High-order harmonic generation in cyclic organic molecules

High-order harmonic generation from the interaction of high-intensity femtosecond laser pulses with the organic molecules benzene and cyclohexane has been observed using laser pulses of 240 and 70 fs duration at wavelengths around 800 nm. Harmonic intensities were measured as a function of laser intensity in the range 4 x 10(13)-5 x 10(15) W cm(-2), of laser polarization, and of the density of the medium. Harmonics from the 7th to the 13th order were found to have comparable intensities with those produced by xenon at the same density. The 7th harmonic from cyclohexane was measured to be four times more intense than that from xenon. No saturation of this harmonic was observed in cyclohexane up to a peak intensity of 2 x 10(15) W cm(-2), in contrast with xenon, where saturation was seen above 8 x 10(14) W cm(-2). These results are related to previous studies of ionization and fragmentation of these molecules in the same intensity range. The role of resonances between harmonics and high-lying molecular states is discussed.