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Optica Publishing Group
  • Quantum Electronics and Laser Science Conference
  • OSA Technical Digest (Optica Publishing Group, 1995),
  • paper QFC1

Above-threshold ionization beyond the high harmonic cutoff

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Abstract

Experimental studies of atoms in intense laser fields are often complicated by the large dynamic range of signal covered over an intensity range that meaningful displays the full evolution of the dynamics: from the perturbative to the tunnel ionization regime. We have developed kilohertz repetition rate laser systems which allow us to reach peak intensities of 1016 W/cm2, and detect photoionization yields over 12 orders of magnitude, and so effectively span the relevant intensity and signal ranges. Theoretical models offering a unified description of various strong field phenomena: optical harmonic generation (OHG), above-threshold ionization (ATI), and multiple ionization, may now be critically tested. Semiclassical trajectory calculations suggest that the rescattering of the field-driven electron with the core can result in OHG, high energy ATI, and multiple ionization. Evidence for rescattering is observed in the angular distributions for high-order ATI peaks as off-axis scattering rings. These structures depend strongly on the intensity and scale approximately as 8Up. A quantum-mechanical model based on the single active electron approximation (SAE), which predicts these structures and their intensity dependence, will be compared with the semiclassical model. Strong-field multiple photoionization of helium has been studied using a 100 fs, 780 nm kilohertz laser system. The He+ yields are well predicted by the SAE model; however an enhanced double ionization called nonsequential (NS) ionization, is observed over an intensity range in which the single ionization dynamics evolves from multiphoton to the tunneling regime. However, the NS double ionization yield scales only with the ac tunneling rate for single ionization. The origin of the nonsequential mechanism will be discussed in terms of the rescattering model.

© 1995 Optical Society of America

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