Paula Rivière

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.

Strong field dynamics with ultrashort electron wave packet replicas

We investigate theoretically electron dynamics under a vacuum ultraviolet (VUV) attosecond pulse train, which has a controlled phase delay with respect to an additional strong infrared laser field. Using the strong field approximation and the fact that the attosecond pulse is short compared to the excited electron dynamics, we arrive at a minimal analytical model for the kinetic energy distribution of the electron as well as the photon absorption probability as a function of the phase delay between the fields. We analyze the dynamics in terms of electron wave packet replicas created by the attosecond pulses. The absorption probability shows strong modulations as a function of the phase delay for VUV photons of energy comparable to the binding energy of the electron, while for higher photon energies the absorption probability does not depend on the delay, in line with the experimental observations for helium and argon, respectively.

Even harmonic generation in isotropic media of dissociating homonuclear molecules.

Isotropic gases irradiated by long pulses of intense IR light can generate very high harmonics of the incident field. It is generally accepted that, due to the symmetry of the generating medium, be it an atomic or an isotropic molecular gas, only odd harmonics of the driving field can be produced. Here we show how the interplay of electronic and nuclear dynamics can lead to a marked breakdown of this standard picture: a substantial part of the harmonic spectrum can consist of even rather than odd harmonics. We demonstrate the effect using ab-initio solutions of the time-dependent Schrödinger equation for and its isotopes in full dimensionality. By means of a simple analytical model, we identify its physical origin, which is the appearance of a permanent dipole moment in dissociating homonuclear molecules, caused by light-induced localization of the electric charge during dissociation. The effect arises for sufficiently long laser pulses and the region of the spectrum where even harmonics are produced is controlled by pulse duration. Our results (i) show how the interplay of femtosecond nuclear and attosecond electronic dynamics, which affects the charge flow inside the dissociating molecule, is reflected in the nonlinear response, and (ii) force one to augment standard selection rules found in nonlinear optics textbooks by considering light-induced modifications of the medium during the generation process.

Enhancing high-order harmonic generation in light molecules by using chirped pulses

One of the current challenges in high-harmonic generation is to extend the harmonic cutoff to increasingly high energies while maintaining or even increasing the efficiency of the high-harmonic emission. Here we show that the combined effect of down-chirped pulses and nuclear dynamics in light molecules allows one to achieve this goal, provided that long enough IR pulses are used to allow the nuclei to move well outside the Franck-Condon region. We also show that, by varying the duration of the chirped pulse or by performing isotopic substitution while keeping the pulse duration constant, one can control the extension of the harmonic plateau.

High harmonic spectroscopy of electron localization in the hydrogen molecular ion

Interaction of a laser pulse with a centrally symmetric medium, such as an isotropic gas of atoms, leads to the generation of harmonic emission which contains exclusively odd harmonics of the incident field. This result is the consequence of both the central symmetry of the medium and the temporal symmetry of the oscillating electric field, , where ωl is the laser frequency. In the case of oriented heteronuclear molecules, the spatial symmetry no longer holds and both odd and even harmonics become allowed. Here we show, by solving the time-dependent Schrodinger equation for H, D, and T, that even-order harmonic generation is also possible for sufficiently long infrared (IR) laser pulses in homonuclear molecules. The appearance of even harmonics is a signature of the coupled electron-nuclear dynamics and reflects field-induced electron localization initiated by the strong laser field, which breaks the spatial symmetry in the system. The analysis of even harmonics generated by pulses of different durations might therefore provide information on correlated electron-nuclear dynamics and charge migration in more complex un-oriented molecular ensembles.

Experimental and theoretical study of rotationally inelastic diffraction of D2 from NiAl(110)

We present a detailed experimental and theoretical study of elastic and rotationally inelastic diffraction of D(2) from NiAl(110) in the energy range 85-150 meV. The experiments were performed using a high-resolution, fixed angle geometry apparatus. Quantum and classical dynamical calculations were performed by using a six-dimensional potential energy surface constructed upon interpolation of a set of DFT (density functional theory) data. We show that, although elastic diffraction peak intensities are accurately described by theory in the whole range of incidence energies and angles explored, significant discrepancies are obtained for RID peaks, especially for those involving rotational initial states with j(i) > 0. Possible reasons for this discrepancy are discussed.

Run-Time Reconfiguration to Check Temperature in Custom Computers: An Application of JBits Technology

This paper is a progress report of a novel application of JBits technology. The capability of handling run-time reconfiguration is utilized to add an array of temperature sensors into a working circuit. The thermal map thus obtained provides information not only of the overall chip temperature, but also about the power consumption of each block of the design. The sensors utilized need no external components, and, by means of the partial reprogrammability of the Virtex FPGA family, no permanent occupation of any resources is made. Several experiments are summarized to illustrate the feasibility of the technique.

Electronic excitation by short x-ray pulses: from quantum beats to wave packet revivals

We propose a simple way to determine the periodicities of wave packets (WPs) in quantum systems directly from the energy differences of the states involved. The resulting classical periods and revival times are more accurate than those obtained with the traditional expansion of the energies about the central quantum number , especially when is low. The latter type of WP motion occurs upon excitation of highly charged ions with short XUV or x-ray pulses. Moreover, we formulate the WP dynamics in such a form that it directly reveals the origin of phase shifts in the maxima of the autocorrelation function, a phenomenon most prominent in the low WP dynamics.

Virtual single-photon transition interrupted : Time-gated optical gain and loss

This work was supported by the National Center of Competence in Research Molecular Ultrafast Science and Technology (NCCR MUST), research instrument of the Swiss National Science Foundation. P.R. acknowledges a Juan de la Cierva Contract Grant from MICINN, and the COST Action CM0702. We thank H. R. Reiss and M. Lucchini for fruitful discussions

Time reconstruction of harmonic emission in molecules near the ionization threshold

Using the example of the molecular ion and its isotopes, we show how the Coulomb explosion triggered by ionization of the molecule can be used to reconstruct the emission times and identify the physical mechanism contributing to the emission of near- and below-threshold harmonics (BTH). The key idea is that recombination responsible for the harmonic emission halts Coulomb explosion at different times, depending on the final vibrational state of the molecule. As a consequence, interferences between different vibrational channels show up, enabling enhanced temporal resolution in the harmonic spectrum. The reconstruction of the emission times for BTH shows that they are associated with the so-called short trajectories and that the time–energy mapping for such harmonics is possible, similar to the well-known mapping for the plateau harmonics.