Electronic movement flashing into view Numerous chemical processes begin with ionization: the ejection of an electron from a molecule. What happens in the immediate aftermath of that event? Kraus et al. explored this question in iodoacetylene by detecting and analyzing the spectrum of emitted high harmonics (see the Perspective by Ueda). They traced the migration of the residual positively charged hole along the molecular axis on a time scale faster than a quadrillionth of a second. They thereby characterized the capacity of a laser field to steer the hole's motion in appropriately oriented molecules. Science , this issue p. 790 ; see also p. 740

The last two decades have seen rapid developments in short-pulse x-ray sources, which have enabled the study of chemical dynamics by x-ray spectroscopies with unprecedented sensitivity to nuclear and electronic degrees of freedom on all relevant time scales. In this perspective, some of the major achievements in the study of chemical dynamics with x-ray pulses produced by high-harmonic, free-electron-laser and synchrotron sources on time scales from attoseconds to nanoseconds are reviewed. Major advantages of x-ray spectral probing of chemical dynamics are unprecedented time resolution, element and oxidation state specificity and-depending on the type of x-ray spectroscopy-sensitivity to both the electronic and nuclear structure of the investigated chemical system. Particular dynamic processes probed by x-ray radiation, which are highlighted in this perspective, are the measurement of electronic coherences on attosecond to femtosecond time scales, time-resolved spectroscopy of chemical reactions such as dissociations and pericyclic ring-openings, spin-crossover dynamics, ligand-exchange dynamics, and structural deformations in excited states. X-ray spectroscopic probing of chemical dynamics holds great promise for the future due to the ongoing developments of new types of x-ray spectroscopies such as four-wave mixing and the continuous improvements of the emerging laboratory-based high-harmonic sources, and large-scale facility-based free-electron lasers.

Understanding excited carrier dynamics in semiconductors is crucial for the development of photovoltaics and efficient photonic devices. However, overlapping spectral features in optical pump-probe spectroscopy often render assignments of separate electron and hole carrier dynamics ambiguous. Here, ultrafast electron and hole dynamics in germanium nanocrystalline thin films are directly and simultaneously observed by ultrafast transient absorption spectroscopy in the extreme ultraviolet at the germanium M4,5 edge. We decompose the spectra into contributions of electronic state blocking and photo-induced band shifts at a carrier density of 8 × 1020 cm−3. Separate electron and hole relaxation times are observed as a function of hot carrier energies. A first-order electron and hole decay of ∼1 ps suggests a Shockley–Read–Hall recombination mechanism. The simultaneous observation of electrons and holes with extreme ultraviolet transient absorption spectroscopy paves the way for investigating few- to sub-femtosecond dynamics of both holes and electrons in complex semiconductor materials and across junctions.

The transfer of charge at the molecular level plays a fundamental role in many areas of chemistry, physics, biology and materials science. Today, more than 60 years after the seminal work of R. A. Marcus, charge transfer is still a very active field of research. An important recent impetus comes from the ability to resolve ever faster temporal events, down to the attosecond time scale. Such a high temporal resolution now offers the possibility to unravel the most elementary quantum dynamics of both electrons and nuclei that participate in the complex process of charge transfer. This review covers recent research that addresses the following questions. Can we reconstruct the migration of charge across a molecule on the atomic length and electronic time scales? Can we use strong laser fields to control charge migration? Can we temporally resolve and understand intramolecular charge transfer in dissociative ionization of small molecules, in transition-metal complexes and in conjugated polymers? Can we tailor molecular systems towards specific charge-transfer processes? What are the time scales of the elementary steps of charge transfer in liquids and nanoparticles? Important new insights into each of these topics, obtained from state-of-the-art ultrafast spectroscopy and/or theoretical methods, are summarized in this review.

The thermalization of hot carriers and phonons gives direct insight into the scattering processes that mediate electrical and thermal transport. Obtaining the scattering rates for both hot carriers and phonons currently requires multiple measurements with incommensurate timescales. Here, transient extreme-ultraviolet (XUV) spectroscopy on the silicon 2p core level at 100 eV is used to measure hot carrier and phonon thermalization in Si(100) from tens of femtoseconds to 200 ps, following photoexcitation of the indirect transition to the Δ valley at 800 nm. The ground state XUV spectrum is first theoretically predicted using a combination of a single plasmon pole model and the Bethe-Salpeter equation with density functional theory. The excited state spectrum is predicted by incorporating the electronic effects of photo-induced state-filling, broadening, and band-gap renormalization into the ground state XUV spectrum. A time-dependent lattice deformation and expansion is also required to describe the excited state spectrum. The kinetics of these structural components match the kinetics of phonons excited from the electron-phonon and phonon-phonon scattering processes following photoexcitation. Separating the contributions of electronic and structural effects on the transient XUV spectra allows the carrier population, the population of phonons involved in inter- and intra-valley electron-phonon scattering, and the population of phonons involved in phonon-phonon scattering to be quantified as a function of delay time.

