| 1. |
- Garg, D., et al.
(författare)
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Fragmentation Dynamics of Fluorene Explored Using Ultrafast XUV-Vis Pump-Probe Spectroscopy
- 2022
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Ingår i: Frontiers in Physics. - : Frontiers Media SA. - 2296-424X. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- We report on the use of extreme ultraviolet (XUV, 30.3 nm) radiation from the Free-electron LASer in Hamburg (FLASH) and visible (Vis, 405 nm) photons from an optical laser to investigate the relaxation and fragmentation dynamics of fluorene ions. The ultrashort laser pulses allow to resolve the molecular processes occurring on the femtosecond timescales. Fluorene is a prototypical small polycyclic aromatic hydrocarbon (PAH). Through their infrared emission signature, PAHs have been shown to be ubiquitous in the universe, and they are assumed to play an important role in the chemistry of the interstellar medium. Our experiments track the ionization and dissociative ionization products of fluorene through time-of-flight mass spectrometry and velocity-map imaging. Multiple processes involved in the formation of each of the fragment ions are disentangled through analysis of the ion images. The relaxation lifetimes of the excited fluorene monocation and dication obtained through the fragment formation channels are reported to be in the range of a few tens of femtoseconds to a few picoseconds.
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| 2. |
- Lee, J. W.L., et al.
(författare)
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Time-resolved relaxation and fragmentation of polycyclic aromatic hydrocarbons investigated in the ultrafast XUV-IR regime
- 2021
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- Polycyclic aromatic hydrocarbons (PAHs) play an important role in interstellar chemistry and are subject to high energy photons that can induce excitation, ionization, and fragmentation. Previous studies have demonstrated electronic relaxation of parent PAH monocations over 10-100 femtoseconds as a result of beyond-Born-Oppenheimer coupling between the electronic and nuclear dynamics. Here, we investigate three PAH molecules: fluorene, phenanthrene, and pyrene, using ultrafast XUV and IR laser pulses. Simultaneous measurements of the ion yields, ion momenta, and electron momenta as a function of laser pulse delay allow a detailed insight into the various molecular processes. We report relaxation times for the electronically excited PAH*, PAH(+*) and PAH(2+*) states, and show the time-dependent conversion between fragmentation pathways. Additionally, using recoil-frame covariance analysis between ion images, we demonstrate that the dissociation of the PAH(2+) ions favors reaction pathways involving two-body breakup and/or loss of neutral fragments totaling an even number of carbon atoms. Polycyclic aromatic hydrocarbons play an important role in interstellar chemistry, where interaction with high energy photons can induce ionization and fragmentation reactions. Here the authors, with XUV-IR pump-probe experiments, investigate the ultrafast photoinduced dynamics of fluorene, phenanthrene and pyrene, providing insight into their preferred reaction channels.
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| 3. |
- Cavalieri, A L, et al.
(författare)
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Clocking femtosecond X rays.
- 2005
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Ingår i: Phys Rev Lett. - 0031-9007. ; 94:11
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Tidskriftsartikel (refereegranskat)abstract
- Linear-accelerator-based sources will revolutionize ultrafast x-ray science due to their unprecedented brightness and short pulse duration. However, time-resolved studies at the resolution of the x-ray pulse duration are hampered by the inability to precisely synchronize an external laser to the accelerator. At the Sub-Picosecond Pulse Source at the Stanford Linear-Accelerator Center we solved this problem by measuring the arrival time of each high energy electron bunch with electro-optic sampling. This measurement indirectly determined the arrival time of each x-ray pulse relative to an external pump laser pulse with a time resolution of better than 60 fs rms.
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| 4. |
- Rolles, D., et al.
(författare)
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Femtosecond x-ray photoelectron diffraction on gas-phase dibromobenzene molecules
- 2014
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Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 47:12
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Tidskriftsartikel (refereegranskat)abstract
- We present time-resolved femtosecond photoelectron momentum images and angular distributions of dissociating, laser-aligned 1,4-dibromobenzene (C6H4Br2) molecules measured in a near-infrared pump, soft-x-ray probe experiment performed at an x-ray free-electron laser. The observed alignment dependence of the bromine 2p photoelectron angular distributions is compared to density functional theory calculations and interpreted in terms of photoelectron diffraction. While no clear time-dependent effects are observed in the angular distribution of the Br(2p) photoelectrons, other, low-energy electrons show a pronounced dependence on the time delay between the near-infrared laser and the x-ray pulse.
