Extreme Ultraviolet Probe Research Articles

Time- and angle-resolved photoemission spectroscopy with wavelength-tunable pump and extreme ultraviolet probe enabled by twin synchronized amplifiers.

We describe a setup for time- and angle-resolved photoemission spectroscopy with wavelength-tunable excitation and an extreme ultraviolet probe. It is enabled by using the 10kHz twin Ti:sapphire amplifiers seeded by the common Ti:sapphire oscillator. The typical probe energy is 21.7eV, and the wavelength of the pump excitation is tuned between 2400 and 1200nm by using the optical parametric amplifier. The spectral width of the extreme ultraviolet probe is 53meV, and the time resolution is dependent on the wavelength for the pump, better than 60fs for the pump energy >0.7eV. This system enables the pump energy to be matched with a specific interband transition and to probe a wider energy-momentum space. We present the results for the prototypical materials of highly oriented pyrolytic graphite and Bi2Se3 to show the performance of our system.

Time‐Resolved Photoemission Electron Microscopy on a ZnO Surface Using an Extreme Ultraviolet Attosecond Pulse Pair

AbstractElectrons photoemitted by extreme ultraviolet attosecond pulses derive spatially from the first few atomic surface layers and energetically from the valence band and highest atomic orbitals. As a result, it is possible to probe the emission dynamics from a narrow 2D region in the presence of optical fields, as well as obtain elemental specific information. However, combining this with spatially‐resolved imaging is a long‐standing challenge because of the large inherent spectral width of attosecond pulses, as well as the difficulty of making them at high repetition rates. Here, this work demonstrates an attosecond interferometry experiment on a zinc oxide (ZnO) surface using spatially and energetically resolved photoelectrons. Photoemission electron microscopy is combined with near‐infrared pump ‐ extreme ultraviolet probe laser spectroscopy and the instantaneous phase of an infrared field is resolved with high spatial resolution. Results show how the core level states with low binding energy of ZnO are well suited to perform spatially resolved attosecond interferometry experiments. A distinct phase shift of the attosecond beat signal is observed across the laser focus which is attributed to wavefront differences between the pump and the probe fields at the surface. This work demonstrates a clear pathway for attosecond interferometry with high spatial resolution at atomic scale surface regions opening up for a detailed understanding of nanometric light‐matter interaction.

Formation of Long-Lived Dark States during Electronic Relaxation of Pyrimidine Nucleobases Studied Using Extreme Ultraviolet Time-Resolved Photoelectron Spectroscopy.

Ultrafast electronic relaxation of nucleobases from 1ππ* states to the ground state (S0) is considered essential for the photostability of DNA. However, transient absorption spectroscopy (TAS) has indicated that some nucleobases in aqueous solutions create long-lived 1nπ*/3ππ* dark states from the 1ππ* states with a high quantum yield of 0.4-0.5. We investigated electronic relaxation in pyrimidine nucleobases in both aqueous solutions and the gas phase using extreme ultraviolet (EUV) time-resolved photoelectron spectroscopy. Femtosecond EUV probe pulses cause ionization from all electronic states involved in the relaxation process, providing a clear overview of the electronic dynamics. The 1nπ* quantum yields for aqueous cytidine and uracil (Ura) derivatives were found to be considerably lower (<0.07) than previous estimates reported by TAS. On the other hand, aqueous thymine (Thy) and thymidine exhibited a longer 1ππ* lifetime and a higher quantum yield (0.12-0.22) for the 1nπ* state. A similar trend was found for isolated Thy and Ura in the gas phase: the 1ππ* lifetimes are 39 and 17 fs and the quantum yield for 1nπ* are 1.0 and 0.45 for Thy and Ura, respectively. The result indicates that single methylation to the C5 position hinders the out-of-plane deformation that drives the system to the conical intersection region between 1ππ* and S0, providing a large impact on the photophysics/photochemistry of a pyrimidine nucleobase. The significant reduction of 1nπ* yield in aqueous solution is ascribed to the destabilization of the 1nπ* state induced by hydrogen bonding.

Nanoscale-Resolved Surface-to-Bulk Electron Transport in CsPbBr3 Perovskite.

