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Recent developments in attosecond technology led to table-top x-ray spectroscopy in the soft x-ray range, thus uniting the element- and state-specificity of core-level x-ray absorption spectroscopy with the time resolution to follow electronic dynamics in real-time. We describe recent work in attosecond technology and investigations into materials such as Si, SiO2, GaN, Al2O3, Ti, and TiO2, enabled by the convergence of these two capabilities. We showcase the state-of-the-art on isolated attosecond soft x-ray pulses for x-ray absorption near-edge spectroscopy to observe the 3d-state dynamics of the semi-metal TiS2 with attosecond resolution at the Ti L-edge (460 eV). We describe how the element- and state-specificity at the transition metal L-edge of the quantum material allows us to unambiguously identify how and where the optical field influences charge carriers. This precision elucidates that the Ti:3d conduction band states are efficiently photo-doped to a density of 1.9 × 1021 cm−3. The light-field induces coherent motion of intra-band carriers across 38% of the first Brillouin zone. Lastly, we describe the prospects with such unambiguous real-time observation of carrier dynamics in specific bonding or anti-bonding states and speculate that such capability will bring unprecedented opportunities toward an engineered approach for designer materials with pre-defined properties and efficiency. Examples are composites of semiconductors and insulators like Si, Ge, SiO2, GaN, BN, and quantum materials like graphene, transition metal dichalcogens, or high-Tc superconductors like NbN or LaBaCuO. Exiting are prospects to scrutinize canonical questions in multi-body physics, such as whether the electrons or lattice trigger phase transitions. [ABSTRACT FROM AUTHOR]

We report the use of time-resolved X-ray absorption spectroscopy in the ns–μs time scale to track the light induced two electron transfer processes in a multi-component photocatalytic system, consisting of [Ru(bpy)3]2+/ a diiron(iii,iii) model/triethylamine. EXAFS analysis with DFT calculations confirms the structural configurations of the diiron(iii,iii) and reduced diiron(ii,ii) states. [ABSTRACT FROM AUTHOR]

We present a theoretical study of high-order harmonic generation (HHG) and propagation driven by an infrared field carrying orbital angular momentum (OAM). Our calculations unveil the following relevant phenomena: extreme-ultraviolet harmonic vortices are generated and survive to the propagation effects, vortices transport high-OAM multiples of the corresponding OAM of the driving field and, finally, the different harmonic vortices are emitted with similar divergence. We also show the possibility of combining OAM and HHG phase locking to produce attosecond pulses with helical pulse structure. [ABSTRACT FROM AUTHOR]

We have unveiled coherent multiphoton interferences originating from different quantum paths taken by the Auger electron induced by a high-intensity x-ray/extreme ultraviolet pulse under the presence of a strong optical field. These interferences give rise to a clear signature in the angle-resolved Auger electron spectrum: an asymmetry with respect to the energy of the Auger decay channel. In order to illustrate this effect we have considered the resonant Auger decay of the transition 2p5 ↔1s-12p6 in Ne+. The simulations show that these interferences are very sensitive to the parameters of the x-ray and optical fields. [ABSTRACT FROM AUTHOR]

(c) XANES measures unoccupied valence states (VS) using a core-electron excited by a high energy SXR photon and is, therefore, energetically relevant around absorption edges. EXAFS spectroscopy instead probes a larger energy window above the edge where modulations in the absorption spectrum stem from photoelectrons with enough excess energy to scatter multiple times and auto-interfere with itself. [Extracted from the article]

Excitons play a key role in the linear optical response of two-dimensional (2D) materials. However, their role in the nonlinear response to intense, nonresonant, low-frequency light is often overlooked as strong fields are expected to tear the electron-hole pair apart. Using high-harmonic generation as a spectroscopic tool, we theoretically study their formation and role in the nonlinear optical response. We show that the excitonic contribution is prominent and that excitons remain stable even when the driving laser field surpasses the strength of the Coulomb field binding the electron-hole pair. We demonstrate a parallel between the behavior of strongly laser-driven excitons in 2D solids and strongly driven Rydberg states in atoms, including the mechanisms of their formation and stability. Last, we show how the excitonic contribution can be singled out by encapsulating the 2D material in a dielectric, tuning the excitonic energy and its contribution to the high-harmonic spectrum. [ABSTRACT FROM AUTHOR]