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| 5. |
- Kunnus, K., et al.
(författare)
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Identification of the dominant photochemical pathways and mechanistic insights to the ultrafast ligand exchange of Fe(CO)(5) to Fe(CO)(4)EtOH
- 2016
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Ingår i: Structural Dynamics. - : AIP Publishing. - 2329-7778. ; 3:4
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Tidskriftsartikel (refereegranskat)abstract
- We utilized femtosecond time-resolved resonant inelastic X-ray scattering and ab initio theory to study the transient electronic structure and the photoinduced molecular dynamics of a model metal carbonyl photocatalyst Fe(CO)(5) in ethanol solution. We propose mechanistic explanation for the parallel ultrafast intra-molecular spin crossover and ligation of the Fe(CO)(4) which are observed following a charge transfer photoexcitation of Fe(CO)(5) as reported in our previous study [ Wernet et al., Nature 520, 78 (2015)]. We find that branching of the reaction pathway likely happens in the (1)A(1) state of Fe(CO)(4). A sub-picosecond time constant of the spin crossover from B-1(2) to B-3(2) is rationalized by the proposed B-1(2) -> (1)A(1) -> B-3(2) mechanism. Ultrafast ligation of the B-1(2) Fe(CO)(4) state is significantly faster than the spin-forbidden and diffusion limited ligation process occurring from the B-3(2) Fe(CO)(4) ground state that has been observed in the previous studies. We propose that the ultrafast ligation occurs via B-1(2) -> (1)A(1) -> (1)A'Fe(CO)(4)EtOH pathway and the time scale of the (1)A(1) Fe(CO)(4) state ligation is governed by the solute-solvent collision frequency. Our study emphasizes the importance of understanding the interaction of molecular excited states with the surrounding environment to explain the relaxation pathways of photoexcited metal carbonyls in solution.
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| 6. |
- Kunnus, K., et al.
(författare)
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Quantifying covalent interactions with resonant inelastic soft X-ray scattering : Case study of Ni2+ aqua complex
- 2017
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 669, s. 196-201
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Tidskriftsartikel (refereegranskat)abstract
- We analyze the effects of covalent interactions in Ni 2p3d resonant inelastic X-ray scattering (RIXS) spectra from aqueous Ni2+ ions and find that the relative RIXS intensities of ligand-to-metal charge-transfer final states with respect to the ligand-field final states reflect the covalent mixing between Ni 3d and water orbitals. Specifically, the experimental intensity ratio at the Ni L-3-edge allows to determine that the Ni 3d orbitals have on average 5.5% of water character. We propose that 2p3d RIXS at the Ni L-3-edge can be utilized to quantify covalency in Ni complexes without the use of external references or simulations.
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| 7. |
- Allum, Felix, et al.
(författare)
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Coulomb explosion imaging of CH3I and CH2CII photodissociation dynamics
- 2018
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 149:20
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Tidskriftsartikel (refereegranskat)abstract
- The photodissociation dynamics of CH3I and CH2CII at 272 nm were investigated by time-resolved Coulomb explosion imaging, with an intense non-resonant 815nmprobe pulse. Fragment ion momenta over a widem/z range were recorded simultaneously by coupling a velocity map imaging spectrometer with a pixel imaging mass spectrometry camera. For both molecules, delay-dependent pump-probe features were assigned to ultraviolet-induced carbon-iodine bond cleavage followed by Coulomb explosion. Multi-mass imaging also allowed the sequential cleavage of both carbon-halogen bonds in CH2ClI to be investigated. Furthermore, delay-dependent relative fragment momenta of a pair of ions were directly determined using recoil-frame covariance analysis. These results are complementary to conventional velocity map imaging experiments and demonstrate the application of time-resolved Coulomb explosion imaging to photoinduced real-time molecular motion.
