Transient resonant Auger-Meitner spectra of photoexcited thymine.
We present the first investigation of excited state dynamics by resonant Auger-Meitner spectroscopy (also known as resonant Auger spectroscopy) using the nucleobase thymine as an example. Thymine is photoexcited in the UV and probed with X-ray photon energies at and below the oxygen K-edge. After initial photoexcitation to a pipi* excited state, thymine is known to undergo internal conversion to an npi* excited state with a strong resonance at the oxygen K-edge, red-shifted from the ground state pi* resonances of thymine (see our previous study Wolf, et al., Nat. Commun., 2017, 8, 29). We resolve and compare the Auger-Meitner electron spectra associated both with the excited state and ground state resonances, and distinguish participator and spectator decay contributions. Furthermore, we observe simultaneously with the decay of the npi* state signatures the appearance of additional resonant Auger-Meitner contributions at photon energies between the npi* state and the ground state resonances. We assign these contributions to population transfer from the npi* state to a pipi* triplet state via intersystem crossing on the picosecond timescale based on simulations of the X-ray absorption spectra in the vibrationally hot triplet state. Moreover, we identify signatures from the initially excited pipi* singlet state which we have not observed in our previous study.
View details for DOI 10.1039/d0fd00112k
View details for PubMedID 33566045
High-sensitivity x-ray/optical cross-correlator for next generation free-electron lasers
2020; 28 (16): 23545–53
We design and realize an arrival time diagnostic for ultrashort X-ray pulses achieving unprecedented high sensitivity in the soft X-ray regime via cross-correlation with a ≈1550 nm optical laser. An interferometric detection scheme is combined with a multi-layer sample design to greatly improve the sensitivity of the measurement. We achieve up to 275% of relative signal change when exposed to 1.6 mJ/cm2 of soft X-rays at 530 eV, more than a hundred-fold improvement in sensitivity as compared to previously reported techniques. The resolution of the arrival time measurement is estimated to around 2.8 fs (rms). The demonstrated X-ray arrival time monitor paves the way for sub-10 fs-level timing jitter at high repetition rate X-ray facilities.
View details for DOI 10.1364/OE.398048
View details for Web of Science ID 000560931200042
View details for PubMedID 32752349
- Orbital dynamics during an ultrafast insulator to metal transition PHYSICAL REVIEW RESEARCH 2020; 2 (2)
- Tunable isolated attosecond X-ray pulses with gigawatt peak power from a free-electron laser NATURE PHOTONICS 2020; 14 (1): 30-+
- Evidence for photoinduced sliding of the charge-order condensate in La1.875Ba0.125CuO4 PHYSICAL REVIEW B 2019; 100 (20)
Ultrafast time-resolved x-ray scattering reveals diffusive charge order dynamics in La2-x Ba x CuO4.
2019; 5 (8): eaax3346
Charge order is universal among high-T c cuprates, but its relation to superconductivity is unclear. While static order competes with superconductivity, dynamic order may be favorable and even contribute to Cooper pairing. Using time-resolved resonant soft x-ray scattering at a free-electron laser, we show that the charge order in prototypical La2-x Ba x CuO4 exhibits transverse fluctuations at picosecond time scales. These sub-millielectron volt excitations propagate by Brownian-like diffusion and have an energy scale remarkably close to the superconducting T c. At sub-millielectron volt energy scales, the dynamics are governed by universal scaling laws defined by the propagation of topological defects. Our results show that charge order in La2-x Ba x CuO4 exhibits dynamics favorable to the in-plane superconducting tunneling and establish time-resolved x-rays as a means to study excitations at energy scales inaccessible to conventional scattering techniques.
View details for DOI 10.1126/sciadv.aax3346
View details for PubMedID 31453340
- Evaporation of an anisotropic nanoplasma E D P SCIENCES. 2019
High-sensitivity X-ray Optical Cross-Correlator for Next Generation Free-Electron Lasers
View details for Web of Science ID 000482226301138
Atom-specific activation in CO oxidation.
The Journal of chemical physics
2018; 149 (23): 234707
We report on atom-specific activation of CO oxidation on Ru(0001) via resonant X-ray excitation. We show that resonant 1s core-level excitation of atomically adsorbed oxygen in the co-adsorbed phase of CO and oxygen directly drives CO oxidation. We separate this direct resonant channel from indirectly driven oxidation via X-ray induced substrate heating. Based on density functional theory calculations, we identify the valence-excited state created by the Auger decay as the driving electronic state for direct CO oxidation. We utilized the fresh-slice multi-pulse mode at the Linac Coherent Light Source that provided time-overlapped and 30 fs delayed pairs of soft X-ray pulses and discuss the prospects of femtosecond X-ray pump X-ray spectroscopy probe, as well as X-ray two-pulse correlation measurements for fundamental investigations of chemical reactions via selective X-ray excitation.
View details for PubMedID 30579301
- Ultrafast dynamics of vibrational symmetry breaking in a charge-ordered nickelate SCIENCE ADVANCES 2017; 3 (11)
Ultrafast dynamics of vibrational symmetry breaking in a charge-ordered nickelate.
