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An attosecond molecular interferometer is proposed by using a XUV–XUV pump-probe scheme. The interferograms resulting in the photoelectron distributions enable the full reconstruction of the molecular wave packet associated to excited states using a quantum state holographic approach that, to our knowledge, has only been proposed for simple atomic targets combining attosecond XUV pulses with IR light. In contrast with existing works, we investigate schemes where one- and two-photon absorption paths contribute to ionize the hydrogen molecule and show that it is possible to retrieve the excitation dynamics even when imprinted in a minority channel. Furthermore, we provide a systematic analysis of the time-frequency maps that reveal the distinct, but tightly coupled, motion of electrons and nuclei.

The ultrafast decay of highly excited electronic states is resolved with a molecular clock technique, using the vibrational motion associated to the ionic bound states as a time-reference. We demonstrate the validity of the method in the context of autoionization of the hydrogen molecule, where nearly exact full dimensional ab-initio calculations are available. The vibrationally resolved photoionization spectrum provides a time-energy mapping of the autoionization process into the bound states that is used to fully reconstruct the decay in time. A resolution of a fraction of the vibrational period is achieved. Since no assumptions are made on the underlying coupled electron-nuclear dynamics, the reconstruction procedure can be applied to describe the general problem of the decay of highly excited states in other molecular targets, We acknowledge the financial support from the FP7 Marie Curie ITN CORINF, the European Research Council under the ERC grant 290853 XCHEM, the European COST Action CM1204 XLIC and the MINECO Project No. FIS201342002-R. AP acknowledges Ramón y Cajal Programme from MINECO (Spain). LM acknowledges support from DFG priority programme 1840 QUTIF. Theoretical calculations were obtained at the Mare Nostrum BSC and CCC-UAM computer centers

Atomic autoionization following photoabsorption is a typical example of quantum interferences governed by electron-electron correlation. Coherence between direct photoionization and autoionization paths results in "Fano profiles", widely explored in atoms in the last 60 years. The advent of femto- and attosecond laser technology made time-resolved images of the delayed electron ejection in autoionization accessible, leading to the reemergence of such studies in atomic systems. The counterpart molecular phenomena show the richness, as well as the complexity, added by nuclear motion, which may proceed on similar time scales. However, Fano profiles are usually absent in measured molecular photoionization cross sections and an unequivocal parametrization of molecular autoionization signatures, similar to that introduced by Fano in atoms [U. Fano, Phys. Rev. 1961, 124, 1866] has not yet been achieved. In this work we introduce a simple semiclassical model that accounts for all the features observed in H2 photoionization and demonstrate that the interference structures observed in dissociative ionization spectra are almost exclusively due to the phase accumulated in the nuclear motion. Furthermore, we show that the temporal build-up of these structures in the energy-differential cross sections is also determined by nuclear motion. We validate our models by comparing with full-dimensional ab initio calculations solving the time-dependent Schrödinger equation.

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′Σu1+ electronic state of H2 via the laser-perturbed transient absorption spectrum, This material is based on work supported by the DARPA PULSE program through a grant from AMRDEC under Award No. W31P4Q1310017; the Army Research Office under Awards No. W911NF-11-1-0297, No. WN911NF-14- 1-0383, and No. FA9550-16-1-0013; the Air Force Office of Scientific Research under Awards No. FA9550-15-1-0037 and No. FA9550-16-1-0149; the National Science Foundation under Award No. PHY-1506345, an Advanced Grant of the European Research Council XCHEM 290853; and European Grants MC-ITN CORINF, the European COST Action XLIC CM1204, the MINECO Project No. FIS2013-42002-R; and the ERA-Chemistry Project PIM2010EEC-00751

The relative stability of the different isomers of cyclopentadienyl derivatives CpXH3 (X = C, Si, Ge) and their intrinsic acidities have been investigated by means of B3LYP/6-311+G(3df,2p)//CCSD/6-311+G(d,p) density functional theory calculations. Whereas for the methylcyclopentadiene the 1- and 2- substituted isomers are almost equally stable and much more stable than the 5-substituted isomer, for the germyl derivatives the 5-substituted compound is the global minimum, due to the stabilization of the system through a hyperconjugative π-aromaticity effect, which is the larger the more electropositive the XH3 substituent is. As a consequence CpXH3 (X = Si, Ge) are more aromatic than cyclopentadiene. The silyl and germyl derivatives are more fluxional than the methyl derivative, the 1,2-XH3 shift activation barriers being around 60 kJ mol−1. For all the isomers, the most favourable deprotonation process corresponds to the loss of the proton attached to the sp3 carbon atom of the five membered ring. For Si a...

We extend attosecond transient absorption spectroscopy (ATAS) to the study of hydrogen molecules, demonstrating the potential of the technique to resolve – simultaneously and with state resolution – both the electronic and nuclear dynamics.

