Snapshots of a phase transitionTime-resolved x-ray scattering can be used to investigate the dynamics of materials during the switch from one structural phase to another. So far, methods provide an ...ensemble average and may miss crucial aspects of the detailed mechanisms at play. Wall et al. used a total-scattering technique to probe the dynamics of the ultrafast insulator-to-metal transition of vanadium dioxide (VO2) (see the Perspective by Cavalleri). Femtosecond x-ray pulses provide access to the time- and momentum-resolved dynamics of the structural transition. Their results show that the photoinduced transition is of the order-disorder type, driven by an ultrafast change in the lattice potential that suddenly unlocks the vanadium atoms and yields large-amplitude uncorrelated motions, rather than occurring through a coherent displacive mechanism.Science, this issue p. 572; see also p. 525Many ultrafast solid phase transitions are treated as chemical reactions that transform the structures between two different unit cells along a reaction coordinate, but this neglects the role of disorder. Although ultrafast diffraction provides insights into atomic dynamics during such transformations, diffraction alone probes an averaged unit cell and is less sensitive to randomness in the transition pathway. Using total scattering of femtosecond x-ray pulses, we show that atomic disordering in photoexcited vanadium dioxide (VO2) is central to the transition mechanism and that, after photoexcitation, the system explores a large volume of phase space on a time scale comparable to that of a single phonon oscillation. These results overturn the current understanding of an archetypal ultrafast phase transition and provide new microscopic insights into rapid evolution toward equilibrium in photoexcited matter.
The description of ultrafast nonadiabatic chemical dynamics during molecular photo-transformations remains challenging because electronic and nuclear configurations impact each other and cannot be ...treated independently. Here we gain experimental insights, beyond the Born-Oppenheimer approximation, into the light-induced spin-state trapping dynamics of the prototypical Fe(bpy)
compound by time-resolved X-ray absorption spectroscopy at sub-30-femtosecond resolution and high signal-to-noise ratio. The electronic decay from the initial optically excited electronic state towards the high spin state is distinguished from the structural trapping dynamics, which launches a coherent oscillating wave packet (265 fs period), clearly identified as molecular breathing. Throughout the structural trapping, the dispersion of the wave packet along the reaction coordinate reveals details of intramolecular vibronic coupling before a slower vibrational energy dissipation to the solution environment. These findings illustrate how modern time-resolved X-ray absorption spectroscopy can provide key information to unravel dynamic details of photo-functional molecules.
The multifunctional protein cytochrome c (cyt c) plays key roles in electron transport and apoptosis, switching function by modulating bonding between a heme iron and the sulfur in a methionine ...residue. This Fe–S(Met) bond is too weak to persist in the absence of protein constraints. We ruptured the bond in ferrous cyt c using an optical laser pulse and monitored the bond reformation within the protein active site using ultrafast x-ray pulses from an x-ray free-electron laser, determining that the Fe–S(Met) bond enthalpy is ~4 kcal/mol stronger than in the absence of protein constraints. The 4 kcal/mol is comparable with calculations of stabilization effects in other systems, demonstrating how biological systems use an entatic state for modest yet accessible energetics to modulate chemical function.
The X‐ray Pump–Probe instrument achieves femtosecond time‐resolution with hard X‐ray methods using a free‐electron laser source. It covers a photon energy range of 4–24 keV. A femtosecond optical ...laser system is available across a broad spectrum of wavelengths for generating transient states of matter. The instrument is designed to emphasize versatility and the scientific goals encompass ultrafast physical, chemical and biological processes involved in the transformation of matter and transfer of energy at the atomic scale.
We report x-ray free electron laser experiments addressing ground-state structural dynamics of the diplatinum anion Pt_{2}POP_{4} following photoexcitation. The structural dynamics are tracked with ...<100 fs time resolution by x-ray scattering, utilizing the anisotropic component to suppress contributions from the bulk solvent. The x-ray data exhibit a strong oscillatory component with period 0.28 ps and decay time 2.2 ps, and structural analysis of the difference signal directly shows this as arising from ground-state dynamics along the PtPt coordinate. These results are compared with multiscale Born-Oppenheimer molecular dynamics simulations and demonstrate how off-resonance excitation can be used to prepare a vibrationally cold excited-state population complemented by a structure-dependent depletion of the ground-state population which subsequently evolves in time, allowing direct tracking of ground-state structural dynamics.
