THz Radiation Using Gases/Plasmas
Published in Hitendra K. Malik, Laser-Matter Interaction for Radiation and Energy, 2021
Ionization involves an electron escape from the atom (or molecule) via crossing the potential barrier. Classically, an electron must have a higher amount of energy compared with that of the potential barrier and only then it is allowed to leave the atom (or molecule). But with the application of an external field, the height of the potential barrier can be suppressed, and the electron can easily tunnel through the potential instead of going all over the way due to its wave nature. This distortion of potential barrier, due to intense electric field, permits the electron to escape from the atom. This quantum mechanical phenomenon where ionization happens due to quantum tunneling, which is forbidden by the classical laws, is known as tunnel ionization. Tunnel ionization allows the particle to escape from the distorted Coulomb potential barrier with non-zero probability. The probability of an electron's tunneling through the barrier with the width of the potential barrier drops off exponentially. Therefore, an electron with higher energy encounters a thinner potential barrier, which further increases the tunneling probability.
Material Processing with Femtosecond Lasers
Published in Shalom Eliezer, Kunioki Mima, Applications of Laser–Plasma Interactions, 2008
where I0 is the ionization potential. In the case of Γ > 1 or Ip > Up > hν0, where hν0 is the photon energy, multiphoton absorption becomes a dominant process and ionization, called above threshold ionization, happens. On the contrary, in the case of Γ < 1, ionization with distorted nuclear potential occurs. If the distortion of the potential is small, electrons try to escape from atoms through the tunneling effect, which is called tunnel ionization. For more distortion, the potential will be suppressed even lower than the electron energy level and barrier-suppression ionization comes off. Figure 7.1 shows schematics of the potential distortion and resulting ionization. When such a high field is applied to molecules in femtosecond timescale, one can ionize them by keeping their original structure, which is useful for detecting environmentally hazardous molecules like dioxins [3].
Theoretical Atto-nano Physics
Published in Klaus D. Sattler, st Century Nanoscience – A Handbook, 2019
Conceptually, HHG is easily understood using the three-step model [27,59,60,62,127]: (i) tunnel ionization due to the intense and low-frequency laser field; (ii) acceleration of the free electron by the laser electric field and (iii) re-collision with the parent atom or molecular ion. The kinetic energy gained by the electron in its travel, under the presence of the laser oscillatory electric field, is converted into a high-energy photon and can be easily calculated starting from semiclassical assumptions.
Attoclock revisited on electron tunnelling time
Published in Journal of Modern Optics, 2019
C. Hofmann, A. S. Landsman, U. Keller
Even if multi-electron effects are negligible once the ionized electron is already far away from the parent ion, there still might be significant electron-electron interaction during the actual tunnel ionization step, while the tunnelling electron is still at a comparable distance to the nucleus relative to the other bound electron. A similar analysis for the tunnel ionization step, however, is challenging to perform since it requires a fully quantum mechanical treatment. Near-circular, but not perfectly circular, polarization prohibits coordinate reduction based on symmetry arguments, making the numerical solution of the TDSE computationally very expensive. Recently, Majety and Scrinzi (48) published an approach for reducing the necessary basis functions with higher orbital angular momentum. The results of (48) show that, similar to the propagation in the continuum, the tunnelling step can also be approximated with a single active electron for the case of helium. They could not find any observable differences in the final angular momentum spectrum between a SAE calculation and multi-channel calculations (48).