2017; 3 (11): e1600735
The ability to probe symmetry-breaking transitions on their natural time scales is one of the key challenges in nonequilibrium physics. Stripe ordering represents an intriguing type of broken symmetry, where complex interactions result in atomic-scale lines of charge and spin density. Although phonon anomalies and periodic distortions attest the importance of electron-phonon coupling in the formation of stripe phases, a direct time-domain view of vibrational symmetry breaking is lacking. We report experiments that track the transient multi-terahertz response of the model stripe compound La1.75Sr0.25NiO4, yielding novel insight into its electronic and structural dynamics following an ultrafast optical quench. We find that although electronic carriers are immediately delocalized, the crystal symmetry remains initially frozen-as witnessed by time-delayed suppression of zone-folded Ni-O bending modes acting as a fingerprint of lattice symmetry. Longitudinal and transverse vibrations react with different speeds, indicating a strong directionality and an important role of polar interactions. The hidden complexity of electronic and structural coupling during stripe melting and formation, captured here within a single terahertz spectrum, opens new paths to understanding symmetry-breaking dynamics in solids.
View details for DOI 10.1126/sciadv.1600735
View details for PubMedID 29202025
View details for PubMedCentralID PMC5706742
Femtosecond X-ray magnetic circular dichroism absorption spectroscopy at an X-ray free electron laser
REVIEW OF SCIENTIFIC INSTRUMENTS
2016; 87 (3)
X-ray magnetic circular dichroism spectroscopy using an X-ray free electron laser is demonstrated with spectra over the Fe L(3,2)-edges. The high brightness of the X-ray free electron laser combined with high accuracy detection of incident and transmitted X-rays enables ultrafast X-ray magnetic circular dichroism studies of unprecedented sensitivity. This new capability is applied to a study of all-optical magnetic switching dynamics of Fe and Gd magnetic sublattices in a GdFeCo thin film above its magnetization compensation temperature.
View details for DOI 10.1063/1.4944410
View details for Web of Science ID 000373713300011
View details for PubMedID 27036761
Sub-nanosecond time-resolved ambient-pressure X-ray photoelectron spectroscopy setup for pulsed and constant wave X-ray light sources
REVIEW OF SCIENTIFIC INSTRUMENTS
2014; 85 (9): 093102
An apparatus for sub-nanosecond time-resolved ambient-pressure X-ray photoelectron spectroscopy studies with pulsed and constant wave X-ray light sources is presented. A differentially pumped hemispherical electron analyzer is equipped with a delay-line detector that simultaneously records the position and arrival time of every single electron at the exit aperture of the hemisphere with ~0.1 mm spatial resolution and ~150 ps temporal accuracy. The kinetic energies of the photoelectrons are encoded in the hit positions along the dispersive axis of the two-dimensional detector. Pump-probe time-delays are provided by the electron arrival times relative to the pump pulse timing. An average time-resolution of (780 ± 20) ps (FWHM) is demonstrated for a hemisphere pass energy E(p) = 150 eV and an electron kinetic energy range KE = 503-508 eV. The time-resolution of the setup is limited by the electron time-of-flight (TOF) spread related to the electron trajectory distribution within the analyzer hemisphere and within the electrostatic lens system that images the interaction volume onto the hemisphere entrance slit. The TOF spread for electrons with KE = 430 eV varies between ~9 ns at a pass energy of 50 eV and ~1 ns at pass energies between 200 eV and 400 eV. The correlation between the retarding ratio and the TOF spread is evaluated by means of both analytical descriptions of the electron trajectories within the analyzer hemisphere and computer simulations of the entire trajectories including the electrostatic lens system. In agreement with previous studies, we find that the by far dominant contribution to the TOF spread is acquired within the hemisphere. However, both experiment and computer simulations show that the lens system indirectly affects the time resolution of the setup to a significant extent by inducing a strong dependence of the angular spread of electron trajectories entering the hemisphere on the retarding ratio. The scaling of the angular spread with the retarding ratio can be well approximated by applying Liouville's theorem of constant emittance to the electron trajectories inside the lens system. The performance of the setup is demonstrated by characterizing the laser fluence-dependent transient surface photovoltage response of a laser-excited Si(100) sample.
View details for PubMedID 25273702
Atomic-Scale Perspective of Ultrafast Charge Transfer at a Dye-Semiconductor Interface
JOURNAL OF PHYSICAL CHEMISTRY LETTERS
2014; 5 (15): 2753-2759
Understanding interfacial charge-transfer processes on the atomic level is crucial to support the rational design of energy-challenge relevant systems such as solar cells, batteries, and photocatalysts. A femtosecond time-resolved core-level photoelectron spectroscopy study is performed that probes the electronic structure of the interface between ruthenium-based N3 dye molecules and ZnO nanocrystals within the first picosecond after photoexcitation and from the unique perspective of the Ru reporter atom at the center of the dye. A transient chemical shift of the Ru 3d inner-shell photolines by (2.3 ± 0.2) eV to higher binding energies is observed 500 fs after photoexcitation of the dye. The experimental results are interpreted with the aid of ab initio calculations using constrained density functional theory. Strong indications for the formation of an interfacial charge-transfer state are presented, providing direct insight into a transient electronic configuration that may limit the efficiency of photoinduced free charge-carrier generation.
View details for DOI 10.1021/jz501264x
View details for Web of Science ID 000340222200044
View details for PubMedID 26277975
- Time-Resolved X-Ray Photoelectron Spectroscopy Techniques For Real-Time Studies Of Interfacial Charge Transfer Dynamics 22nd International Conference on the Application of Accelerators in Research and Industry (CAARI) AMER INST PHYSICS. 2013: 475–479
- Discontinuity of the ultrafast electronic response of underdoped superconducting Bi2Sr2CaCu2O8+delta strongly excited by ultrashort light pulses PHYSICAL REVIEW B 2009; 79 (22)