Describing the nanoscale charge carrier transport at surfaces and interfaces is fundamental for designing high-performance optoelectronic devices. To achieve this, we employ time- and angle-resolved photoelectron spectroscopy with ultraviolet pump and extreme ultraviolet probe pulses. The resulting high surface sensitivity reveals an ultrafast carrier population decay associated with surface-to-bulk transport, which was tracked with a sub-nanometer spatial resolution normal to the surface, and on a femtosecond time scale, in the case of the inorganic CsPbBr3 lead halide perovskite. The decay time exhibits a pronounced carrier density dependence, which is attributed via modeling to enhanced diffusive transport and concurrent recombination. The transport is found to approach an ordinary diffusive regime, limited by electron–hole scattering, at the highest excitation fluences. This approach constitutes an important milestone in our capability to probe hot-carrier transport at solid interfaces with sub-nanometer resolution in a theoretically and experimentally challenging, yet technologically relevant, high-carrier-density regime.

Tracing the electron motion in using attosecond photoelectron spectroscopy

Charge migration during light–matter interaction is one of the most fundamental processes which plays a key role in chemical and biological processes of molecule. The measurement of this process requires the extreme temporal and subatomic spatial resolutions. Here, we show that a scheme based on infrared pump and short extreme ultraviolet probe technique enables us to trace the laser-controlled electron motion with high spatiotemporal resolution. By numerically solving the time-dependent Schrödinger equation for combined with a simple double-slit model, we demonstrate that the laser-controlled charge migration can be directly reconstructed from the interference patterns of the photoelectron spectrum.

Photodissociation dynamics of methyl iodide across the A-band probed by femtosecond extreme ultraviolet photoelectron spectroscopy

The dissociation dynamics of CH3I at three UV pump wavelengths (279 nm, 254 nm, 243 nm) are measured using an extreme ultraviolet probe in a time-resolved photoelectron spectroscopy experiment. The results are compared with previously published data at a pump wavelength of 269 nm, [2020, Phys. Chem. Chem. Phys., 22, 25695], with complementary photoelectron spectroscopy experiments performed using a multiphoton ionization (MPI) probe [2019, Phys. Chem. Chem. Phys., 21, 11142] and with the recent action spectroscopy measurements of Murillo-Sánchez et al [2020, J. Chem. Phys., 152, 014304]. The measurements at 279 nm and 243 nm show signals that are consistent with rapid dissociation along the C–I bond occurring on timescales that are consistent with previous measurements. The measurements at 254 nm show a significantly longer excited state lifetime with a secondary feature appearing after 100 fs which is indicative of more complex dynamics in the excited state. The time-dependence of the changes are consistent with the previously measured MPI photoelectron spectroscopy measurements of Warne et al, [2019, Phys. Chem. Chem. Phys., 21, 11142]. The consistency of the signal appearance across ionization processes suggests that the extended observation time at 254 nm is not an artefact of the previously used MPI process but is caused by more complex dynamics on the excited state potential. Whether this is caused by complex vibrational dynamics on the dominant 3 Q 0 state or is due to enhanced population and dynamics on the 1 Q 1 state remains an open question.

Suppression of the vacuum space-charge effect in fs-photoemission by a retarding electrostatic front lens.

The performance of time-resolved photoemission experiments at fs-pulsed photon sources is ultimately limited by the e-e Coulomb interaction, downgrading energy and momentum resolution. Here, we present an approach to effectively suppress space-charge artifacts in momentum microscopes and photoemission microscopes. A retarding electrostatic field generated by a special objective lens repels slow electrons, retaining the k-image of the fast photoelectrons. The suppression of space-charge effects scales with the ratio of the photoelectron velocities of fast and slow electrons. Fields in the range from -20 to -1100 V/mm for Ekin = 100eV to 4 keV direct secondaries and pump-induced slow electrons back to the sample surface. Ray tracing simulations reveal that this happens within the first 40 to 3 μm above the sample surface for Ekin = 100eV to 4 keV. An optimized front-lens design allows switching between the conventional accelerating and the new retarding mode. Time-resolved experiments at Ekin = 107eV using fs extreme ultraviolet probe pulses from the free-electron laser FLASH reveal that the width of the Fermi edge increases by just 30 meV at an incident pump fluence of 22 mJ/cm2 (retarding field -21 V/mm). For an accelerating field of +2 kV/mm and a pump fluence of only 5 mJ/cm2, it increases by 0.5eV (pump wavelength 1030nm). At the given conditions, the suppression mode permits increasing the slow-electron yield by three to four orders of magnitude. The feasibility of the method at high energies is demonstrated without a pump beam at Ekin = 3830eV using hard x rays from the storage ring PETRA III. The approach opens up a previously inaccessible regime of pump fluences for photoemission experiments.

