For the past several decades, we have been able to directly probe the motion of atoms that is associated with chemical transformations and which occurs on the femtosecond (10(-15)-s) timescale. However, studying the inner workings of atoms and molecules on the electronic timescale has become possible only with the recent development of isolated attosecond (10(-18)-s) laser pulses. Such pulses have been used to investigate atomic photoexcitation and photoionization and electron dynamics in solids, and in molecules could help explore the prompt charge redistribution and localization that accompany photoexcitation processes. In recent work, the dissociative ionization of H(2) and D(2) was monitored on femtosecond timescales and controlled using few-cycle near-infrared laser pulses. Here we report a molecular attosecond pump-probe experiment based on that work: H(2) and D(2) are dissociatively ionized by a sequence comprising an isolated attosecond ultraviolet pulse and an intense few-cycle infrared pulse, and a localization of the electronic charge distribution within the molecule is measured that depends-with attosecond time resolution-on the delay between the pump and probe pulses. The localization occurs by means of two mechanisms, where the infrared laser influences the photoionization or the dissociation of the molecular ion. In the first case, charge localization arises from quantum mechanical interference involving autoionizing states and the laser-altered wavefunction of the departing electron. In the second case, charge localization arises owing to laser-driven population transfer between different electronic states of the molecular ion. These results establish attosecond pump-probe strategies as a powerful tool for investigating the complex molecular dynamics that result from the coupling between electronic and nuclear motions beyond the usual Born-Oppenheimer approximation.

Manipulation of the molecular-axis distribution is an important ingredient in experiments aimed at understanding and controlling molecular processes 1-6 . Samples of aligned or oriented molecules can be obtained following the interaction with an intense laser field 7-9 , enabling experiments in the molecular rather than the laboratory frame 10-12 . However, the degree of impulsive molecular orientation and alignment that can be achieved using a single laser field is limited 13 and crucially depends on the initial states, which are thermally populated. Here we report the successful demonstration of a new technique for laser-field-free orientation and alignment of molecules that combines an electrostatic field, non-resonant femtosecond laser excitation 14 and the preparation of state-selected molecules using a hexapole 2 . As a unique quantum-mechanical wavepacket is formed, a large degree of orientation and alignment is observed both during and after the femtosecond laser pulse, which is even further increased (to cos θ = −0.74 and cos 2 θ = 0.82, respectively) by tailoring the shape of the femtosecond laser pulse. This work should enable new applications such as the study of reaction dynamics or collision experiments in the molecular frame, and orbital tomography 11 of heteronuclear molecules.The outcome of molecular collision experiments is strongly affected by the angular anisotropies in the initial molecular axis distribution. In bimolecular and molecule-surface collisions, collision cross-sections sensitively depend on the relative arrangement of the collision partners 1,2 . Likewise, photon-molecule collisions such as X-ray diffraction and photodissocation experiments aimed at the elucidation of molecular structure or photochemical activity depend on and can benefit from angular confinement of the sample 3,4 . The two most important moments of the molecular axis distribution are the 'alignment' ( cos 2 θ ) and 'orientation' ( cosθ ), where θ is the angle between the molecular axis and a reference axis.First attempts to orient and align molecules relied on electrostatic fields. A hexapole electric field can be used to stateselect polar molecules and orient them through their first-order Stark effect 2,5 using a moderate field strength. The orientation is limited by the selected state. 'Brute-force orientation' uses a strong homogeneous electrostatic field and relies on the second-and higher-order Stark effect 6 . It requires molecules with a large dipole moment and extremely high electric-field strengths.As a part of extensive efforts aimed at achieving laser-controlled chemistry 15-17 , laser-controlled alignment 8 has attracted considerable attention. Suitably chosen laser fields can exert torques on molecules, exploiting the interaction of the laser field with the molecular polarizability. Both adiabatic alignment, where molecules are exposed to a slowly varying laser field 18 , and non-adiabatic (impulsive) alignment, where molecules align after receiving a kick by a short laser pulse 7 , have been succe...

We report experiments where hydrogen molecules were dissociatively ionized by an attosecond pulse train in the presence of a near-infrared field. Fragment ion yields from distinguishable ionization channels oscillate with a period that is half the optical cycle of the IR field. For molecules aligned parallel to the laser polarization axis, the oscillations are reproduced in two-electron quantum simulations, and can be explained in terms of an interference between ionization pathways that involve different harmonic orders and a laser-induced coupling between the 1s g and 2p u states of the molecular ion. This leads to a situation where the ionization probability is sensitive to the instantaneous polarization of the molecule by the IR electric field and demonstrates that we have probed the IR-induced electron dynamics with attosecond pulses. The prospect of observing and controlling ultrafast electron dynamics in molecular systems is the basis of the current interest to apply attosecond (1 as ¼ 10 À18 s) laser pulses to physical chemistry. Since the first demonstration of attosecond pulses [1,2], pioneering experiments have demonstrated their potential in atoms [3,4], solid state systems [5], and, most recently, molecules [6], where interest has been stimulated by numerical studies which suggest that an electronic (i.e., attosecond or fewfemtosecond) time scale may be important in fundamental chemical processes [7,8]. The inherent multielectron nature of the electron dynamics in many molecular systems is a formidable challenge to theoreticians and experimentalists alike, and requires the development of novel theoretical and experimental techniques.Attosecond pump-probe spectroscopy is based on the generation of attosecond light pulses by high harmonic generation. Presently, attosecond pulses exist as attosecond pulse trains (APTs) [1] and as isolated attosecond pulses [2]. The first application of attosecond pulses to follow rapid electron dynamics in a molecule revealed that the dissociative ionization of hydrogen by a two-color extreme-ultraviolet ðXUVÞ þ IR field results in a localization of the bound electron in the molecular ion that depends with attosecond time resolution on the time delay between the attosecond XUV pulse and the IR laser pulse [6]. This could be observed via an asymmetry of the ejected fragments in the laboratory frame, i.e., after the dissociation was complete [9]. A similar experimental result was also obtained using an APT [10]. In these experiments the attosecond pulses initiated electron dynamics that was subsequently addressed by an IR pulse. A next challenge is to use attosecond pulses as a probe of ultrafast molecular electron dynamics. In this Letter we do so by investigating how a moderately intense IR field influences the electronic states that are accessed in photoionization of hydrogen using an APT.In the experiment, an XUV APT (with two pulses per IR cycle) and a 30 fs FWHM 780 nm (IR) pulse (3 Â 10 13 W=cm 2 ) with identical linear polarization were collinearly propagated and...

