Access and use of this website and the material on it are subject to the Terms and Conditions set forth at NRC Publications Record / Notice d'Archives des publications de CNRC:http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?action=rtdoc&an=16925491&lang=en http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?action=rtdoc&an=16925491&lang=fr READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/jsp/nparc_cp.jsp?lang=en Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n'arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. Questions? Contact the NRC Publications Archive team atPublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information. NRC Publications Archive Archives des publications du CNRCThis publication could be one of several versions: author's original, accepted manuscript or the publisher's version. / La version de cette publication peut être l'une des suivantes : la version prépublication de l'auteur, la version acceptée du manuscrit ou la version de l'éditeur. For the publisher's version, please access the DOI link below./ Pour consulter la version de l'éditeur, utilisez le lien DOI ci-dessous.http://doi.org/10.1038/nature09185 Nature, 466, 7306, pp. 604-607, 2010-07-01 LETTERS Following a chemical reaction using high-harmonic interferometryThe study of chemical reactions on the molecular (femtosecond) timescale typically uses pump laser pulses to excite molecules and subsequent probe pulses to interrogate them. The ultrashort pump pulse can excite only a small fraction of molecules, and the probe wavelength must be carefully chosen to discriminate between excited and unexcited molecules. The past decade has seen the emergence of new methods that are also aimed at imaging chemical reactions as they occur, based on X-ray diffraction 1 , electron diffraction 2 or laser-induced recollision 3,4 -with spectral selection not available for any of these new methods. Here we show that in the case of high-harmonic spectroscopy based on recollision, this apparent limitation becomes a major advantage owing to the coherent nature of the attosecond high-harmonic pulse generation. The coherence allows the unexcited molecules to act as local oscillators against which the dynamics are observed, so a transient grating technique 5,6 can be used to reconstruct the amplitude and phase of emission from the excited molecules. We then extract structural information from the amplitude, which encodes the internuclear separation, by quantum interference at short times and by scattering of the recollision electron at longer times. The phase records the attosecond dynamics of the electrons, giving access to the evolving ionization potentials...

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Conical intersections play a crucial role in the chemistry of most polyatomic molecules, ranging from the simplest bimolecular reactions to the photostability of DNA. The real-time study of the associated electronic dynamics poses a major challenge to the latest techniques of ultrafast measurement. We show that high-harmonic spectroscopy reveals oscillations in the electronic character that occur in nitrogen dioxide when a photoexcited wave packet crosses a conical intersection. At longer delays, we observe the onset of statistical dissociation dynamics. The present results demonstrate that high-harmonic spectroscopy could become a powerful tool to highlight electronic dynamics occurring along non-adiabatic chemical reaction pathways. 1The outcome of chemical reactions is determined by the valence electronic structure of molecules. Therefore, the elucidation of elementary reaction mechanisms requires an understanding of the valence electron dynamics. Recently developed techniques that are efficient in probing valence electron dynamics include attosecond transient absorption (1), extreme ultraviolet photoelectron spectroscopy (XUV-PES) (2), high-order harmonic spectroscopy (HHS) (3-5) and strong-field ionization (6). Both time-resolved PES (7) and time-resolved HHS are sensitive to valence electron dynamics through the molecular photoionization matrix elements.Electronic dynamics in molecules are particularly challenging to observe when they are strongly coupled to nuclear dynamics. Such situations often arise in polyatomic molecules where conical intersections between the potential energy surfaces induce very rapid radiationless transitions at particular nuclear configurations (see inset of Fig. 1) (8, 9). These features channel electronic excitation into atomic motion in such diverse contexts as the primary steps of vision (10) and the dynamics underlying electron transfer and the photostability of DNA bases (11).Here we show that high-harmonic spectroscopy reveals the variations in electronic character during the conical intersection dynamics and the subsequent dissociation of nitrogen dioxide (NO 2 ). We chose NO 2 , a radical, because of its model status for theories of unimolecular dissociation (12-14) and conical intersection dynamics (15)(16)(17)(18)(19). Our results translate the previously recognized sensitivity of HHS to electronic structure into a tool for elucidating chemical reaction dynamics.High-harmonic spectroscopy can be factored into three steps: removal of an electron by an intense femtosecond laser field, acceleration of the electron in the laser field and photorecombination (20, 21). Each step contributes an amplitude and a phase to the emitted XUV radiation (20,(22)(23)(24)(25). The measurement relies on a coherent detection scheme in a transient grating geometry, using unexcited molecules as a local oscillator (4, 5). It is thus sensitive to 2 both amplitude and phase of the photorecombination matrix elements, a quantity that has recently attracted a lot of interest (26,27). Time-resolved...