Towards full surface Brillouin zone mapping by coherent multi-photon photoemission

We report a novel approach for coherent multi-photon photoemission in the entire Brillouin zone with infrared light that is readily implemented in a laboratory setting. We excite a solid state material, Ag(110), with intense femtosecond laser pulses to excite higher-order multi-photon photoemission; angle-resolved electron spectroscopic acquisition records photoemission at large in-plane momenta involving optical transitions from the occupied to unoccupied bands of the sample that otherwise might remain hidden by the photoemission horizon. We propose this as a complementary ultrafast method to time- and angle-resolved two-color, e.g. infrared pump and extreme ultraviolet probe, photoemission spectroscopy, with the advantage of being able to measure and control the coherent electron dynamics.

Free Electron Laser Measurement of Liquid Carbon Reflectivity in the Extreme Ultraviolet

Ultrafast time-resolved extreme ultraviolet (EUV) reflectivity measurements of optically pumped amorphous carbon (a-C) have been performed with the FERMI free electron laser (FEL). This work extends the energy range used in previous reflectivity studies and adds polarization dependence. The EUV probe is known to be sensitive to lattice dynamics, since in this range the reflectivity is essentially unaffected by the photo-excited surface plasma. The exploitation of both s- and p-polarized EUV radiation permits variation of the penetration depth of the probe; a significant increase in the characteristic time is observed upon increasing the probing depth (1 vs. 5 ps) due to hydrodynamic expansion and consequent destruction of the excited region, implying that there is only a short window during which the probed region is in the isochoric regime. A weak wavelength dependence of the reflectivity is found, consistent with previous measurements and implying a maximum electronic temperature of 0.8 eV ± 0.4.

Photodissociation dynamics of methyl iodide probed using femtosecond extreme ultraviolet photoelectron spectroscopy.

Femtosecond pump-probe photoelectron spectroscopy measurements using an extreme ultraviolet probe have been made on the photodissociation dynamics of UV (269 nm) excited CH3I. The UV excitation leads to population of the 3Q0 state which rapidly dissociates. The dissociation is manifested as shifts in the measured photoelectron kinetic energy that map the extending C-I bond. The increased energy available in the XUV probe relative to a UV probe means the dynamics are followed over the chemically important region as far as C-I bond lengths of approximately 4 Å.

Photoinduced valence tautomerism of a cobalt-dioxolene complex revealed with femtosecond M-edge XANES.

Cobalt complexes that undergo charge-transfer induced spin-transitions or valence tautomerism from low spin CoIII to high spin (HS) CoII are potential candidates for magneto-optical switches. We use M2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy with 40 fs time resolution to measure the excited-state dynamics of CoIII(Cat-N-SQ)(Cat-N-BQ), where Cat-N-BQ and Cat-N-SQ are the singly and doubly reduced forms of the 2-(2-hydroxy-3,5-di-tert-butylphenyl-imino)-4,6-di-tert-butylcyclohexa-3,5-dienone ligand. The extreme ultraviolet probe pulses, produced using a tabletop high-harmonic generation light source, measure 3p → 3d transitions and are sensitive to the spin and oxidation state of the Co center. Photoexcitation at 525 nm produces a low-spin CoII ligand-to-metal charge transfer state which undergoes intersystem crossing to high-spin CoII in 67 fs. Vibrational cooling from this hot HS CoII state competes on the hundreds-of-fs time scale with back-intersystem crossing to the ground state, with 60% of the population trapped in a cold HS CoII state for 24 ps. Ligand field multiplet simulations accurately reproduce the ground-state spectra and support the excited-state assignments. This work demonstrates the ability of M2,3-edge XANES to measure ultrafast photophysics of molecular Co complexes.

