Frankfurt 2006 – scientific programme

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SYIF: Intense field interaction with molecules and clusters

SYIF 1: Intense field interaction with molecules and clusters 1

SYIF 1.3: Invited Talk

Friday, March 17, 2006, 14:36–15:04, HV

Multiphoton photonics and applications in ultrafast chemical spectroscopies — •Margaret Murnane, Nick Wagner, Andrea Wuest, Ivan Christov, and Henry Kapteyn — Department of Physics and JILA, and NSF Engineering Research Center in Extreme-Ultraviolet Science and Technology, University of Colorado and National Institute of Standards and Technology, Boulder, CO 80309-0440

Coherent beams at soft-x-ray wavelengths[1] can be generated by focusing intense femtosecond, laser pulses into a gas. This process, called high-order harmonic generation (HHG), takes nonlinear optics to an extreme. By combining together tens to hundreds of visible laser photons, high harmonic generation upshifts laser light from the visible into the ultraviolet and soft-x-ray regions, to energies in excess of 1keV. High harmonics are usually generated from noble gas atoms. Recently however, there has been great interest in using high-harmonic generation from molecules as a probe of molecular structure and dynamics. To date, HHG from rotational wavepackets has been observed, and static, tomographic, structural information has been extracted in the case of a diatomic molecule.[2,3] In this talk, I will discuss the first observation of intramolecular dynamics using electrons rescattered during the process of high harmonic generation.[4] We first excite coherent vibrations in SF6 using impulsive Raman scattering with a short laser pulse. A second, more-intense laser pulse generates high-order harmonics of the fundamental laser, at wavelengths of  20-50 nm. The high-order harmonic yield is observed to oscillate, at frequencies corresponding to all the Raman-active modes of SF6, with an asymmetric breathing mode most visible. This is in contrast to conventional impulsive stimulated Raman spectroscopy where only the symmetric breathing mode of the molecule is easily observed. The data also show evidence of relaxation dynamics following impulsive excitation of the molecule. Our results indicate that high harmonic generation is a sensitive probe of vibrational dynamics and may yield more information simultaneously than conventional ultrafast spectroscopic techniques. Since the de Broglie wavelength of the recolliding electron is on the order of interatomic distances, i.e.   1.5 Å, small changes in the shape of the molecule lead to large changes in the high harmonic yield. This work therefore demonstrates a new spectroscopic technique for probing ultrafast internal dynamics in molecules that uniquely combines ultrafast time resolution with atomic-scale structural information.

[1] H. Kapteyn et al., "Extreme Nonlinear Optics: Coherent X-Rays from Lasers", Physics Today, March (2006).

[2] R. Velotta et al., "High-order harmonic generation in aligned molecules", PRL 8718 (2001).

[3] J. Itatani et al. Tomographic imaging of molecular orbitals. Nature 432, 867-871 (2004).

[4] N. Wagner et al., "Monitoring Molecular Dynamics using Coherent Electrons from High-Harmonic Generation", submitted for publication (2005).