We present an interferometric pump-probe technique for the characterization of attosecond electron wave packets (WPs) that uses a free WP as a reference to measure a bound WP. We demonstrate our method by exciting helium atoms using an attosecond pulse (AP) with a bandwidth centered near the ionization threshold, thus creating both a bound and a free WP simultaneously. After a variable delay, the bound WP is ionized by a few-cycle infrared laser precisely synchronized to the original AP. By measuring the delay-dependent photoelectron spectrum we obtain an interferogram that contains both quantum beats as well as multipath interference. Analysis of the interferogram allows us to determine the bound WP components with a spectral resolution much better than the inverse of the AP duration.

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We present the design of a velocity map imaging spectrometer where the target gas is injected from a capillary that is integrated in the repeller plate of the ion optics assembly that drives electrons/ions formed by ionization or dissociation to a two-dimensional detector. The geometry of this design allows the use of gas densities in the interaction region that are two to three orders of magnitude higher than the densities that are used in standard velocity map imaging spectrometers, making the detector suitable for working with weak light sources such as newly developed attosecond pulse sources, or (quasi-)cw sources such as synchrotrons. In a test where monoenergetic photoelectrons were generated by six-photon ionization of Xe (utilizing the second harmonic of a neodymium doped Nd:YAG), the kinetic energy resolution of the spectrometer was found to be DeltaE/E=1.8%. This number was found to be in good agreement with Monte Carlo simulations.

We demonstrate that dissociative ionization of O 2 can be controlled by the relative delay between an attosecond pulse train (APT) and a copropagating infrared (IR) field. Our experiments reveal a dependence of both the branching ratios between a range of electronic states and the fragment angular distributions on the extreme ultraviolet (XUV) to IR time delay. The observations go beyond adiabatic propagation of dissociative wave packets on IR-induced quasistatic potential energy curves and are understood in terms of an IR-induced coupling between electronic states in the molecular ion.

The exact nature of the low temperature electronic phase of the manganite materials family, and hence the origin of their colossal magnetoresistant (CMR) effect, is still under heavy debate. By combining new photoemission and tunneling data, we show that in La 2-2x Sr 1+2x Mn 2 O 7 the polaronic degrees of freedom win out across the CMR region of the phase diagram. This means that the generic ground state is that of a system in which strong electron-lattice interactions result in vanishing coherent quasi-particle spectral weight at the Fermi level for all locations in k-space. The incoherence of the charge carriers offers a unifying explanation for the anomalous charge-carrier dynamics seen in transport, optics and electron spectroscopic data. The stacking number N is the key factor for true metallic behavior, as an intergrowth-driven breakdown of the polaronic domination to give a metal possessing a traditional Fermi surface is seen in the bilayer system.Competition between local lattice distortions leading to anti-ferromagnetic, charge and orbital (CO) ordering on the one hand, and mixed valence character promoting metallic ferromagnetic double exchange on the other, determines the transport transport properties LSMO [5], whereas the bilayer analogue is metallic only in a narrow Sr-doping and temperature regime [6], giving rise to the largest CMR effect [7]. The more strongly 2D, single layer compound shows neither metallic nor CMR behavior [8].Focusing on bilayer LSMO within the CMR-region of * These authors contributed equally to this work the phase diagram, the prevailing picture from structural studies is one of polarons existing above T C . On cooling towards T C , these short range versions of the CO order typical of the insulating compositions, become increasingly correlated [9, 10]. Eventually double exchange, leading to an itinerant, metallic state, takes over.This metallic state for x = 0.4 has been shown to support small quasi-particles (QPs) in the spectral function measured by angle-resolved photoemission (ARPES) [11]. These signal coherent electronic excitations, albeit strongly dressed with lattice distortions, and are seen as evidence for a novel and elusive state of matter known as a polaronic metal [11, 12].Other ARPES studies paint a different picture, with stronger QP features observed at low T that persist up to temperatures of order 1.5T C [13][14][15], despite the system being nominally insulating. In contrast, scanning tunneling microscopy / spectroscopy (STM/S) studies reported gaps in the local density of states near-E F for x = 0.30 [16] and 0.325 [17], both in the metallic and insulating temperature regimes. Finally, new neutron diffraction data for x = 0.4 has shown that even far below T C -at 10 K in the metallic state -polarons remain as fluctuations that strongly broaden and soften phonons near the wave vectors where the charge order peaks would appear in the insulating phase [18].Here, a combination of ARPES and STM/S reveals that the bilayer manganites still have a number of...