Excitons play an essential role in the optical response of two-dimensional materials. These are bound states showing up in the band gaps of many-body systems and are conceived as quasiparticles formed by an electron and a hole. By performing real-time simulations in hBN, we show that an ultrashort (few-fs) UV pulse can produce a coherent superposition of excitonic states that induces an oscillatory motion of electrons and holes between different valleys in reciprocal space, leading to a sizeable exciton migration in real space. We also show that an ultrafast spectroscopy scheme based on the absorption of an attosecond pulse in combination with the UV pulse can be used to read out the laser-induced coherences, hence to extract the characteristic time for exciton migration. This work opens the door towards ultrafast electronics and valleytronics adding time as a control knob and exploiting electron coherence at the early times of excitation. Charge migration is one of the key applications of attosecond science mainly studied in molecular systems. Here the authors propose the extension of this process to two-dimensional materials by exploiting the excitonic interactions typical on these systems, performing numerical simulations to demonstrate the possibility to produce and read exciton migration in monolayer hBN. [ABSTRACT FROM AUTHOR]

Photoionization cross sections and partial ion yields of Xe and XeF2 from Xe 3d5/2, Xe 3d3/2, and F 1s subshells in the 660-740 eV range are compared to explore effects of the F ligands. The Xe 3d-f continuum shape resonances dominate the photoionization cross sections of both the atom and molecule, but prominent resonances appear in the XeF2 cross section due to nominal excitation of Xe 3d and F 1s electrons to the lowest unoccupied molecular orbital (LUMO), a delocalized anti-bonding MO. Comparisons of the ion products from the atom and molecule following Xe 3d photoionization show that the charge-state distribution of Xe ions is shifted to lower charge states in the molecule along with production of energetic F+ and F2+ ions. This suggests that, in decay of a Xe 3d core hole, charge is redistributed to the F ligands and the system dissociates due to Coulomb repulsion. The ion products from excitation of the F 1s-LUMO resonance are different and show strong increases in the yields of Xe+ and F+ ions. The subshell ionization thresholds, the LUMO resonance energies, and their oscillator strengths are calculated by relativistic coupled-cluster methods and agree well with measurements. [ABSTRACT FROM AUTHOR]

We demonstrate that the standard picture of strong-field tunnel-ionization from molecules should be reformulated. The extended nature of the molecular potential implies the separation of some of the molecular sites from the edge of the ionization barrier. We show that the dependence of the tunnel probability with the distance to the barrier is translated into the ionized wavepacket, modifying substantially the high-order harmonic emission. The introduction of the dependence of tunnel ionization with the molecular site significantly improves the theoretical description of high-order harmonic generation in molecules, which is used as a cornerstone in high-harmonic spectroscopy and attosecond imaging. [ABSTRACT FROM AUTHOR]

[Display omitted] • 2,6-diethylphenol measured in a jet supersonic expansion using rotational spectroscopy. • The rotational constants for two conformers and 13C isotopologues have been obtained. • The structure of 2,6-diethylphenol is determined by DFT calculations and validated by Kraitchman's equation. • The large amplitude motion of hydroxyl group has been determined around 100 MHz. Phenol and some of its derivatives exhibit interesting tunneling motions consisting of two groups of transitions separated by a few hundred MHz. Recently, one of its derivatives, 2,6-di- tert -butylphenol, has shown additional hyperfine tunneling components, the origin of which remains unclear. In this work, another member of the family, 2,6-diethylphenol, has been investigated through its rotational spectrum. The jet-cooled broadband chirped-pulse Fourier transform microwave spectra in the 2–8 GHz frequency region revealed the presence of two conformers. The comparison with the equilibrium structure obtained by computational calculations at the B3LYP-D3(BJ)/Def2-TZVP level validates the structural determination and the orientation of the lateral ethyl groups. Additional observation of all the singly-substituted 13C isotopologues for the most stable ones allowed the determination of the substitution structure by means of the Kraitchman equations. Both conformers exhibited tunneling that was reproduced using an advanced 1D model, which provides an estimate of the barrier height for both conformers. [ABSTRACT FROM AUTHOR]

Time‐resolved X‐ray (Tr‐XAS) and optical transient absorption (OTA) spectroscopy on the pico‐microsecond timescale coupled with density functional theory calculations are applied to study the light‐induced spin crossover processes of a Fe‐based macrocyclic complex in solution. Tr‐XAS analysis after light illumination shows the formation of a seven‐coordinated high‐spin quintet metastable state, which relaxes to a six‐coordinated high‐spin configuration before decaying to the ground state. Kinetic analysis of the macrocyclic complex reveals an unprecedented long‐lived decay lifetime of approximately 42.6 μs. Comparative studies with a non‐macrocyclic counterpart illustrate a significantly shortened approximately 568‐fold decay lifetime of about 75 ns, and highlight the importance of the ligand arrangement in stabilizing the reactivity of the excited state. Lastly, OTA analysis shows the seven‐coordinated high‐spin state to be formed within approximately 6.2 ps. These findings provide a complete understanding of the spin crossover reaction and relaxation pathways of the macrocyclic complex, and reveal the importance of a flexible coordination environment for their rational design. [ABSTRACT FROM AUTHOR]

