Tunnel ionization

The interaction of even simple diatomic molecules with strong laser fields is considerably more complicated than the interaction with atoms. In atoms, nearly all of the observed phenomena can be explained with a simple three-step model [1], at least in the tunneling regime (1) The laser field releases the least bound electron through tunneling ionization (2) the free electron evolves in the laser field and (3) under certain conditions, the electron can return to the vicinity of the ion core, and either collisionally ionize a second electron [2], scatter off the core and gain additional kinetic energy [3], or recombine with the core and produce a harmonic photon [4]. 

Fig. 1.6. Results from simulations showing the simultaneous increase in vibrational amplitude AR and decrease in f avg for Lochfrass in thermal molecules. The pulse duration is 25 fs and the calculations assume tunneling ionization (i.e., no...

Lastly, we mention one more excitation mechanism that has been observed in molecules. It is well-established that following strong field ionization in atoms and molecules, under certain conditions, the ionized electron can be driven back to the ion core where it can recombine to produce high-harmonic radiation, induce further ionization, or experience inelastic scattering. However, there is also the possibility of collisional excitation. Such excitation was observed in [43] in N2 and O2. In both molecules, one electron is tunnel ionized by the strong laser field. When the electron rescatters with the ion core, it can collisionally ionize and excite the molecular ion, creating either N + or Ol+ in an excited state. When the double ion dissociates, its initial state can... 

A weakly bound electron submitted to an intense pulse of IR radiation can be ionized by a variety of mechanisms, from multiphoton absorption to tunnel or even over the barrier ionization, depending of the intensity regime, [16]. At higher intensities tunnel ionization and over the barrier processes are dominant and, as a consequence, the ejected electrons are initially relatively slow. This implies that the changes induced by relativity in the corresponding photoionization spectra are expected to be small. 

Figure 2.1 shows the ionization mechanisms for atoms in high intensity laser fields. Non-resonant multiphoton ionization (NRMPI) is expected at an irradiation intensity of around 1013 W cm 2. Optical field ionization (OFI), which comprises tunneling ionization (TI) and barrier suppression ionization (BSI), occurs at an intensity above 1014Wcm 2. The original Coulomb potential is distorted enough for the electron to either tunnel out through or escape over the barrier. The threshold intensity of BSI for atoms can be estimated by (2.1) [14] ... 

S. L. Chin, From Multiphoton to Tunnel Ionization, in Advances in Multiphoton Processes and Spectroscopy, eds. S. H. Lin, A. A. Villaeys and Y. Fujimura, World Scientific, Singapore, 16, 249-272 (2004)... 

A. Talebpour, J. Yang, S.L. Chin, Semi-empirical model for the rate of tunnel ionization of N% and O g molecules in an intense Ti Sapphire laser pulse, Optics Communications 163, 29 (1999)... 

T.T. Nguyen-Dang, F. Chateauneuf, S. Manoli, O. Atabek, A. Keller, Tunnel ionization 0/H2 in a low-frequency laser field A wave-packet approach, Phys. Rev. A 56 (1997) 2142. 

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