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| 8. |
- Brasse, Felix, et al.
(författare)
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Time-resolved inner-shell photoelectron spectroscopy : From a bound molecule to an isolated atom
- 2018
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Ingår i: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - 2469-9926 .- 2469-9934. ; 97:4
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Tidskriftsartikel (refereegranskat)abstract
- Due to its element and site specificity, inner-shell photoelectron spectroscopy is a widely used technique to probe the chemical structure of matter. Here, we show that time-resolved inner-shell photoelectron spectroscopy can be employed to observe ultrafast chemical reactions and the electronic response to the nuclear motion with high sensitivity. The ultraviolet dissociation of iodomethane (CH3I) is investigated by ionization above the iodine 4d edge, using time-resolved inner-shell photoelectron and photoion spectroscopy. The dynamics observed in the photoelectron spectra appear earlier and are faster than those seen in the iodine fragments. The experimental results are interpreted using crystal-field and spin-orbit configuration interaction calculations, and demonstrate that time-resolved inner-shell photoelectron spectroscopy is a powerful tool to directly track ultrafast structural and electronic transformations in gas-phase molecules.
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| 9. |
- Burt, Michael, et al.
(författare)
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Coulomb-explosion imaging of concurrent CH2BrI photodissociation dynamics
- 2017
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Ingår i: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - 2469-9926 .- 2469-9934. ; 96:4
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics following laser-induced molecular photodissociation of gas-phase CH2BrI at 271.6 nm were investigated by time-resolved Coulomb-explosion imaging using intense near-IR femtosecond laser pulses. The observed delay-dependent photofragment momenta reveal that CH2BrI undergoes C-I cleavage, depositing 65.6% of the available energy into internal product states, and that absorption of a second UV photon breaks the C-Br bond of C(H)2Br. Simulations confirm that this mechanism is consistent with previous data recorded at 248 nm, demonstrating the sensitivity of Coulomb-explosion imaging as a real-time probe of chemical dynamics.
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| 10. |
- Engel, Robin Y., et al.
(författare)
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Electron population dynamics in resonant non-linear x-ray absorption in nickel at a free-electron laser
- 2023
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Ingår i: Structural Dynamics. - : American Institute of Physics (AIP). - 2329-7778. ; 10:5
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Tidskriftsartikel (refereegranskat)abstract
- Free-electron lasers provide bright, ultrashort, and monochromatic x-ray pulses, enabling novel spectroscopic measurements not only with femtosecond temporal resolution: The high fluence of their x-ray pulses can also easily enter the regime of the non-linear x-ray-matter interaction. Entering this regime necessitates a rigorous analysis and reliable prediction of the relevant non-linear processes for future experiment designs. Here, we show non-linear changes in the L-3-edge absorption of metallic nickel thin films, measured with fluences up to 60 J/cm(2). We present a simple but predictive rate model that quantitatively describes spectral changes based on the evolution of electronic populations within the pulse duration. Despite its simplicity, the model reaches good agreement with experimental results over more than three orders of magnitude in fluence, while providing a straightforward understanding of the interplay of physical processes driving the non-linear changes. Our findings provide important insights for the design and evaluation of future high-fluence free-electron laser experiments and contribute to the understanding of non-linear electron dynamics in x-ray absorption processes in solids at the femtosecond timescale.
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| 11. |
- Kockert, Hansjochen, et al.
(författare)
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UV-induced dissociation of CH2BrI probed by intense femtosecond XUV pulses
- 2022
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Ingår i: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 55:1
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Tidskriftsartikel (refereegranskat)abstract
- The ultraviolet (UV)-induced dissociation and photofragmentation of gas-phase CH2BrI molecules induced by intense femtosecond extreme ultraviolet (XUV) pulses at three different photon energies are studied by multi-mass ion imaging. Using a UV-pump-XUV-probe scheme, charge transfer between highly charged iodine ions and neutral CH2Br radicals produced by C-I bond cleavage is investigated. In earlier charge-transfer studies, the center of mass of the molecules was located along the axis of the bond cleaved by the pump pulse. In the present case of CH2BrI, this is not the case, thus inducing a rotation of the fragment. We discuss the influence of the rotation on the charge transfer process using a classical over-the-barrier model. Our modeling suggests that, despite the fact that the dissociation is slower due to the rotational excitation, the critical interatomic distance for charge transfer is reached faster. Furthermore, we suggest that charge transfer during molecular fragmentation may be modulated in a complex way.
