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High harmonic emission occurs when an electron, liberated from a molecule by an incident intense laser field, gains energy from the field and recombines with the parent molecular ion. The emission provides a snapshot of the structure and dynamics of the recombining system, encoded in the amplitudes, phases and polarization of the harmonic light. Here we show with CO 2 molecules that high harmonic interferometry can retrieve this structural and dynamic information: by measuring the phases and amplitudes of the harmonic emission, we reveal ‘fingerprints’ of multiple molecular orbitals participating in the process and decode the underlying attosecond multi-electron dynamics, including the dynamics of electron rearrangement upon ionization. These findings establish high harmonic interferometry as an effective approach to resolving multi-electron dynamics with sub-Ångström spatial resolution arising from the de Broglie wavelength of the recombining electron, and attosecond temporal resolution arising from the timescale of the recombination event. Electronic movies: attosecond sight The high harmonic emission that accompanies the recombination of an electron with its parent molecular ion in an intense laser field provides a snapshot of the structure and dynamics of the recombining system. Experiments with CO 2 molecules now show that high harmonic interferometry can retrieve this structural and dynamic information by measuring the phases and amplitudes of the harmonic emission. The resulting 'fingerprints' of the multiple molecular orbitals participating in the process can be used to decode the underlying attosecond multi-electron dynamics, including the dynamics of electron rearrangement upon ionization. The light emitted from the system contains images of moving electrons that can be processed into a movie. These findings establish high harmonic interferometry as an effective approach to resolving multi-electron dynamics with sub-Ångström spatial resolution arising from the de-Broglie wavelength of the recombining electron, and attosecond temporal resolution arising from the timescale of the recombination event. The high harmonic emission that accompanies the recombination of an electron with its parent molecular ion in an intense laser field provides a snapshot of the structure and dynamics of the recombining system. Experiments on CO 2 molecules now show how to extract information from the properties of the emitted light about the underlying multi-electron dynamics with sub-Ångström spatial resolution and attosecond temporal resolution

Today laser pulses with electric fields comparable to or higher than the electrostatic forces binding valence electrons in atoms and molecules have become a routine tool with applications in laser acceleration of electrons and ions, generation of short wavelength emission from plasmas and clusters, laser fusion, etc. Intense fields are also naturally created during laser filamentation in the air or due to local field enhancements in the vicinity of metal nanoparticles. One would expect that very intense fields would always lead to fast ionization of atoms or molecules. However, recently observed acceleration of neutral atoms [Eichmann et al. (2009) Nature 461: 1261-1264] at the rate of 10¹⁵ m/s² when exposed to very intense IR laser pulses demonstrated that substantial fraction of atoms remained stable during the pulse. Here we show that the electronic structure of these stable \"laser-dressed\" atoms can be directly imaged by photoelectron spectroscopy. Our findings open the way to visualizing and controlling bound electron dynamics in strong laser fields and reexamining its role in various strong-field processes, including microscopic description of high order Kerr nonlinearities and their role in laser filamentation [Béjot et al. (2010) Phys Rev Lett 104:103903].

Interferometry has been at the heart of wave optics since its early stages, resolving the coherence of the light field and enabling the complete reconstruction of the optical information it encodes. Transferring this concept to the attosecond time domain shed new light on fundamental ultrafast electron phenomena. Here we introduce attosecond-gated interferometry and probe one of the most fundamental quantum mechanical phenomena, field-induced tunnelling. Our experiment probes the evolution of an electronic wavefunction under the tunnelling barrier and records the phase acquired by an electron as it propagates in a classically forbidden region. We identify the quantum nature of the electronic wavepacket and capture its evolution within the optical cycle. Attosecond-gated interferometry has the potential to reveal the underlying quantum dynamics of strong-field-driven atomic, molecular and solid-state systems.Attosecond-gated interferometry is developed by combining sub-cycle temporal gating and extreme-ultraviolet interferometry. By measuring the electron’s relative phase and amplitude under a tunnelling barrier, the quantum nature of the electronic wavepacket is identified.

At the fundamental level, full description of light-matter interaction requires quantum treatment of both matter and light. However, for standard light sources generating intense laser pulses carrying quadrillions of photons in a coherent state, the classical description of light during intense laser-matter interaction has been expected to be adequate. Here, we show how nonlinear optical response of matter can be controlled to generate dramatic deviations from this standard picture, including generation of several squeezed and entangled harmonics of the incident laser light. In particular, such nontrivial quantum states of harmonics are generated as soon as one of the harmonics induces a transition between different laser-dressed states of the material system. Such transitions generate an entangled light-matter wave function, which can generate quantum states of harmonics even in the absence of a quantum driving field or material correlations. In turn, entanglement of the material system with a single harmonic generates and controls entanglement between different harmonics. Hence, nonlinear media that are near resonant with at least one of the harmonics appear to be quite attractive for controlled generation of massively entangled quantum states of light. Our analysis opens remarkable opportunities at the interface of attosecond physics and quantum optics, with implications for quantum information science.

