Role of tunnel ionization in high harmonic generation from substituted benzenes.

We theoretically study high-harmonic generation in toluene, ortho-xylene and fluorobenzene driven by a 1.8 mm ultrashort pulse. We find that the chemical substitutions have a strong influence on the amplitude and phase of the emission from the highest occupied molecular orbital, despite having a small influence on the orbital itself. We show that this influence is due to the tunnel ionization step, which depends critically on the sign and amplitude of the asymptotic part of the wave function. We discuss how these effects would manifest in phase-sensitive high-harmonic generation spectroscopy experiments.