High harmonic light sources make it possible to access attosecond timescales, thus opening up the prospect of manipulating electronic wave packets for steering molecular dynamics. However, two decades after the birth of attosecond physics, the concept of attosecond chemistry has not yet been realized; this is because excitation and manipulation of molecular orbitals requires precisely controlled attosecond waveforms in the deep UV, which have not yet been synthesized. Here, we present a unique approach using attosecond vacuum UV pulse-trains to coherently excite and control the outcome of a simple chemical reaction in a deuterium molecule in a non-Born-Oppenheimer regime. By controlling the interfering pathways of electron wave packets in the excited neutral and singly ionized molecule, we unambiguously show that we can switch the excited electronic state on attosecond timescales, coherently guide the nuclear wave packets to dictate the way a neutral molecule vibrates, and steer and manipulate the ionization and dissociation channels. Furthermore, through advanced theory, we succeed in rigorously modeling multiscale electron and nuclear quantum control in a molecule. The observed richness and complexity of the dynamics, even in this very simplest of molecules, is both remarkable and daunting, and presents intriguing new possibilities for bridging the gap between attosecond physics and attochemistry.chemical dynamics | electron dynamics | ultrafast T he coherent manipulation of quantum systems on their natural timescales, as a means to control the evolution of a system, is an important goal for a broad range of science and technology, including chemical dynamics and quantum information science. In molecules, these timescales span from attosecond timescales characteristic of electronic dynamics, to femtosecond timescales characteristic of vibrations and dissociation, to picosecond timescales characteristic of rotations in molecules. With the advent of femtosecond lasers, observing the transition state in a chemical reaction (1), and controlling the reaction itself, became feasible. Precisely timed femtosecond pulse sequences can be used to selectively excite vibrations in a molecule, allow it to evolve, and finally excite or deexcite it into an electronic state not directly accessible from the ground state (2). Alternatively, interferences between different quantum pathways that end up in the same final state can be used to control the outcome of a chemical reaction (3)(4)(5)(6)(7)(8)(9).In recent years, coherent high harmonic sources with bandwidths sufficient to generate either attosecond pulse trains or a single isolated attosecond pulses have been developed that are also perfectly synchronized to the driving femtosecond laser (10-12). This new capability provides intriguing possibilities for coherently and simultaneously controlling both the electronic and nuclear dynamics in a molecule in regimes where the BornOppenheimer approximation is no longer valid, to select specific reaction pathways or products. Here...

Understanding the coupled electronic and nuclear dynamics in molecules by using pump-probe schemes requires not only the use of short enough laser pulses but also wavelengths and intensities that do not modify the intrinsic behavior of the system. In this respect, extreme UV pulses of few-femtosecond and attosecond durations have been recognized as the ideal tool because their short wavelengths ensure a negligible distortion of the molecular potential. In this work, we propose the use of two twin extreme UV pulses to create a molecular interferometer from direct and sequential two-photon ionization processes that leave the molecule in the same final state. We theoretically demonstrate that such a scheme allows for a complete identification of both electronic and nuclear phases in the wave packet generated by the pump pulse. We also show that although total ionization yields reveal entangled electronic and nuclear dynamics in the bound states, doubly differential yields (differential in both electronic and nuclear energies) exhibit in addition the dynamics of autoionization, i.e., of electron correlation in the ionization continuum. Visualization of such dynamics is possible by varying the time delay between the pump and the probe pulses.attosecond molecular dynamics | free electron lasers | high harmonic generation | XUV pump-probe spectroscopy A dvances in optical technology in the last 2 decades have enabled a variety of applications in which ultrashort laser pulses can be used to probe, manipulate, and eventually control electron and fast nuclear dynamics in molecules. A successful approach, inspired by femtochemistry (1), is to use a combination of ultrashort pump and probe pulses, in which the dynamics are launched by the pump pulse and the imaging of these dynamics is obtained by scanning the time delay between both pulses. By controlling optical coherence, early experimental works made use of intense infrared (IR) pulses with durations of the order of femtoseconds (fs) to steer electronic motion in atoms and molecules and to probe it, in a subfemtosecond time scale, by analyzing either the high harmonic emission, the photoelectron spectrum resulting from rescattered electrons, or the kinetic energy of the generated ions (2-4). More recent experimental approaches have developed pump-probe schemes in which attosecond extreme UV (XUV) pulses have been used to ionize and induce some dynamics in the target, and phase-locked IR probe pulses have been used to trace it, both in atoms (5-7) and molecules (8, 9). A step forward in pump-probe schemes has been the advent of new detection and analysis strategies, such as interferometric methods to reconstruct electron wave packets in atoms (10) or the attosecond streak camera (7,11,12,13) to retrieve phases and time-varying intensity envelopes of attosecond XUV pulses from their imprint in photoelectron spectra (14). The optical characterization of attosecond pulses is thus directly linked to the understanding of photoionization (13,15,16).In all of the above schemes,...

