High-order harmonic generation from organic molecules in ultra-short pulses |
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Authors: | N Hay R de Nalda T Halfmann KJ Mendham MB Mason M Castillejo JP Marangos |
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Institution: | (1) The Blackett Laboratory, Imperial College of Science, Technology and Medicine, London SW7 2BZ, UK, GB;(2) Instituto de Química Física Rocasolano, CSIC, Serrano 119, 28006 Madrid, Spain, ES |
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Abstract: | We have studied high-order harmonic generation (HHG) from organic molecules irradiated with near-infrared high intensity laser
pulses of 70 fs and 240 fs duration. The molecular systems studied were the aromatics benzene and naphthalene and the alkanes
cyclopropane and cyclohexane (cyclic) and n-hexane (linear). Harmonic intensities were measured both as a function of laser intensity (in the range 5×1013-5×1015 W cm-2) and as a function of ellipticity of the laser field polarisation. The results were compared with those from the xenon atom.
For 70 fs pulses, harmonic generation from the organic systems was similar to that of xenon, revealing an atom-like behaviour
for molecules when the laser pulse duration is shorter than the fragmentation timescale of the molecule. We note significant
differences between molecules with respect to HHG efficiencies and the suppression of HHG in larger species. We discuss these
differences in the context of the molecular properties, electronic structure and behaviour of ionisation and fragmentation
that result in enhancement of field ionisation in larger systems. Study of the polarisation ellipticity dependence of HHG
shows that the harmonic yield in molecules is less sensitive to the polarisation than for atoms (xenon). This is consistent
with the expected behaviour given the larger recollision cross-section presented by the core in the molecular system compared
to the atom. Our results suggest that study of HHG from molecules exposed to ultra-short pulses is potentially a powerful
tool for understanding the electron dynamics of molecules exposed to an intense field.
Received 14 September 2000 and Received in final form 6 December 2000 |
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Keywords: | PACS 42 65 Ky Harmonic generation frequency conversion – 42 50 Hz Strong-field excitation of optical transitions in quantum systems multi-photon processes dynamic Stark shift |
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