Author
Maik Kaiser
Other affiliations: École Polytechnique Fédérale de Lausanne
Bio: Maik Kaiser is an academic researcher from Paul Scherrer Institute. The author has contributed to research in topics: Femtosecond & Absorption spectroscopy. The author has an hindex of 14, co-authored 26 publications receiving 1422 citations. Previous affiliations of Maik Kaiser include École Polytechnique Fédérale de Lausanne.
Papers
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TL;DR: These results resolve a long-standing issue about the population mechanism of quintet states in iron(II)-based complexes, which are identified as a simple 1MLCT→3 MLCT→5T cascade from the initially excited state.
Abstract: X-ray absorption spectroscopy is a powerful probe of molecular structure, but it has previously been too slow to track the earliest dynamics after photoexcitation. We investigated the ultrafast formation of the lowest quintet state of aqueous iron(II) tris(bipyridine) upon excitation of the singlet metal-to-ligand-charge-transfer ( 1 MLCT) state by femtosecond optical pump/x-ray probe techniques based on x-ray absorption near-edge structure (XANES). By recording the intensity of a characteristic XANES feature as a function of laser pump/x-ray probe time delay, we find that the quintet state is populated in about 150 femtoseconds. The quintet state is further evidenced by its full XANES spectrum recorded at a 300-femtosecond time delay. These results resolve a long-standing issue about the population mechanism of quintet states in iron(II)-based complexes, which we identify as a simple 1 MLCT→ 3 MLCT→ 5 T cascade from the initially excited state. The time scale of the 3 MLCT→ 5 T relaxation corresponds to the period of the iron-nitrogen stretch vibration.
489 citations
TL;DR: The SwissFEL X-ray Free Electron Laser (XFEL) facility as discussed by the authors started construction at the Paul Scherrer Institute (Villigen, Switzerland) in 2013 and will be ready to accept its first users in 2018 on the Aramis hard Xray branch.
Abstract: The SwissFEL X-ray Free Electron Laser (XFEL) facility started construction at the Paul Scherrer Institute (Villigen, Switzerland) in 2013 and will be ready to accept its first users in 2018 on the Aramis hard X-ray branch. In the following sections we will summarize the various aspects of the project, including the design of the soft and hard X-ray branches of the accelerator, the results of SwissFEL performance simulations, details of the photon beamlines and experimental stations, and our first commissioning results.
295 citations
TL;DR: The structural relaxation from the high spin to the low spin state was followed over the entire lifetime of the excited state and an Fe-N bond elongation of 0.2 A in the quintet state compared to the singlet ground state is delivered.
Abstract: Structural changes of the Fe(II)-tris-bipyridine ([FeII(bpy)3]2+) complex induced by ultrashort pulse excitation and population of its short-lived (?0.6 ns) quintet high spin state were detected by picosecond x-ray absorption spectroscopy. The structural relaxation from the high spin to the low spin state was followed over the entire lifetime of the excited state. A combined anal. of the x-ray-absorption near-edge structure and extended x-ray-absorption fine structure spectroscopy features delivers an Fe-N bond elongation of 0.2 A in the quintet state compared to the singlet ground state. [on SciFinder (R)]
212 citations
TL;DR: Grazing-incidence x-ray diffraction is employed to characterize the coherent, femtosecond laser-induced lattice motion of a bismuth crystal as a function of depth from the surface with a temporal resolution of 193+/-8 fs.
Abstract: We employ grazing-incidence femtosecond x-ray diffraction to characterize the coherent, femtosecond laser-induced lattice motion of a bismuth crystal as a function of depth from the surface with a temporal resolution of 193 +/- 8 fs. The data show direct consequences on the lattice motion from carrier diffusion and electron-hole interaction, allowing us to estimate an effective diffusion rate of D=2.3 +/- 0.3 cm(2)/s for the highly excited carriers and an electron-hole interaction time of 260 +/- 20 fs.
131 citations
TL;DR: In this article, the first lasing results of SwissFEL, a hard X-ray free-electron laser (FEL) that recently came into operation at the Paul Scherrer Institute in Switzerland, were presented.