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| 12. |
- Lindenberg, AM, et al.
(författare)
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Atomic-scale visualization of inertial dynamics
- 2005
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 1095-9203 .- 0036-8075. ; 308:5720, s. 392-395
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Tidskriftsartikel (refereegranskat)abstract
- The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.
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| 13. |
- Bari, Sadia, et al.
(författare)
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Soft X-ray Spectroscopy as a Probe for Gas-Phase Protein Structure : Electron Impact Ionization from Within
- 2018
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539. ; 24:30, s. 7631-7636
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Tidskriftsartikel (refereegranskat)abstract
- Preservation of protein conformation upon transfer into the gas phase is key for structure determination of free single molecules, for example using X-ray free-electron lasers. In the gas phase, the helicity of melittin decreases strongly as the protein's protonation state increases. We demonstrate the sensitivity of soft X-ray spectroscopy to the gas-phase structure of melittin cations ([melittin+qH]q+, q=2-4) in a cryogenic linear radiofrequency ion trap. With increasing helicity, we observe a decrease of the dominating carbon 1s-π* transition in the amide C=O bonds for non-dissociative single ionization and an increase for non-dissociative double ionization. As the underlying mechanism we identify inelastic electron scattering. Using an independent atom model, we show that the more compact nature of the helical protein conformation substantially increases the probability for off-site intramolecular ionization by inelastic Auger electron scattering.
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| 14. |
- Boll, Rebecca, et al.
(författare)
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Imaging molecular structure through femtosecond photoelectron diffraction on aligned and oriented gas-phase molecules
- 2014
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Ingår i: Faraday Discussions. - : Royal Society of Chemistry (RSC). - 1364-5498. ; 171, s. 57-80
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Tidskriftsartikel (refereegranskat)abstract
- This paper gives an account of our progress towards performing femtosecond time-resolved photoelectron diffraction on gas-phase molecules in a pump-probe setup combining optical lasers and an X-ray free-electron laser. We present results of two experiments aimed at measuring photoelectron angular distributions of laser-aligned 1-ethynyl-4-fluorobenzene (C8H5F) and dissociating, laser-aligned 1,4-dibromobenzene (C6H4Br2) molecules and discuss them in the larger context of photoelectron diffraction on gas-phase molecules. We also show how the strong nanosecond laser pulse used for adiabatically laser-aligning the molecules influences the measured electron and ion spectra and angular distributions, and discuss how this may affect the outcome of future time-resolved photoelectron diffraction experiments.
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| 15. |
- Kunnus, Kristjan, et al.
(författare)
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From Ligand Fields to Molecular Orbitals : Probing the Local Valence Electronic Structure of Ni2+ in Aqueous Solutions with Resonant Inelastic X-ray Scattering
- 2013
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 117, s. 16512-16521
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Tidskriftsartikel (refereegranskat)abstract
- Bonding of the Ni2+(aq) complex is investigated with an unprecedented combination of resonant inelastic X-ray scattering (RIXS) measurements and ab initio calculations at the Ni L absorption edge. The spectra directly reflect the relative energies of the ligand-field and charge-transfer valence-excited states. They give element-specific access with atomic resolution to the ground-state electronic structure of the complex and allow quantification of ligand-field strength and 3d–3d electron correlation interactions in the Ni2+(aq) complex. The experimentally determined ligand-field strength is 10Dq = 1.1 eV. This and the Racah parameters characterizing 3d–3d Coulomb interactions B = 0.13 eV and C = 0.42 eV as readily derived from the measured energies match very well with the results from UV–vis spectroscopy. Our results demonstrate how L-edge RIXS can be used to complement existing spectroscopic tools for the investigation of bonding in 3d transition-metal coordination compounds in solution. The ab initio RASPT2 calculation is successfully used to simulate the L-edge RIXS spectra.