We study the emission of even and odd high-harmonic orders from oriented OCS molecules. We use an intense, nonresonant femtosecond laser pulse superimposed with its phase-controlled second harmonic field to impulsively align and orient a dense sample of molecules from which we subsequently generate high-order harmonics. The even harmonics appear around the full revivals of the rotational dynamics. We demonstrate perfect coherent control over their intensity through the subcycle delay of the two-color fields. The odd harmonics are insensitive to the degree of orientation, but modulate with the degree of axis alignment, in agreement with calculated photorecombination dipole moments. We further compare the shape of the even and odd harmonic spectra with our calculations and determine the degree of orientation.

We introduce and demonstrate a new approach to measuring coherent electron wave packets using high-harmonic spectroscopy. By preparing a molecule in a coherent superposition of electronic states, we show that electronic coherence opens previously unobserved high-harmonic-generation channels that connect distinct but coherently related electronic states. Performing the measurements in dynamically aligned nitric oxide (NO) molecules we observe the complex temporal evolution of the electronic coherence under coupling to nuclear motion. Choosing a weakly allowed transition to prepare the wave packet, we demonstrate an unprecedented sensitivity that arises from optical interference between coherent and incoherent pathways. This mechanism converts a 0.1 % excitation fraction into a ∼20 % signal modulation.Measuring the motion of valence-shell electrons in molecules is one of the central goals of modern ultrafast science. The last decade has witnessed very fundamental progress in this area with the development of attosecond streaking [1,2] and interferometric techniques [3] to time resolve electronic dynamics as well as transient absorption [4,5] and strong-field ionization [6-9] to probe electronic wave packets in atomic ions. All of these experiments have been performed on highly-excited states, in the continuum or in ionic species. Electronic dynamics involving the ground state and a low-lying electronically excited state of a neutral molecule have not been observed to date.Here, we introduce a new all-optical technique that allows the measurement of an electronic wave packet in the valence shell of a neutral molecule for the first time. In our pump-probe experiment, an electronic wave packet is created through stimulated Raman scattering and probed by the generation of high-order harmonics (orders 9 to 23) of an infrared laser pulse. Figure 1A illustrates the concept of our measurement. High-harmonic emission from a coherent superposition of two electronic states can be described as a superposition of radiation produced in four channels illustrated by arrows in Fig. 1A. Ionization from and recombination to the same electronic state gives rise to two channels (blue arrows) that are independent of the electronic coherence. Ionization from one state and recombination to the other state gives rise to two additional channels (red arrows) that only contribute to an observable high-harmonic signal if the two states are coherently related. These channels are the key to probing electronic coherence and have not been observed previously.The two channels connecting the same initial and final states (blue arrows) emit radiation that is insensitive to the quantum phase of the initial state. The amplitude of the radiation generated in each of these channels is proportional to the population in each state. These pathways have been exploited to time resolve photochemical dynamics [10,11]. In contrast, the two cross-channels (red arrows) read out the relative quantum phases and encode their difference in the phase of the emitted ra...

We report the observation of macroscopic field-free orientation, i.e. more than 73 % of CO molecules pointing in the same direction. This is achieved through an all-optical scheme operating at high particle densities (> 10 17 cm −3 ) that combines a one-color (ω) and a two-color (ω + 2ω) non-resonant femtosecond laser pulses. We show that the achieved orientation solely relies on the hyperpolarizability interaction as opposed to an ionization-depletion mechanism thus opening a wide range of applications. The achieved strong orientation enables us to reveal the molecularframe anisotropies of the photorecombination amplitudes and phases caused by a shape resonance. The resonance appears as a local maximum in the even-harmonic emission around 28 eV. In contrast, the odd-harmonic emission is suppressed in this spectral region through the combined effects of an asymmetric photorecombination phase and a sub-cycle Stark effect, generic for polar molecules, that we experimentally identify.Techniques for fixing molecules in space are invaluable tools for a broad range of experiments in ultrafast science [1,2]. The availability of transiently aligned molecular samples has particularly advanced strong-field and attosecond spectroscopies, providing new insights into the electronic structure of molecules and its temporal evolution [3][4][5][6][7]. High-harmonic spectroscopy (HHS) provides a new access to the rich structures of photoionization continua, such as Cooper minima [8][9][10][11] and shape resonances [11][12][13]. The investigation of the inherent structural and dynamical anisotropies of polar molecules has however been prevented by the difficulty of orienting molecules. Interesting phenomena tied to polar molecules include the predicted recombination-site dependence of structural minima [14,15] and attosecond charge migration [16][17][18] triggered by strong-field ionization. Here, we demonstrate a protocol for molecular orientation which achieves macroscopic field-free orientation and exploit this progress to probe the anisotropy of photorecombination dipole moments at a molecular shape resonance.Successful approaches to laser-induced molecular orientation include the combination of an electrostatic field with a rapidly turned-off laser field [19], alignment in combination with quantum-state selection and a weak dc-field [20][21][22][23], adiabatic [24] and impulsive two-color orientation [25][26][27]. All of these techniques are subject to substantial limitations: The presence of electric fields may alter the electronic structure of the molecule, its photo-induced dynamics or the subsequent probing process. The low particle densities available after quantumstate selection make the application of such techniques to high-harmonic and attosecond spectroscopies challenging to impossible. The two-color scheme [22,25], recently applied to HHS [26,27], relies on an ionization-depletion mechanism [28] and thus ties the achievable degree of orientation to the ionization fraction of the sample. This fact does not only limi...