We investigate the use of UV-pump-UV-probe schemes to trace the evolution of nuclear wave packets in excited molecular states by analyzing the asymmetry of the electron angular distributions resulting from dissociative ionization. The asymmetry results from the coherent superposition of gerade and ungerade states of the remaining molecular ion in the region where the nuclear wave packet launched by the pump pulse in the neutral molecule is located. Hence, the variation of this asymmetry with the time delay between the pump and the probe pulses parallels that of the moving wave packet and, consequently, can be used to clock its field-free evolution. The performance of this method is illustrated for the H 2 molecule. DOI: 10.1103/PhysRevLett.108.063009 PACS numbers: 33.80.Àb, 82.53.Eb The rapid development of ultrashort laser pulses combined with optical pump-probe spectroscopy has opened the way for controlling and manipulating electron (and nuclear) dynamics in atoms (and molecules). For molecules, recent advances in coherent pulse control, as carrierenvelope phase stabilization, has triggered the interest in steering and tracing electron motion upon ionization by means of intense IR fields [1][2][3][4][5][6][7]. They have also suggested that the analysis of the vibrational population of the remaining molecular ion can be used as a clock for the recollision events [5,6]. More recent experiments have made use of few-cycle IR probe pulses on molecules ionized by a single attosecond pulse [8] or a train of attosecond pulses [9,10]. In all the above experiments, H 2 and D 2 molecular targets were used, which is the natural choice in view of the many variables potentially intervening in these kinds of problems and the need for theoretical support to understand the ensuing dynamics and to suggest alternative mechanisms of control [8,10,11].In the control schemes mentioned above, the dynamics is mostly driven by the strong IR field. An alternative procedure that allows one to explore field-free molecular dynamics is to use XUV pump-XUV probe schemes [12,13], which have been successfully employed to image nuclear wave packets (NWPs) of the D þ 2 ion in experiments leading to the Coulomb explosion (CE) of D 2 [14,15]. Free electron lasers provide such femtosecond (fs) pulses in vacuum ultraviolet and soft x-ray energy domains [16] and new facilities are expected to generate fs pulses in a wider photon energy range (12-124 eV) [17]. In spite of the relatively high intensity of the light generated by these lasers, its short wavelength ensures that the field-induced potential is much smaller than the electron-nucleus and electron-electron potentials (perturbative regime [18]), so the dynamics of the NWP is exclusively driven by the unperturbed molecular potential.In this work we focus on the use of UV pulses to trace the NWP generated in excited states of neutral molecules. We will show how to use a UV-pump-UV-probe scheme with identical single pulses to trace the evolution of the NWP generated by the pump pulse in the ...

Attosecond science promises to allow new forms of quantum control in which a broadband isolated attosecond pulse excites a molecular wave packet consisting of a coherent superposition of multiple excited electronic states. This electronic excitation triggers nuclear motion on the molecular manifold of potential energy surfaces and can result in permanent rearrangement of the constituent atoms. Here, we demonstrate attosecond transient absorption spectroscopy (ATAS) as a viable probe of the electronic and nuclear dynamics initiated in excited states of a neutral molecule by a broadband vacuum ultraviolet pulse. Owing to the high spectral and temporal resolution of ATAS, we are able to reconstruct the time evolution of a vibrational wave packet within the excited B 1 u + electronic state of H 2 via the laser-perturbed transient absorption spectrum.

Dissociative single ionization of H 2 induced by extreme ultraviolet photons from an attosecond pulse train has been studied in a kinematically complete experiment. Depending on the electron kinetic energy and the alignment of the molecule with respect to the laser polarization axis, we observe pronounced asymmetries in the relative emission directions of the photoelectron and the H þ ion. The energydependent asymmetry pattern is explained by a semiclassical model and further validated by fully quantum mechanical calculations, both in very good agreement with the experiment. DOI: 10.1103/PhysRevLett.110.213002 PACS numbers: 33.80.Eh Molecular hydrogen is the simplest molecule that exhibits electron correlation and doubly excited states. Having a total energy above the ionization threshold, they are embedded in the single-ionization continuum and thus autoionize via emission of an electron on a time scale that is comparable to the dissociation time. The dynamical interplay between the electronic and nuclear motion in autoionization has been the subject of research [1,2]. It has raised particular attention with the advent of ultrashort light pulses whose durations may permit real-time imaging and control of these processes [3].Theoretical investigations suggested that the symmetry of hydrogenic molecules (H 2 , D 2 , HD) can be broken after light-induced ionization [4][5][6]. This is a consequence of interferences arising from a coherent superposition of ionic molecular states with different parities resulting in the localization of the remaining bound electron. Several experiments demonstrated this for multiphoton processes [3,[7][8][9][10] as well as for single-photon transitions [6,11,12]. However, a fully differential analysis of the asymmetry arising from electron localization in photoionization induced by spectrally broad laser pulses has never been reported.Here, we present experimental data on H 2 photoionization using an attosecond pulse train. Using a semiclassical model [13][14][15] based on the WKB approximation, we demonstrate that both the origin of the observed asymmetry and its oscillations as a function of the kinetic energies of the charged fragments can be accounted for fully. The present findings are further confirmed by a fully quantum mechanical calculation.In the experiment ð35 AE 10Þ fs IR pulses, provided by a commercial laser system, are spectrally broadened in a hollow core fiber and recompressed to ð15 AE 5Þ fs using chirped mirrors. Using high harmonic generation [16] in argon followed by an aluminum filter to remove the fundamental light, we reach extreme ultraviolet (XUV) photon energies between 16 and 37 eV. The high harmonics are emitted as attosecond pulse trains where the number of pulses depends on the duration of the generating laser pulse and is here estimated to be less than 10. The XUV beam is then focused into a supersonic jet of hydrogen gas. The focal spot is placed in the center of a reaction microscope [17] enabling the measurement of the individual momenta of all c...