We report ab initio calculations on laser-assisted photoionization of the hydrogen molecule in the energy region where autoionization from doubly excited states is expected to occur. We use a UV-pump/IR-probe scheme in which an isolated attosecond UV pulse and a 750 nm IR pulse are combined. The IR pulse has a relatively low intensity (1012 W cm−2), which allows us to perform a perturbative analysis of the calculated ionization probabilities differential in either electron or nuclear energy or both. We show that, for dissociative ionization, the electron energy distributions as a function of time delay exhibit unusual streaking patterns that are due to the presence of autoionizing states. These patterns significantly differ from the standard ones observed in direct single ionization of atoms and molecules. We also show that, by using such a pump–probe scheme, one can suppress autoionization from doubly excited states for time delays between 0 and 4 fs.

Título del postprint: Attosecond VUV Coherent Control of Molecular Dynamics, 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, Work partially supported by the Advanced Grant of the European Research Council XCHEM 290853, the European grant MC-RG ATTOTREND, the European COST Actions CM0702 and CM1204, the European ITN CORINF, the MICINN project Nos. FIS201015127 and CSD 2007-00010 (Spain), and the ERA-Chemistry project PIM2010EEC-00751. XMT was supported by Grand-in-Aid for Scientific Researches (No. C24540421) from the Japan Society for the Promotion of Science and HA-PACS Project for advanced interdisciplinary computational sciences by exa-scale computing technology

Manipulation and control of molecular electron dynamics is currently in the spotlight for numerous multidisciplinary applications in physics, chemistry and biology. During the last decade, free electron lasers and sources based on high-order harmonic generation have been successfully developed to enable the generation of femtosecond and attosecond intense radiation pulses in the ultraviolet and soft X-ray regions. These tools have lead to an outbreak of pump-probe experiments suited to explore structural dynamics in atoms and molecules with spatial and temporal resolutions on the atomic length and intrinsic electronic time scales, respectively. Such experiments, using hydrogen molecules (H2, D2) as prototypical examples, have been performed to study molecular dissociative single and multi-photon ionization, photon-induced symmetry breaking in molecular dissociation, and time-resolved imaging of coherent nuclear wave-packets. The counterpart state-of-the-art time-dependent theoretical methods, able to provide a solid groundwork for describing and interpreting the underlying ultrafast physical molecular dynamics in such experiments, are still scarce. The difficulty is to achieve an accurate description accounting for the full dimensionality of the problem. A proper treatment of nuclear degrees of freedom is already indispensable to study multiphoton single ionization of diatomic molecules. This is discussed in the present manuscript in different applications. We first examine the role of the coupled electronic and nuclear motions in problems that probe coherent nuclear wave-packets using intense UV pulses and in resonance-enhanced multiphoton single ionization (REMPI) processes, whose rates are underestimated when using approaches within the fixed nuclei approximation. Later, we show that for highly intense fields the presence of vibrational structure leads to step-ladder Rabi oscillations that are probed in the REMPI probabilities. Finally, we demonstrate the suitability of these time-dependent full-dimensional treatments to provide time-resolved images of autoionization of H2, following the time evolution of both electron and proton distributions after the interaction with ultrashort pulses.

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.

Dissociative single ionization of H2 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+ ion. The energy-dependent asymmetry pattern is explained by a semiclassical model and further validated by fully quantum mechanical calculations, both in very good agreement with the experiment, A. G.-C., A. P., and F. M. acknowledge allocation of computer time by CCC-UAM and BSC Mare Nostrum and financial support from the Advanced Grant of the European Research Council XCHEM 290853, the European grants MC-RG ATTOTREND and ITN CORINF, the European COST Actions CM0702 (CUSPFEL) and CM1204 (XLIC), the MICINN projects No. FIS2010-15127 and No. CSD 2007-00010, and the ERA-Chemistry project No. PIM2010EEC-00751

Vibrational wave packets in the excited electronic states of H2 created by a single UV pulse are probed by a time-delayed second pulse of the same characteristics. The motion of the vibrational wave packet is traced by analyzing the signature of the 2pσu dissociative ionization channel in the proton kinetic energy distributions at different time delays between the pulses. Moreover, since the pump and probe pulses have the same characteristics, replicas of the vibrational wave packet are created at different times leading to interference patterns in the vibrational distributions of the excited states that are probed in the ionization channels.

In contrast with atomic photoionization or molecular direct photoionization, in the autoionization region, electron and proton kinetic-energy differential probabilities resulting from a short pulse cannot be reconstructed by the incoherent superposition of those resulting from long pulses.