Experimental station for ultrafast extreme ultraviolet spectroscopy for non-equilibrium dynamics in warm dense matter

Recent interest in highly excited matter generated by intense femtosecond laser pulses has led to experimental methods that directly investigate ultrafast non-equilibrium electronic and structural dynamics. We present a tabletop experimental station for the extreme ultraviolet (EUV) spectroscopy used to trace L-edge dynamics in warm dense aluminum with a temporal resolution of a hundred femtoseconds. The system consists of the EUV probe generation part via a high-order harmonic generation process of femtosecond laser pulses with atomic clusters, a beamline with high-throughput optics and a sample-refreshment system of nano-foils utilizing the full repetition rate of the probe, and a flat-field EUV spectrograph. With the accumulation of an order of a hundred shots, a clear observation of the change in the aluminum L-shell absorption was achieved with a temporal resolution of 90 fs in a 600-fs window. The signature of a non-equilibrium electron distribution over a 10-eV range and its evolution to a 1-eV Fermi distribution are observed. This demonstrates the capability of this apparatus to capture the non-equilibrium electron–hole dynamics in highly excited warm dense matter conditions.

Spin-Orbit State-Selective C-I Dissociation Dynamics of the CH3I+ X̃ Electronic State Induced by Intense Few-Cycle Laser Fields.

Studies of ultrafast molecular dynamics induced by intense laser fields can reveal new approaches to manipulating chemical reactions in the strong-field regime. Here, we show that intense few-cycle laser pulses can induce the spin-orbit state-selective C-I dissociation of the iodomethane cation (CH3I+) in the X̃ electronic state. Irradiation of CH3I by 6 fs laser pulses with peak intensities of 1.9 × 1014 W/cm2 followed by femtosecond extreme ultraviolet probing of the iodine 4d core-level transitions reveals dissociation of the CH3I+ X̃ 2E1/2 state with a time constant of 0.76 ± 0.16 ps. By contrast, the X̃ 2E3/2 spin-orbit ground state does not exhibit any appreciable dissociation on the picosecond time scale. The observed spin-orbit state-selective dissociation of the X̃ state is rationalized in terms of the laser-induced coupling to the à state. Our results suggest that the intense-laser control of photodissociation channels can be potentially extended to spin-orbit split states.

Transient EUV Reflectivity Measurements of Carbon upon Ultrafast Laser Heating

Time resolved extreme ultraviolet (EUV) transient reflectivity measurements on non-equilibrium amorphous carbon (a-C) have been carried out by combining optical and free electron laser (FEL) sources. The EUV probing was specifically sensitive to lattice dynamics, since the EUV reflectivity is essentially unaffected by the photo-excited surface plasma. Data have been interpreted in terms of the dynamics of an expanding surface, i.e., a density gradient rapidly forming along the normal surface. This allowed us to determine the characteristic time ( τ ≲ 1 ps) for hydrodynamic expansion in photo-excited a-C. This finding suggests an extremely narrow time window during which the system can be assumed to be in the isochoric regime, a situation that may complicate the study of photo-induced metastable phases of carbon. Data also showed a weak dependence on the probing EUV wavelength, which was used to estimate the electronic temperature ( T e ≈ 0.8 eV) of the excited sample. This experimental finding compares fairly well with the results of calculations, while a comparison of our data and calculations with previous transient optical reflectivity measurements highlights the complementarities between optical and EUV probing.

Spin-resolved photoelectron spectroscopy using femtosecond extreme ultraviolet light pulses from high-order harmonic generation.

The fundamental mechanism responsible for optically induced magnetization dynamics in ferromagnetic thin films has been under intense debate since almost two decades. Currently, numerous competing theoretical models are in strong need for a decisive experimental confirmation such as monitoring the triggered changes in the spin-dependent band structure on ultrashort time scales. Our approach explores the possibility of observing femtosecond band structure dynamics by giving access to extended parts of the Brillouin zone in a simultaneously time-, energy- and spin-resolved photoemission experiment. For this purpose, our setup uses a state-of-the-art, highly efficient spin detector and ultrashort, extreme ultraviolet light pulses created by laser-based high-order harmonic generation. In this paper, we present the setup and first spin-resolved spectra obtained with our experiment within an acquisition time short enough to allow pump-probe studies. Further, we characterize the influence of the excitation with femtosecond extreme ultraviolet pulses by comparing the results with data acquired using a continuous wave light source with similar photon energy. In addition, changes in the spectra induced by vacuum space-charge effects due to both the extreme ultraviolet probe- and near-infrared pump-pulses are studied by analyzing the resulting spectral distortions. The combination of energy resolution and electron count rate achieved in our setup confirms its suitability for spin-resolved studies of the band structure on ultrashort time scales.