High-order harmonic generation (HHG) is a powerful tool for probing electronic structure and ultrafast dynamics in matter. Traditionally studied in atomic and molecular gases, HHG has recently been extended to condensed matter, enabling all-optical investigations of electronic and crystal structures. Here, we experimentally demonstrate HHG in a new class of materials: thin organic molecular crystals with perfectly aligned molecules, using pentacene as a model system. Organic molecular crystals, characterized by weak intermolecular coupling, flat electronic bands, and large unit cells, differ fundamentally from conventional covalent or ionic crystals and have attracted significant interest as promising candidates for organic electronics. We show that pentacene crystals endure laser intensities sufficient for efficient HHG up to the 17th order. The harmonic yield as a function of laser polarization reveals a strong dependence on intermolecular interactions, with higher harmonic orders particularly sensitive to both nearest- and next-nearest-neighbor couplings. Model calculations indicate that weaker intermolecular interactions necessitate probing with higher harmonic orders to resolve the crystal structure. These findings suggest that HHG may serve as a powerful tool for probing the electronic structure of organic molecular crystals, enhancing all-optical techniques for studying electronic properties and ultrafast dynamics in complex organic materials. High-harmonic generation (HHG) in organic molecular crystals was previously demonstrated only in the non-perturbative regime. Here, the authors demonstrate HHG up to the 17th order in pentacene crystals, with a strong orientational dependence caused by the neighbouring molecules.

Attosecond photoelectron interferometry, based on the measurement of photoelectron spectra generated by a two-colour field, provides access to the photoionisation dynamics of quantum systems. In general, due to the entanglement between the wave function of the emitted photoelectron and that of the parent ion, the dynamics driven by the infra-red field in the photoion can affect the properties of the photoemitted electronic wave packet, when the measurement protocol corresponds to the projection of the total time-dependent wave function onto a specific final state of the bipartite system. This is particularly relevant for molecules, due to their rich internal electronic and vibrational energy structure. Here we show how the polarisation of the ion influences the photoionisation dynamics by introducing an additional time delay in the photoelectrons emitted from CO 2 molecules. The delay stems from the entanglement between the photoion and the photoelectron created in the photoionisation process. The authors report on an experimental study of attosecond time delays of photoelectrons emitted from CO 2 molecules. This clarifies the role of internal dynamics of ions and ionelectron entanglement on the photoionization dynamics.

Subcycle strong-field ionization (SFI) underlies many emerging spectroscopic probes of atomic or molecular attosecond electronic dynamics. Extending methods such as attosecond high harmonic generation spectroscopy to complex polyatomic molecules requires an understanding of multielectronic excitations, already hinted at by theoretical modeling of experiments on atoms, diatomics, and triatomics. Here, we present a direct method which, independent of theory, experimentally probes the participation of multiple electronic continua in the SFI dynamics of polyatomic molecules. We use saturated -butane) and unsaturated (1,3-butadiene) linear hydrocarbons to show how subcycle SFI of polyatomics can be directly resolved into its distinct electronic-continuum channels by above-threshold ionization photoelectron spectroscopy. Our approach makes use of photoelectron-photofragment coincidences, suiting broad classes of polyatomic molecules.

We present a new methodology for measuring few-femtosecond electronic and nuclear dynamics in both atoms and polyatomic molecules using multidimensional high harmonic generation (HHG) spectroscopy measurements, in which the spectra are recorded as a function of the laser intensity to form a two-dimensional data set. The method is applied to xenon atoms and to benzene molecules, the latter exhibiting significant fast nuclear dynamics following ionization. We uncover the signature of the sub-cycle evolution of the returning electron flux in strong-field ionized xenon atoms, implicit in the strong field approximation but not previously observed directly. We furthermore extract the nuclear autocorrelation function in strong field ionized benzene cations, which is determined to have a decay of τ 0 = 4 ± 1 fs, in good agreement with the τ 0 = 3.5 fs obtained from direct dynamics variational multi-configuration Gaussian calculations. Our method requires minimal assumptions about the system, and is applicable even to un-aligned polyatomic molecules.

Many methods have been proposed for efficient storage of molecular hydrogen for fuel cell applications. However, despite intense research efforts, the twin U.S. Department of Energy goals of 6.5% mass ratio and 62 kg/ m3volume density has not been achieved either experimentally or via theoretical simulations on reversible model systems. Carbon-based materials, such as carbon nanotubes, have always been regarded as the most attractive physisorption substrates for the storage of hydrogen. Theoretical studies on various model graphitic systems, however, failed to reach the elusive goal. Here, we show that insufficiently accurate carbon- H2interaction potentials, together with the neglect and incomplete treatment of the quantum effects in previous theoretical investigations, led to misleading conclusions for the absorption capacity. A proper account of the contribution of quantum effects to the free energy and the equilibrium constant for hydrogen adsorption suggest that the U.S. Department of Energy specification can be approached in a graphite-based physisorption system. The theoretical prediction can be realized by optimizing the structures of nano-graphite platelets (graphene), which are light-weight, cheap, chemically inert, and environmentally benign.

Using ab-initio simulations, we demonstrate amplification of extreme-ultraviolet (XUV) radiation during transient absorption in a high-harmonic generation type process using the example of the hydrogen atom. The strong IR driving field rapidly depletes the initial ground state while populating excited electronic states through frustrated tunnelling, thereby creating a population inversion. Concomitant XUV lasing is demonstrated by explicit inclusion of the XUV seed in our simulations, allowing a thorough analysis in terms of this transient absorption setup. Possibilities for increasing this gain, e.g. through preexcitation of excited states, change of the atomic gain medium or through multi-center effects in molecules, are demonstrated. Our findings should lead to a reinterpretation of recent experiments.