Ionization and high-order harmonic generation in aligned benzene by a short intense circularly polarized laser pulse

We present a first-principles study of ionization and high-order harmonic generation by benzene aligned in the polarization plane of a short circularly polarized laser pulse. Time-dependent density-functional theory within the adiabatic local-density approximation is employed to describe the 30 valence-electron dynamics in three dimensions. The multielectron approach enables us to study the effect of very strong laser fields, 10¹⁴–10¹⁵W cm^-2, where multiple ionization and high-order harmonic generation interplay. Large ionization currents are formed, causing ionization of 1–4 electron charges, while strong high-order harmonic generation is observed. The well-known recollision mechanism of high-order harmonic generation plays a part for moderate laser intensities but is fully suppressed for strong laser fields. The harmonic generation spectra are characterized by two distinguishable plateaus, where the structure of the first plateau is dominated by the 6k±1 (k=0,1,…) selection rule. The number of harmonics in the second plateau is insensitive to the duration of the pulse. The peaks appear in pairs or in threesomes, depending on the pulse duration.