| 1. |
- Grånäs, Oscar, 1979-, et al.
(författare)
-
Ultrafast modification of the electronic structure of a correlated insulator
- 2022
-
Ingår i: Physical Review Research. - : American Physical Society. - 2643-1564. ; 4:3
-
Tidskriftsartikel (refereegranskat)abstract
- A nontrivial balance between Coulomb repulsion and kinematic effects determines the electronic structure of correlated electron materials. The use of electromagnetic fields strong enough to rival these native microscopic interactions allows us to study the electronic response as well as the time scales and energies involved in using quantum effects for possible applications. We use element-specific transient x-ray absorption spectroscopy and high-harmonic generation to measure the response to ultrashort off-resonant optical fields in the prototypical correlated electron insulator NiO. Surprisingly, fields of up to 0.22 V/angstrom lead to no detectable changes in the correlated Ni 3d orbitals contrary to previous predictions. A transient directional charge transfer is uncovered, a behavior that is captured by first-principles theory. Our results highlight the importance of retardation effects in electronic screening and pinpoints a key challenge in functionalizing correlated materials for ultrafast device operation.
|
|
| 2. |
- Iacocca, Ezio, 1986, et al.
(författare)
-
Spin-current-mediated rapid magnon localisation and coalescence after ultrafast optical pumping of ferrimagnetic alloys
- 2019
-
Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 10:1
-
Tidskriftsartikel (refereegranskat)abstract
- Sub-picosecond magnetisation manipulation via femtosecond optical pumping has attracted wide attention ever since its original discovery in 1996. However, the spatial evolution of the magnetisation is not yet well understood, in part due to the difficulty in experimentally probing such rapid dynamics. Here, we find evidence of a universal rapid magnetic order recovery in ferrimagnets with perpendicular magnetic anisotropy via nonlinear magnon processes. We identify magnon localisation and coalescence processes, whereby localised magnetic textures nucleate and subsequently interact and grow in accordance with a power law formalism. A hydrodynamic representation of the numerical simulations indicates that the appearance of noncollinear magnetisation via optical pumping establishes exchange-mediated spin currents with an equivalent 100% spin polarised charge current density of 10 7 A cm −2 . Such large spin currents precipitate rapid recovery of magnetic order after optical pumping. The magnon processes discussed here provide new insights for the stabilization of desired meta-stable states.
|
|
| 3. |
- Kjellsson, Ludvig, et al.
(författare)
-
Resonant Inelastic X-Ray Scattering Reveals Hidden Local Transitions of the Aqueous OH Radical
- 2020
-
Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 124:23
-
Tidskriftsartikel (refereegranskat)abstract
- Resonant inelastic x-ray scattering (RIXS) provides remarkable opportunities to interrogate ultra-fast dynamics in liquids. Here we use RIXS to study the fundamentally and practically important hydroxyl radical in liquid water, OH(aq). Impulsive ionization of pure liquid water produced a short-lived population of OH(aq), which was probed using femtosecond x-rays from an x-ray free-electron laser. We find that RIXS reveals localized electronic transitions that are masked in the ultraviolet absorption spectrum by strong charge-transfer transitions-thus providing a means to investigate the evolving electronic structure and reactivity of the hydroxyl radical in aqueous and heterogeneous environments. First-principles calculations provide interpretation of the main spectral features.
|
|
| 4. |
- Loh, Z-H, et al.
(författare)
-
Observation of the fastest chemical processes in the radiolysis of water
- 2020
-
Ingår i: Science. - : AMER ASSOC ADVANCEMENT SCIENCE. - 0036-8075 .- 1095-9203. ; 367:6474, s. 179-182
-
Tidskriftsartikel (refereegranskat)abstract
- Elementary processes associated with ionization of liquid water provide a framework for understanding radiation-matter interactions in chemistry and biology. Although numerous studies have been conducted on the dynamics of the hydrated electron, its partner arising from ionization of liquid water, H2O+, remains elusive. We used tunable femtosecond soft x-ray pulses from an x-ray free electron laser to reveal the dynamics of the valence hole created by strong-field ionization and to track the primary proton transfer reaction giving rise to the formation of OH. The isolated resonance associated with the valence hole (H2O+/OH) enabled straightforward detection. Molecular dynamics simulations revealed that the x-ray spectra are sensitive to structural dynamics at the ionization site. We found signatures of hydrated-electron dynamics in the x-ray spectrum.
|
|
| 5. |
- Shen, L., et al.
(författare)
-
Decoupling spin-orbital correlations in a layered manganite amidst ultrafast hybridized charge-transfer band excitation
- 2020
-
Ingår i: Physical Review B. - 2469-9950. ; 101:20
-
Tidskriftsartikel (refereegranskat)abstract
- In the mixed-valence manganites, a near-infrared laser typically melts the orbital and spin order simultaneously, corresponding to the photoinduced d1d0→d0d1 excitations in the Mott-Hubbard bands of manganese. Here, we use ultrafast methods-both femtosecond resonant X-ray diffraction and optical reflectivity-to demonstrate that the orbital response in the layered manganite Nd1-xSr1+xMnO4(x=2/3) does not follow this scheme. At the photoexcitation saturation fluence, the orbital order is only diminished by a few percent in the transient state. Instead of the typical d1d0→d0d1 transition, a near-infrared pump in this compound promotes a fundamentally distinct mechanism of charge transfer, the d0→d1L, where L denotes a hole in the oxygen band. This finding may pave a different avenue for selectively manipulating specific types of order in complex materials of this class.
|
|
| 6. |
- Porat, G., et al.
