High harmonic generation in butane and butadiene

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Abstract
We have investigated the generation of the high order harmonics (q=5-15) of a 150 fs Ti:S laser operating at 780 nm in the intensity range 2.5*1013-1015 W cm-2. We report, we believe for the first time, high order harmonic generation from gases of pure organic molecules (butane C4H10 and 1,3-butadiene C4H6). By comparison with xenon under nominally the same irradiance and gas density conditions, we infer that the conversion efficiency in the molecular systems is approximately an order of magnitude less than that from xenon. We report broad similarity between butane and butadiene under these conditions, with butane being more efficient at higher irradiances. In contrast, at the lowest irradiances investigated (2.5*1013 W cm-2) harmonic generation in butadiene becomes relatively more efficient, when compared to butane, which we tentatively attribute to the presence of the pi delocalized electrons in this molecule.