Abstract: We present the first lasing results of SwissFEL, a hard X-ray free-electron laser (FEL) that recently came into operation at the Paul Scherrer Institute in Switzerland. SwissFEL is a very stable, compact and cost-effective X-ray FEL facility driven by a low-energy and ultra-low-emittance electron beam travelling through short-period undulators. It delivers stable hard X-ray FEL radiation at 1-A wavelength with pulse energies of more than 500 μJ, pulse durations of ~30 fs (root mean square) and spectral bandwidth below the per-mil level. Using special configurations, we have produced pulses shorter than 1 fs and, in a different set-up, broadband radiation with an unprecedented bandwidth of ~2%. The extremely small emittance demonstrated at SwissFEL paves the way for even more compact and affordable hard X-ray FELs, potentially boosting the number of facilities worldwide and thereby expanding the population of the scientific community that has access to X-ray FEL radiation. The first lasing results at SwissFEL, an X-ray free-electron laser, are presented, highlighting the facility’s unique capabilities. A general comparison to other major facilities is also provided.
118 citations
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TL;DR: A laser accelerator that produces electron beams with an energy spread of a few per cent, low emittance and increased energy (more than 109 electrons above 80 MeV) and opens the way for compact and tunable high-brightness sources of electrons and radiation.
Abstract: Laser-driven accelerators, in which particles are accelerated by the electric field of a plasma wave (the wakefield) driven by an intense laser, have demonstrated accelerating electric fields of hundreds of GV m-1 (refs 1–3) These fields are thousands of times greater than those achievable in conventional radio-frequency accelerators, spurring interest in laser accelerators4,5 as compact next-generation sources of energetic electrons and radiation To date, however, acceleration distances have been severely limited by the lack of a controllable method for extending the propagation distance of the focused laser pulse The ensuing short acceleration distance results in low-energy beams with 100 per cent electron energy spread1,2,3, which limits potential applications Here we demonstrate a laser accelerator that produces electron beams with an energy spread of a few per cent, low emittance and increased energy (more than 109 electrons above 80 MeV) Our technique involves the use of a preformed plasma density channel to guide a relativistically intense laser, resulting in a longer propagation distance The results open the way for compact and tunable high-brightness sources of electrons and radiation
1,749 citations
TL;DR: This critical review discusses recent work in the field of molecule-based spin crossover materials with a special focus on these emerging issues, including chemical synthesis, physical properties and theoretical aspects as well (223 references).
Abstract: Recently we assisted a strong renewed interest in the fascinating field of molecular spin crossover complexes by (1) the emergence of nanosized spin crossover materials through direct synthesis of coordination nanoparticles and nanopatterned thin films as well as by (2) the use of novel sophisticated high spatial and temporal resolution experimental techniques and theoretical approaches for the study of spatiotemporal phenomena in cooperative spin crossover systems. Besides generating new fundamental knowledge on size-reduction effects and the dynamics of the spin crossover phenomenon, this research aims also at the development of practical applications such as sensor, display, information storage and nanophotonic devices. In this critical review, we discuss recent work in the field of molecule-based spin crossover materials with a special focus on these emerging issues, including chemical synthesis, physical properties and theoretical aspects as well (223 references).
1,084 citations
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Abstract: (Article begins on next page) Anyone can freely access the full text of works made available as \"Open Access\". Works made available under a Creative Commons license can be used according to the terms and conditions of said license. Use of all other works requires consent of the right holder (author or publisher) if not exempted from copyright protection by the applicable law. Availability: This is the author's manuscript
528 citations
TL;DR: The theory and fundamental principles of the spin-vibronic mechanism for ISC are presented, followed by empirical rules to estimate the rate of ISC within this regime.
Abstract: Intersystem crossing (ISC), formally forbidden within nonrelativistic quantum theory, is the mechanism by which a molecule changes its spin state. It plays an important role in the excited state decay dynamics of many molecular systems and not just those containing heavy elements. In the simplest case, ISC is driven by direct spin–orbit coupling between two states of different multiplicities. This coupling is usually assumed to remain unchanged by vibrational motion. It is also often presumed that spin-allowed radiationless transitions, i.e. internal conversion, and the nonadiabatic coupling that drives them, can be considered separately from ISC and spin–orbit coupling owing to the vastly different time scales upon which these processes are assumed to occur. However, these assumptions are too restrictive. Indeed, the strong mixing brought about by the simultaneous presence of nonadiabatic and spin–orbit coupling means that often the spin, electronic, and vibrational dynamics cannot be described independe...
505 citations