(författare)
-
Phase-matched extreme-ultraviolet frequency-comb generation
- 2019
-
Ingår i: Ultrafast Optics XII. - 1996-756X .- 0277-786X. - 9781510635128 ; 11370, s. 94-95
-
Konferensbidrag (refereegranskat)abstract
- Extreme ultraviolet (XUV) laser radiation is commonly produced via high-harmonic generation (HHG) in gases. The lasers that drive this process typically operate at low pulse repetition rates (<100 kHz). Under these operating conditions, the plasma generated by each laser pulse clears the generation volume before the next pulse arrives. Therefore, each laser pulse interacts with fresh plasma-free gas, where phase-matching facilitates efficient HHG. However, applications requiring high counting statistics or frequency-comb precision make high repetition rates (>10 MHz) necessary. Unfortunately, at high repetition rates, plasma accumulates in the XUV generation region and prevents phase-matching, resulting in low HHG efficiency. We use high-temperature gas mixtures to increase the gas translational velocity, thus reduce plasma accumulation and facilitate phase-matching. We experimentally achieve phase-matched HHG at a repetition rate of 77 MHz, generating record power of ~2 mW at 97 nm and ~0.9 mW at 67 nm.
|
|
| 7. |
- Schreck, Simon, et al.
(författare)
-
Atom-specific activation in CO oxidation
- 2018
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 149:23
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 8. |
- Wolf, T. J. A., et al.
(författare)
-
Probing molecular photoinduced dynamics by ultrafast soft x-rays
- 2017
-
Ingår i: 2017 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC). 25-29 June 2017, Munich, Germany. - : IEEE. - 9781509067367 - 9781509067374
-
Konferensbidrag (refereegranskat)abstract
- Summary form only given. Molecules selectively transform light energy from the sun into other forms of energy like heat, electricity, or chemical energy with high quantum efficiency. The energy conversion process is the result of a correlated motion of electrons and nuclei after photoexcitation, often under breakdown of the Born-Oppenheimer approximation. The element and site selectivity of x-rays allows observing molecular processes from a different point of view compared to ultrafast optical probes [1,2]. I will concentrate on time resolved x-ray absorption spectroscopy. The method provides high selectivity on the transient electronic structure of a molecule. Recently, we establishes this method in the soft x-ray domain for probing ππ* to nπ* transitions, a general and important process for molecular energy conversion. Fig. 1 shows a sketch of thymine, used in the experiment, with one of the oxygen 1s core orbitals and the π,n and π* valence orbitals. While valence orbitals are generally delocalized over the whole molecular body, the lone pair n orbital is essentially an oxygen 2p orbital. An x-ray induced transition from the oxygen 1s to the n orbital will result in a strong absorption maximum in the pre-edge region. We use this feature to probe the molecular dynamics after photoexcitation.
|
|
| 9. |
- Wolf, T.J.A., et al.
(författare)
-
Probing ultrafast ππ*/nπ* internal conversion in organic chromophores via K-edge resonant absorption
- 2017
-
Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 8:1
-
Tidskriftsartikel (refereegranskat)abstract
- Many photoinduced processes including photosynthesis and human vision happen in organic molecules and involve coupled femtosecond dynamics of nuclei and electrons. Organic molecules with heteroatoms often possess an important excited-state relaxation channel from an optically allowed ππ* to a dark nπ* state. The ππ*/nπ* internal conversion is difficult to investigate, as most spectroscopic methods are not exclusively sensitive to changes in the excited-state electronic structure. Here, we report achieving the required sensitivity by exploiting the element and site specificity of near-edge soft X-ray absorption spectroscopy. As a hole forms in the n orbital during ππ*/nπ* internal conversion, the absorption spectrum at the heteroatom K-edge exhibits an additional resonance. We demonstrate the concept using the nucleobase thymine at the oxygen K-edge, and unambiguously show that ππ*/nπ* internal conversion takes place within (60 ± 30) fs. High-level-coupled cluster calculations confirm the method’s impressive electronic structure sensitivity for excited-state investigations.
|
|
| 10. |
- Wolf, T.J.A., et al.
(författare)
-
Transient Resonant Auger-Meitner Spectra of Photoexcited Thymine
- 2021
-
Ingår i: Faraday discussions. - : Royal Society of Chemistry (RSC). - 1364-5498 .- 1359-6640. ; 228, s. 555-70
-
Tidskriftsartikel (refereegranskat)abstract
- 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 ππ* excited state, thymine is known to undergo internal conversion to an nπ* excited state with a strong resonance at the oxygen K-edge, red-shifted from the groundstate π* 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 participato rand spectator decay contributions. Furthermore, we observe simultaneously with the decay of the nπ* state signatures the appearance of additional resonant Auger-Meitner contributions at photon energies between the nπ* state and the ground state resonances. We assign these contributions to population transfer from the nπ* state to a ππ* 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 ππ* singlet state which we have not observed in our previous study.
|
|