Abstract: We report the light‐induced electronic and geometric changes taking place within a heteroleptic CuI photosensitizer, namely [(xant)Cu(Me2phenPh2)]PF6 (xant=xantphos, Me2phenPh2=bathocuproine), by time‐resolved X‐ray absorption spectroscopy in the ps–μs time regime. Time‐resolved X‐ray absorption near edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS) analysis enabled the elucidation of the electronic and structural configuration of the copper center in the excited state as well as its decay dynamics in different solvent conditions with and without triethylamine acting as a sacrificial electron donor. A three‐fold decrease in the decay lifetime of the excited state is observed in the presence of triethylamine, showing the feasibility of the reductive quenching pathway in the latter case. A prominent pre‐edge feature is observed in the XANES spectrum of the excited state upon metal to charge ligand transfer transition, showing an increased hybridization of the 3d states with the ligand p orbitals in the tetrahedron around the Cu center. EXAFS and density functional theory illustrate a significant shortening of the Cu−N and an elongation of the Cu−P bonds together with a decrease in the torsional angle between the xantphos and bathocuproine ligand. This study provides mechanistic time‐resolved understanding for the development of improved heteroleptic CuI photosensitizers, which can be used for the light‐driven production of hydrogen from water. [ABSTRACT FROM AUTHOR]

We investigate theoretically the generation of extreme-ultraviolet (EUV) beams carrying fractional orbital angular momentum. To this end, we drive high-order harmonic generation with infrared conical refraction (CR) beams. We show that the high-order harmonic beams emitted in the EUV/soft x-ray regime preserve the characteristic signatures of the driving beam, namely ringlike transverse intensity profile and CR-like polarization distribution. As a result, through orbital and spin angular momentum conservation, harmonic beams are emitted with fractional orbital angular momentum, and they can be synthesized into structured attosecond helical beams -or 'structured attosecond light springs'- with rotating linear polarization along the azimuth. Our proposal overcomes the state of the art limitations for the generation of light beams far from the visible domain carrying non-integer orbital angular momentum and could be applied in fields such as diffraction imaging, EUV lithography, particle trapping, and super-resolution imaging. [ABSTRACT FROM AUTHOR]

High-order harmonic generation (HHG) has been recently proven to produce extreme-ultraviolet (XUV) vortices from the nonlinear conversion of infrared twisted beams. Previous works have demonstrated a linear scaling law of the vortex charge with the harmonic order. We demonstrate that this simple law hides an unexpectedly rich scenario for the buildup of orbital angular momentum (OAM) due to the nonperturbative behavior of HHG. The complexity of these twisted XUV beams appears only when HHG is driven by nonpure vortex modes, where the XUV OAM content is dramatically increased. We explore the underlying mechanisms for this diversity and derive a general conservation rule for the nonperturbative OAM buildup. The simple scaling found in previous works corresponds to the collapse of this scenario for the particular case of pure (single-mode) OAM driving fields. [ABSTRACT FROM AUTHOR]

Quantum optics with X-rays has long been a somewhat exotic activity, but it is now rapidly becoming relevant as precision x-ray optics and novel X-ray light sources, and high-intensity lasers are becoming available. This article gives an overview of the current state of the field and an outlook to future prospects. [ABSTRACT FROM AUTHOR]

High-order harmonic generation stands as a unique nonlinear optical up-conversion process, mediated by a laser-driven electron recollision mechanism, which has been shown to conserve energy, linear momentum, and spin and orbital angular momentum. Here, we present theoretical simulations that demonstrate that this process also conserves a mixture of the latter, the torus-knot angular momentum Jγ, by producing high-order harmonics with driving pulses that are invariant under coordinated rotations. We demonstrate that the charge Jγ of the emitted harmonics scales linearly with the harmonic order, and that this conservation law is imprinted onto the polarization distribution of the emitted spiral of attosecond pulses. We also demonstrate how the nonperturbative physics of high-order harmonic generation affect the torus-knot angular momentum of the harmonics, and we show that this configuration harnesses the spin selection rules to channel the full yield of each harmonic into a single mode of controllable orbital angular momentum. [ABSTRACT FROM AUTHOR]

Optical vortices are structures of the electromagnetic field with a spiral phase ramp about a point-phase singularity, carrying orbital angular momentum (OAM). Recently, OAM has been imprinted to short-wavelength radiation through high-order harmonic generation (HHG), leading to the emission of attosecond twisted beams in the extreme-ultraviolet (XUV) regime. We explore the details of the mapping of the driving vortex to its harmonic spectrum. In particular, we show that the geometry of the harmonic vortices is convoluted, arising from the superposition of the contribution from the short and long quantum paths responsible of HHG. Finally, we show how to take advantage of transverse phase-matching to select twisted attosecond beams with different spatiotemporal properties. [ABSTRACT FROM AUTHOR]