Alberto González-Castrillo

Attosecond vacuum UV coherent control of molecular dynamics

Significance We show that we can precisely control molecular dynamics on both nuclear (i.e., femtosecond) and electronic (i.e., attosecond) timescales. By using attosecond vacuum UV light pulse trains that are tunable in the frequency domain, we show that it is possible to switch population between electronically excited states of a neutral molecule on attosecond time scales, and use this ability to coherently control excitation and ionization through specific pathways. This paper represents a milestone advance because almost two decades after attosecond physics was demonstrated, attosecond chemistry has not yet been fully established because the wavelength and bandwidth of attosecond pulses did not well match molecular quantum states. The richness and complexity of the dynamics, even in a simple molecule, is remarkable and daunting. 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.

Molecular interferometer to decode attosecond electron–nuclear dynamics

Significance Current attosecond technologies designed to study fast electron and nuclear motion in molecules make use, at some stage, of infrared laser pulses that strongly perturb the molecular potential, thus modifying the dynamics inherent to the system. To access the actual dynamics of the molecule, gentler laser sources must be used. Very recently, extreme UV pulses have been successfully combined to provide a picture of the intrinsic electron dynamics in atoms with subfemtosecond time resolution. Here we show that by using two identical extreme UV pulses, one can also obtain a complete description of the coupled electronic and nuclear dynamics in molecules. Visualization of such dynamics is possible by varying the wavelength and/or the time delay between the two 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.

Signatures of attosecond electronic?nuclear dynamics in the one-photon ionization of molecular hydrogen: analytical model versus ab initio calculations

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.

Wave packet dynamics in molecular excited electronic states

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.

Electron streaking and dissociation in laser-assisted photoionization of molecular hydrogen

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.

The role of hyperconjugative π-aromaticity in the enhanced acidity of methyl-, silyl and germylcyclopentadienes

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 and Ge containing compounds this behaviour differs from that observed for saturated and α,β-unsaturated compounds, which behave as Si or Ge acids in the gas phase. CpXH3 (X = C, Si, Ge) compounds are predicted to be stronger acids in the gas phase than the unsubstituted parent compound, due to a significant anionic hyperconjugation effect which reinforces the C–X bond upon deprotonation and favours the conjugation of the C–X π-bond with the π-system associated to the five membered ring.

Attosecond transient absorption spectroscopy of molecular hydrogen

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.

Electron streaking in the autoionization region of H2

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.

Molecular interferometer using XUV attosecond pulses to unravel electron and nuclear dynamics

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.

XUV Lasers for Ultrafast Electronic Control in H2

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.