Pulse-length dependence of high-order harmonic generation in dissociating cyclic organic molecules

We compare high-order harmonic generation in gas jets of benzene, cyclohexane, and xenon using ∼800-nm laser pulses of 70 fs and 240 fs duration. The harmonic intensities of the 7th to 15th orders are studied as a function of laser intensity in the range $5\times {10}^{13}$ to $5\times {10}^{15}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{-}2}.$ Using 240-fs pulses, the dependence of the harmonic intensity on the laser intensity of the organic molecules exhibits significant deviations from the behavior of atoms, and shows strong evidence for harmonic generation from molecular fragments as well as the parent molecule. When the pulse duration is reduced to 70 fs, the intensity dependences of the harmonics are similar to those of xenon. This is attributed to the significant reduction in dissociation occurring during the shorter pulse and suggests that high-order harmonic generation in the ultrafast regime will be important in the understanding of molecular electron dynamics in strong fields.