J.P.Marangos, S.Baker, R.Torres, N.Kajumba, C.Haworth, J.Robinson, J.W.G.
Tisch, Blackett Laboratory, Imperial College ,London SW7 2BW, United Kingdom
C.Vozzi, F. Calegari, E. Benedetti, G. Sansone, S. Stagira, M. Nisoli, National Laboratory for Ultrafast and Ultraintense Optical Science-INFM, Dipartimento di Fisica, Politecnico, Milano, Italy
C.Altucci and R.Velotta, Coherentia-INFM and Dipartimento di Scienze Fisiche, Universita di Napoli "Federico II", Napoli, Italy
Our recent work has looked in particular at the signal from high order harmonic generation which contains rich information about the structure and intra-molecular dynamics of small molecules. This we will illustrate by two types of experiment; (a) measurements of HHG from aligned molecular samples to observe two-centre recombination interference and electronic structure dependence of the angle dependent yield, (b) reconstruction of intra-molecular proton dynamics from the spectral dependence of the HHG using the intrinsic chirp of recolliding electrons.We experimentally investigate the process of intramolecular quantum interference in high-order harmonic generation in impulsively aligned CO2 molecules. The recombination interference effect is clearly seen through the order dependence of the harmonic yield in an aligned sample. This confirms that the effective de Broglie wavelength of the returning electron wave is not significantly altered by acceleration in the Coulomb field of the molecular ion. For the first time, to our knowledge, we demonstrate that such interference effects can be effectively controlled by changing the ellipticity of the driving laser field . We demonstrate a new technique using high order harmonic generation in molecules to probe nuclear dynamics and structural rearrangement on a sub-femtosecond timescale. The chirped nature of the electron wavepacket produced by laser ionization in a strong field gives rise to a similar chirp in the photons emitted upon electron-ion recombination. Use of this chirp in the emitted light allows information about nuclear dynamics to be gained with 100 attosecond temporal resolution, from excitation by an 8 fs pulse, in a single laser shot. Measurements on H2 and D2 agree well with calculations of ultra-fast nuclear dynamics in the H2+ molecule, confirming the validity of the method. Guided by this result, we have measured harmonic spectra from CH4 and CD4 to demonstrate a few-femtosecond timescale for the onset of proton rearrangement in methane upon ionization .
 C.Vozzi et al., "Controlling two-center interference in molecular high harmonic generation", Phys. Rev. Lett., 95, 153902 (2005).
 S.Baker, et al., "Probing proton dynamics in molecules on an attosecond timer scale", Science, 312 424 (2006).
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