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The overarching goal of much of ultrafast science is to uncover the dynamics of molecular-scale processes. An ideal experiment could monitor the instantaneous positions of atoms within a molecule as a reaction occurs. A variety of x-ray, electron, and laser scattering techniques are being developed seeking to approach this ideal. In our work, we present the first direct observation of the motion of atoms within a molecule using electrons generated from the molecule itself. We achieve this using the attosecond "rescattering" of electrons that occurs during high-harmonic generation. Our experiment not only probes internal dynamics in a molecule, but also corresponds to a new manifestation of a widely used spectroscopic technique - coherent Raman spectroscopy, and gives a richer data set than is obtained using visible lasers.
In our work, we first vibrationally excite SF6 molecules using impulsive Raman scattering with an ultrashort laser pulse. After the excitation pulse, the molecule starts to vibrate coherently. Later, a more-intense laser pulse is used to pluck an electron from the molecule. After this electron is released, it oscillates in the laser field, and can return to the molecule in < 1 femtosecond to recombine, coherently generating an x-ray. Because the deBroglie wavelength of this electron, at ~1.5 Angstroms, is matched to the interatomic separations in the molecule, the strength of the x-ray emission is exquisitely sensitive (to miliangstroms) to the instantaneous position of the atoms within the molecule. A Fourier transform of the signal shows which vibrational modes are excited.
This new spectroscopic technique is more sensitive than ultrafast visible Raman spectroscopy because the x-ray generation is modulated by any motion of the molecule. (Ultrafast Raman only detects the strong symmetric mode). This work extends Raman spectroscopies in new directions, adding new capabilities that are not addressed by other approaches because of the potential sensitivity to all motion in a molecule, the ultrafast duration of the probe, and because all vibrational modes are observed simultaneously and coherently. In the future, this approach can be applied to complex phenomena such as dissociation dynamics, vibrational mode coupling, fast configuration changes, nuclear and electron dynamics during structural changes in molecules. Moreover, by monitoring the change in the x-ray signal as a function vof harmonic order (i.e. recolliding electron wavelength), it may also be possible to extend this technique to monitor dynamic molecular structure. As a result, this technique could become a broadly-applicable probe of chemical dynamics, combining the ultrahigh time-resolution with the potential for obtaining structural information complimentary to techniques such as electron diffraction. |
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