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| 16. |
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| 17. |
- Schreck, Simon, et al.
(författare)
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Ground state potential energy surfaces around selected atoms from resonant inelastic x-ray scattering.
- 2016
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- Thermally driven chemistry as well as materials' functionality are determined by the potential energy surface of a systems electronic ground state. This makes the potential energy surface a central and powerful concept in physics, chemistry and materials science. However, direct experimental access to the potential energy surface locally around atomic centers and to its long-range structure are lacking. Here we demonstrate how sub-natural linewidth resonant inelastic soft x-ray scattering at vibrational resolution is utilized to determine ground state potential energy surfaces locally and detect long-range changes of the potentials that are driven by local modifications. We show how the general concept is applicable not only to small isolated molecules such as O2 but also to strongly interacting systems such as the hydrogen bond network in liquid water. The weak perturbation to the potential energy surface through hydrogen bonding is observed as a trend towards softening of the ground state potential around the coordinating atom. The instrumental developments in high resolution resonant inelastic soft x-ray scattering are currently accelerating and will enable broad application of the presented approach. With this multidimensional potential energy surfaces that characterize collective phenomena such as (bio)molecular function or high-temperature superconductivity will become accessible in near future.
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| 18. |
- Schubert, Kaja, et al.
(författare)
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The electronic structure and deexcitation pathways of an isolated metalloporphyrin ion resolved by metal L-edge spectroscopy
- 2021
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Ingår i: Chemical Science. - : Royal Society of Chemistry (RSC). - 2041-6520 .- 2041-6539. ; 12:11, s. 3966-3976
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Tidskriftsartikel (refereegranskat)abstract
- The local electronic structure of the metal-active site and the deexcitation pathways of metalloporphyrins are crucial for numerous applications but difficult to access by commonly employed techniques. Here, we applied near-edge X-ray absorption mass spectrometry and quantum-mechanical restricted active space calculations to investigate the electronic structure of the metal-active site of the isolated cobalt(iii) protoporphyrin IX cation (CoPPIX+) and its deexcitation pathways upon resonant absorption at the cobalt L-edge. The experiments were carried out in the gas phase, thus allowing for control over the chemical state and molecular environment of the metalloporphyrin. The obtained mass spectra reveal that resonant excitations of CoPPIX+at the cobalt L3-edge lead predominantly to the formation of the intact radical dication and doubly charged fragments through losses of charged and neutral side chains from the macrocycle. The comparison between experiment and theory shows that CoPPIX+is in a3A2gtriplet ground state and that competing excitations to metal-centred non-bonding and antibonding σ* molecular orbitals lead to distinct deexcitation pathways.
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| 19. |
- Wernet, Philippe, et al.
(författare)
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Mapping chemical bonding of reaction intermediates with femtosecond X-ray laser spectroscopy
- 2013
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Ingår i: XVIIIth International Conference on Ultrafast Phenomena. - : EDP Sciences. - 9782759809561 ; , s. 05025-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We determine the pathways in the photo-dissociation reactions of Fe(CO)5 both in the gas phase and in solution by mapping the valence electronic structure of the reaction intermediates with femtosecond X-ray laser spectroscopy.
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| 20. |
- Yin, Z., et al.
(författare)
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A new compact soft x-ray spectrometer for resonant inelastic x-ray scattering studies at PETRA III
- 2015
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Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 86:9
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Tidskriftsartikel (refereegranskat)abstract
- We present a newly designed compact grating spectrometer for the energy range from 210 eV to 1250 eV, which would include the K alpha(1,2) emission lines of vital elements like C, N, and O. The spectrometer is based on a grazing incidence spherical varied line spacing grating with 2400 l/mm at its center and a radius of curvature of 58 542 mm. First, results show a resolving power of around 1000 at an energy of 550 eV and a working spectrometer for high vacuum (10(-4) mbar) environment without losing photon intensity.
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