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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...

A ttosecond electron wavepackets are produced when an intense laser field ionizes an atom or a molecule 1 . When the laser field drives the wavepackets back to the parent ion, they interfere with the bound wavefunction, producing coherent subfemtosecond extreme-ultraviolet light bursts. When only a single return is possible 2,3 , an isolated attosecond pulse is generated. Here we demonstrate that by modulating the polarization of a carrier-envelope phase-stabilized short laser pulse 4 , we can finely control the electron-wavepacket dynamics. We use high-order harmonic generation to probe these dynamics. Under optimized conditions, we observe the signature of a single return of the electron wavepacket over a large range of energies. This temporally confines the extreme-ultraviolet emission to an isolated attosecond pulse with a broad and tunable bandwidth. Our approach is very general, and extends the bandwidth of attosecond isolated pulses in such a way that pulses of a few attoseconds seem achievable. Similar temporal resolution could also be achieved by directly using the broadband electron wavepacket. This opens up a new regime for timeresolved tomography of atomic or molecular wavefunctions 5,6 and ultrafast dynamics.During high-order harmonic generation (HHG) in gas 7 , short electron wavepackets (EWPs) are periodically released by high-field ionization. Their subsequent coherent interaction with the remaining bound wavefunction leads to coherent extremeultraviolet (XUV) emission. The T 0 /2 periodicity of this process (T 0 being the laser optical period) ensures that only odd harmonics of the fundamental radiation are emitted. Temporally, the XUV pulses are emitted as a train of chirped attosecond pulses [8][9][10] (1 attosecond = 10 −18 s). For both plateau (low energy) and cut-off (high energy) harmonics, specific focusing conditions ensure that only a single attosecond pulse is emitted every half cycle 11,12 . Extracting an isolated attosecond pulse from this train requires breaking the periodicity of the process, so that XUV emission is only possible within a single half cycle of the fundamental pulse. In this way, isolated 250-attosecond-long Figure 1 Spectra generated in argon. Spectra emitted from an argon medium irradiated with a polarization-modulated pulse (τ = 5 fs, δ = 6.2 fs, β = 0 • ) as a function of the CEP shift. For some CEPs, harmonic peaks appear, whereas for other CEPs, they broaden up to a continuum.pulses were recently obtained 13 by selecting the (highly intensity dependent) cut-off harmonics generated in neon by a 5-fs linearly polarized, fundamental pulse with stabilized carrierenvelope phase (CEP). With this technique, the minimum pulse duration achievable is limited by the (∼10 eV) bandwidth of the selected cut-off harmonics, which prevents us reaching the sub-100-attosecond domain.To isolate a broadband attosecond pulse, we used a different approach 2 . Our approach relies on the strong HHG sensitivity on the ellipticity, ε, of the fundamental field, which is largely nature phy...

We perform high harmonic generation spectroscopy of aligned nitrogen molecules to characterize the attosecond dynamics of multielectron rearrangement during strong-field ionization. We use the spectrum and ellipticity of the harmonic light to reconstruct the relative phase between different ionization continua participating in the ionization, and thus determine the shape and location of the hole left in the molecule by strong-field ionization. Our interferometric technique uses transitions between the ionic states, induced by the laser field on the subcycle time scale.

A. A demographic study of homicide-suicide in the Pretoria region over a 5 year period. Jnl of Forensic and Legal Medicine. Jan 2009. 16:261-265. 3. Kruger C. The influence of conflicting medieval church and social discourses on individual consciousness: Dissociation in the visions of Hadewijch of Brabant (13 th century). October 2009. Studia Historiae Ecclesiasticae. Vol XXXV(2) 239-266. 4. Lippi G., Smit DJ., Jordaan JC., Roos JL. Suicide risk in schizophrenia-a follow-up study after 20 years. Part I: Outcome and associated social factors. October 2009. SAJP. 15(3):56-62. 5. Maydell RJ., Van der Walt C., Roos JL., Scribante L., Ladikos A. Clinical characteristics and premorbid variables in childhood-onset schizophrenia: a descriptive study of twelve cases from a schizophrenia founder population. May 2009. African Journal of Psychiatry. 12(2):144-148. 6.

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International audienceWe photoionize nitrogen molecules with a train of extreme ultraviolet attosecond pulses together with a weak infrared field. We measure the phase of the two-color two-photon ionization transition ͑molecular phase͒ for different states of the ion. We observe a 0.9␲ shift for the electrons produced in the ionization channels leading to the X 2 ⌺ g + , vЈ = 1, and vЈ = 2 states. We relate this phase shift to the presence of a complex resonance in the continuum. By providing both a high spectral and temporal resolution, this general approach gives access to the evolution of extremely short-lived states, which is often not accessible otherwise. DOI: 10.1103/PhysRevA.80.011404 PACS number͑s͒: 33.80.Eh, 33.60.ϩq, 42.65.Ky, 82.53.Kp Ionization of atoms and molecules by absorption of ul-trashort extreme ultraviolet ͑xuv͒ radiation provides rich structural information on the considered species. The ioniza-tion process releases an electron wave packet, which can be described as a coherent superposition of partial waves. The relative contributions and phases of the partial waves can be extracted from photoelectron angular distributions at a given energy ͓1͔. However, the temporal structure of the ejected wave packet, which is imposed by the phase relation between different energy components, is not accessible with such experiments. To access this phase, one needs to couple two energy components of the electron wave packet and record the resulting interference. This can be achieved by absorption of high-order harmonics of an infrared laser pulse in the presence of the fundamental field. An intense laser pulse propagating in a gas jet produces coherent xuv radiation constituted of odd harmonics ͑2q +1͒␻ 0 of the fundamental frequency ␻ 0. These harmonics are all approximately phase locked with the fundamental and form an attosecond pulse train ͑APT͒ ͓2͔. In photoionization experiments with high harmonics, the photoelectron spectrum exhibits equidistant lines resulting from single-photon ionization ͓Fig. 1͑a͔͒. If an additional laser field with frequency ␻ 0 is added, two-photon ionization can occur: absorption of a harmonic photon accompanied by either absorption or stimulated emission of one photon ␻ 0. New lines ͑sidebands͒ appear in the spectrum, in between the harmonics ͓Fig. 1͑a͔͒. Since two coherent quantum paths lead to the same sideband, interferences occur. They are observed in an oscillation of the sideband amplitude as the delay ␶ between the probe ͑ir͒ and harmonic fields is scanned ͓2,3͔. This is the basis of the reconstruction of attosecond beating by interference of two-photon transitions ͑RABBITT͒ technique. The phase of the oscillation is determined by the phase difference between consecutive harmonics ͑phase locking͒ and by additional phase characteristics of the ionization process. The same process can be described in the time domain. The APT creates a train of attosecond electron wave packets. The additional laser field acts as an optical gate on the electrons , which can be used to re...

We study the Cooper minimum in high harmonic generation from argon atoms using long wavelength laser pulses. We find that the minimum in high harmonic spectra is systematically shifted with respect to total photoionization cross section measurements. We use a semi-classical theoretical approach based on Classical Trajectory Monte Carlo and Quantum Electron Scattering methods (CTMC-QUEST) to model the experiment. Our study reveals that the shift between photoionization and high harmonic emission is due to several effects: the directivity of the recombining electrons and emitted polarization, and the shape of the recolliding electron wavepacket.