Loss of harmonic generation in intense laser fields

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Abstract
Harmonic generation and above-threshold ionization are both consequences of the nonlinear response of atoms to intense laser fields. In this paper, we examine what happens to the single-atom harmonic spectrum at high intensities, when multiphoton absorption ceases to be the dominant process of ionization. At these intensities, ionization can occur by the electron wave packet passing directly over the resultant barrier of the atomic potential and the incident electric field. In consequence of this, the harmonics produced are of a lower strength and can be swamped by the production of a large background to the harmonic spectrum. This background is caused by the ionized electron rescattering from the nucleus in the presence of the laser field.