Ultrafast, polarization-selective time-resolved X-ray absorption near-edge structure (XANES) was used to characterize the photochemistry of vitamin B12, cyanocobalamin (CNCbl), in solution. ...Cobalamins are important biological cofactors involved in methyl transfer, radical rearrangement, and light-activated gene regulation, while also holding promise as light-activated agents for spatiotemporal controlled delivery of therapeutics. We introduce polarized femtosecond XANES, combined with UV–visible spectroscopy, to reveal sequential structural evolution of CNCbl in the excited electronic state. Femtosecond polarized XANES provides the crucial structural dynamics link between computed potential energy surfaces and optical transient absorption spectroscopy. Polarization selectivity can be used to uniquely identify electronic contributions and structural changes, even in isotropic samples when well-defined electronic transitions are excited. Our XANES measurements reveal that the structural changes upon photoexcitation occur mainly in the axial direction, where elongation of the axial Co–CN bond and Co–NIm bond on a 110 fs time scale is followed by corrin ring relaxation on a 260 fs time scale. These observations expose features of the potential energy surfaces controlling cobalamin reactivity and deactivation.
Reliably identifying short-lived chemical reaction intermediates is crucial to elucidate reaction mechanisms but becomes particularly challenging when multiple transient species occur simultaneously. ...Here, we report a femtosecond x-ray emission spectroscopy and scattering study of the aqueous ferricyanide photochemistry, utilizing the combined Fe Kβ main and valence-to-core emission lines. Following UV-excitation, we observe a ligand-to-metal charge transfer excited state that decays within 0.5 ps. On this timescale, we also detect a hitherto unobserved short-lived species that we assign to a ferric penta-coordinate intermediate of the photo-aquation reaction. We provide evidence that bond photolysis occurs from reactive metal-centered excited states that are populated through relaxation of the charge transfer excited state. Beyond illuminating the elusive ferricyanide photochemistry, these results show how current limitations of Kβ main line analysis in assigning ultrafast reaction intermediates can be circumvented by simultaneously using the valence-to-core spectral range.
Experimental methods that use free‐electron laser (FEL) sources that can deliver short X‐ray pulses below a 10 fs pulse duration and traditional optical lasers are ideal tools for pump–probe ...experiments. However, these new methods also come with a unique set of challenges, such as how to accurately determine temporal overlap between two sources at the femtosecond scale and how to correct for the pulse‐to‐pulse beam property fluctuations of the FEL light derived from the self‐amplified spontaneous emission process. Over the past several years of performing pump–probe experiments at the Linac Coherent Light Source (LCLS), new methods and tools have been developed to improve the ways experimental timing is measured, monitored and scanned. The aim of this article is to present an overview of the most commonly used techniques at LCLS to perform pump–probe‐type experiments.
Techniques developed at LCLS to make optical laser pump–X‐ray probe experiments both robust and easy to implement at ultrafast X‐ray free‐electron lasers are presented.
Molecular iodine is photoexcited by a strong 800-nm laser driving several channels of multiphoton excitation. The motion following photoexcitation is probed using time-resolved x-ray scattering, ...which produces a scattering mapS(Q,τ). Temporal Fourier-transform methods are employed to obtain a frequency-resolved x-ray-scattering signalS˜(Q,ω). Taken together,S(Q,τ)andS˜(Q,ω)separate different modes of motion so that mode-specific nuclear oscillatory positions, oscillation amplitudes, directions of motion, and times may be measured accurately. Molecular dissociations likewise have a distinct signature, which may be used to identify both velocities and dissociation time shifts and also can reveal laser-induced couplings among the molecular potentials.
The Fe(L222N5)(CN)2 compound, where L222N5 refers to the macrocyclic Schiff‐base ligand, 2,13‐dimethyl‐3,6,9,‐12,18‐pentaazabicyclo12.3.1octadeca‐1(18),2,12,14,‐ 16‐pentaene, is a photomagnetic FeII ...based coordination compound, which undergoes light‐induced excited spin‐state trapping (LIESST). The low spin state is hexacoordinated and the high spin state heptacoordinated. This system also serves as complex for the design of trinuclear or one‐dimensional compounds made of other types of bricks with diverse coordinated metals. Here its ultrafast spin‐state photoswitching dynamics are studied, by combining femtosecond optical spectroscopy and femtosecond X‐ray absorption measurements at the XPP station of the X‐ray free‐electron laser LCLS. DFT and TD‐DFT calculations are used to interpret experimental findings. These studies, performed in the solution phase, show that LIESST in Fe(L222N5)(CN)2 occurs on the 100 fs timescale under different types of photoexcitation. In addition, coherent oscillations were observed, resulting from the structural dynamics accompanying LIESST, which were recently evidenced in more conventional octahedral FeIIN6 systems.
Fast and ferrous! Ultrafast light‐induced excited spin‐state trapping in the photomagnetic complex Fe(L222N5)(CN)2 was studied by femtosecond optical and X‐ray absorption studies. The study, supported by theoretical calculations, reveals different switching pathways involving coherent structural dynamics.
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