Broadband extreme ultraviolet probing of transient gratings in vanadium dioxide.

Nonlinear spectroscopy in the extreme ultraviolet (EUV) and soft x-ray spectral range offers the opportunity for element selective probing of ultrafast dynamics using core-valence transitions (Mukamel et al., Acc. Chem. Res. 42, 553 (2009)). We demonstrate a step on this path showing core-valence sensitivity in transient grating spectroscopy with EUV probing. We study the optically induced insulator-to-metal transition (IMT) of a VO(2) film with EUV diffraction from the optically excited sample. The VO(2) exhibits a change in the 3p-3d resonance of V accompanied by an acoustic response. Due to the broadband probing we are able to separate the two features.

Diagnosis of energy transport in iron buried layer targets using an extreme ultraviolet laser

We demonstrate the use of extreme ultra-violet (EUV) laboratory lasers in probing energy transport in laser irradiated solid targets. EUV transmission through targets containing a thin layer of iron (50 nm) encased in plastic (CH) after irradiation by a short pulse (35 fs) laser focussed to irradiances 3 × 1016 Wcm−2 is measured. Heating of the iron layer gives rise to a rapid decrease in EUV opacity and an increase in the transmission of the 13.9 nm laser radiation as the iron ionizes to Fe5+ and above where the ion ionisation energy is greater than the EUV probe photon energy (89 eV). A one dimensional hydrodynamic fluid code HYADES has been used to simulate the temporal variation in EUV transmission (wavelength 13.9 nm) using IMP opacity values for the iron layer and the simulated transmissions are compared to measured transmission values. When a deliberate pre-pulse is used to preform an expanding plastic plasma, it is found that radiation is important in the heating of the iron layer while for pre-pulse free irradiation, radiation transport is not significant.

Modelling the effect of nuclear motion on the attosecond time-resolved photoelectron spectra of ethylene

Using time-dependent density functional theory we examine the energy, angular and time-resolved photoelectron spectra (TRPES) of ethylene in a pump–probe setup. To simulate TRPES we expose ethylene to an ultraviolet femtosecond pump pulse, followed by a time delayed extreme ultraviolet probe pulse. Studying the photoemission spectra as a function of this delay provides us direct access to the dynamic evolution of the molecule’s electronic levels. Further, by including the nuclei’s motion, we provide direct chemical insight into the chemical reactivity of ethylene. These results show how angular and energy resolved TRPES could be used to directly probe electron and nucleus dynamics in molecules.

Attosecond snapshots: imaging charge transfer in molecules using EUV light

Electron dynamics underlie the charge and energy transfer processes in molecules. These processes, although extremely fast, can be resolved by sub-femtosecond light pulses and the electron motion can, to some extent, be controlled. A novel scheme for electron localization using intense IR lasers has recently been developed using phase-locked multiple pulses, in which the charge centre can be isolated. This process can be identified by the asymmetry in the dissociation products. We propose the use of an extreme ultraviolet probe of the molecular ion prior to dissociation to image the density and phase of the electron orbital. We hope this will provide rich insight into the charge transfer process in small molecules, which may be extended to larger systems.

Extreme ultraviolet interferometry of laser plasma material between the critical and ablation surfaces

Interferometric probing using an extreme ultraviolet (EUV) laser has measured both transmission and phase information through laser-irradiated plastic (parylene-N C 8H 8) targets (thickness 350 nm). Unusually, the probe beam is incident longitudinally in approximately the same direction as the incident optical laser. Agreement of the experimental interferometry results has been obtained with two-dimensional radiation hydrodynamic code (h2d) simulations of EUV (21.2 nm) probe transmissions and phase shifts. We show that the transmission of the EUV probe beam provides a measure of the rate of target ablation, as ablated plasma becomes close to transparent when the photon energy is less than the ionization energy of the predominate ion species. Here C 3+ ions with ionization energy 64.5 eV are transparent, while lower carbon ionization stages, present in the unablated target and close to the ablation surface, absorb the 58.5 eV photons. Similarly, we show that refractive indices η below the solid parylene-N ( η solid = 0.946) and expected plasma values are produced in the warm dense plasma created by laser irradiation due to bound–free absorption in C +.