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 H2 molecule, This work was accomplished with an allocation of computer time from Mare Nostrum BSC and CCC-UAM, and was partially supported by the MICINN Projects No. FIS2010-15127, No. ACI2008-0777 and No. CSD 2007-00010, the ERA-Chemistry Project No. PIM2010EEC-00751, the European grants MC-ITN CORINF and MC-RG ATTOTREND, the European COST Action CM0702, and the Advanced Grant of the European Research Council XCHEM 290853. A. P. acknowledges a Juan de la Cierva contract grant from MICINN

One-photon single ionization of molecules has been at the focus of several discussions concerning the reconstruction of observables obtained with ultrashort pulses from those obtained from continuous wave radiation (and vice versa). A related controversy on the conditions and observables that allow for coherent control in one-photon processes has been recently revisited (Science 2006, 313, 1257; J. Chem. Phys. 2010, 133, 151101). Our benchmark to investigate these issues is photoionization of the hydrogen molecule, where the autoionization events are the time-dependent processes in field-free evolution that could serve as a target for coherent control. We show that the variation of one-photon ionization probabilities with pulse duration are solely due to spectral effects and thus cannot be coherently controlled. We then discuss for which observables and under which conditions phase control of autoionization dynamics is possible.

A time-dependent theoretical method is used to describe a UV pump–UV probe strategy to trace, at a femtosecond time scale, the motion of vibrational wave packets created in excited states of the hydrogen molecule by measuring single ionization probabilities. We use a spectral method to solve the time-dependent Schrodinger equation in full dimensionality, including correlation and all electronic and vibrational degrees of freedom. A pump pulse initially creates a vibrational wave packet in the intermediate electronic excited states of $$\hbox{H}_2$$ . The frequency of the probe is chosen to ionize the target leaving the ion in a bound vibrational state. By varying the time delay between pulses, non-dissociative single ionization is enhanced or suppressed. Energy differential ionization probabilities are reported and compared with a model based on the Franck–Condon approximation.

Two identical XUV attosecond pulses interact with the hydrogen molecule creating an interferometer resulting from the direct and sequential two-photon absorption paths reaching the same final ionized states. The dependence of the ionization yields with the time delay between the pulses allows to reconstruct the pumped vibronic (electronic and vibrational) wave packet created in the singly excited states of the molecule. The use of XUV pulses avoids a laser-induced distortion of the molecular potential, ensuring the characterization of the intrinsic behaviour of the system.

We use a UV-pump/IR-probe scheme, combining a single attosecond UV pulse and a 750 nm IR pulse, to explore laser-assisted photoionization of the hydrogen molecule in the autoionization region. The electron energy distributions exhibit unusual streaking patterns that are explored for different angles of the electron ejection with respect to the polarization vector and the molecular axis. Moreover, by controlling the time delay between the pulses, we observe that one can suppress the autoionization channel.

We present an analytical model based on the time-dependent WKB approximation to reproduce the photoionization spectra of an H2 molecule in the autoionization region. We explore the nondissociative channel, which is the major contribution after one-photon absorption, and we focus on the features arising in the energy differential spectra due to the interference between the direct and the autoionization pathways. These features depend on both the timescale of the electronic decay of the autoionizing state and the time evolution of the vibrational wavepacket created in this state. With full ab initio calculations and with a one-dimensional approach that only takes into account the nuclear wavepacket associated to the few relevant electronic states we compare the ground state, the autoionizing state, and the background continuum electronic states. Finally, we illustrate how these features transform from molecular-like to atomic-like by increasing the mass of the system, thus making the electronic decay time shorter than the nuclear wavepacket motion associated with the resonant state. In other words, autoionization then occurs faster than the molecular dissociation into neutrals, We acknowledge computer time from the CCC-UAM and Mare Nostrum supercomputer centers and financial support by the European Research Council under the ERC Advanced Grant no. 290853 XCHEM, FP7 Marie Curie ITN CORINF, FP7 Marie Curie IRG ATTOTREND, the Ministerio de Economía y Competitividad projects FIS2010-15127, FIS2013-42002 R and ERA-Chemistry PIM2010EEC-00751, the European COST Action CM1204 XLIC, and EPSRC programme no. EP/I032517/1

We theoretically explore the use of UV pump – UV probe schemes to resolve in time the dynamics of nuclear wave packets in excited electronic states of the hydrogen molecule. The pump pulse ignites the dynamics in singly excited states, that will be probed after a given time delay by a second identical pulse that will ionize the molecule. The field-free molecular dynamics is first explored by analyizing the autocorrelation function for the pumped wave packet and the excitation probabilities. We investigate both energy and angle differential ionization probabilities and demonstrate that the asymmetry induced in the electron angular distributions gives a direct map of the time evolution of the pumped wave packet.

We theoretically investigate the use of UV pump - UV probe schemes with two identical pulses to excite and ionize the hydrogen molecule. The nuclear wave packet (NWP) generated in excited states of neutral molecules is traced by analyzing the asymmetry of the electron angular distributions resulting from dissociative ionization. The mapped NWP can be later used to selectively launch the ionization process into different channels.