Kelly J. Gaffney

Imaging Atomic Structure and Dynamics with Ultrafast X-ray Scattering

Measuring atomic-resolution images of materials with x-ray photons during chemical reactions or physical transformations resides at the technological forefront of x-ray science. New x-ray–based experimental capabilities have been closely linked with advances in x-ray sources, a trend that will continue with the impending arrival of x-ray–free electron lasers driven by electron accelerators. We discuss recent advances in ultrafast x-ray science and coherent imaging made possible by linear-accelerator–based light sources. These studies highlight the promise of ultrafast x-ray lasers, as well as the technical challenges and potential range of applications that will accompany these transformative x-ray light sources.

Tracking excited-state charge and spin dynamics in iron coordination complexes

Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics and the flux limitations of ultrafast X-ray sources. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2′-bipyridine)3]2+, where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2′-bipyridine)3]2+ on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.

Large angular jump mechanism observed for hydrogen bond exchange in aqueous perchlorate solution.

Wet Twists and Turns When salts dissolve in water, their constituent positively and negatively charged ions are pulled apart and surrounded by shells of H2O molecules (see the Perspective by Skinner). Ji et al. (p. 1003) looked closely at the motion in these shells, using a type of vibrational spectroscopy sensitive to both the orientation and to the neighbors of the targeted molecules. In agreement with recent theoretical predictions, the individual water molecules shifted orientation between an anion and the surrounding liquid in sudden discrete steps, rather than by making smooth incremental rotations. Tielrooij et al. (p. 1006) compared the relative impacts of cations and anions on the rigidity of the wider water network, using spectroscopic techniques sensitive to the role of each ion. Certain cation/anion combinations, such as magnesium sulfate, appeared to act together to restrict water motion beyond the boundaries of individual shells. Water molecules shift orientation between dissolved ions and the surrounding liquid by taking large, sudden steps. The mechanism for hydrogen bond (H-bond) switching in solution has remained subject to debate despite extensive experimental and theoretical studies. We have applied polarization-selective multidimensional vibrational spectroscopy to investigate the H-bond exchange mechanism in aqueous NaClO4 solution. The results show that a water molecule shifts its donated H-bonds between water and perchlorate acceptors by means of large, prompt angular rotation. Using a jump-exchange kinetic model, we extracted an average jump angle of 49 ± 4°, in qualitative agreement with the jump angle observed in molecular dynamics simulations of the same aqueous NaClO4 solution.

Orbital-specific mapping of the ligand exchange dynamics of Fe(CO)(5) in solution

Transition-metal complexes have long attracted interest for fundamental chemical reactivity studies and possible use in solar energy conversion. Electronic excitation, ligand loss from the metal centre, or a combination of both, creates changes in charge and spin density at the metal site that need to be controlled to optimize complexes for photocatalytic hydrogen production and selective carbon–hydrogen bond activation. An understanding at the molecular level of how transition-metal complexes catalyse reactions, and in particular of the role of the short-lived and reactive intermediate states involved, will be critical for such optimization. However, suitable methods for detailed characterization of electronic excited states have been lacking. Here we show, with the use of X-ray laser-based femtosecond-resolution spectroscopy and advanced quantum chemical theory to probe the reaction dynamics of the benchmark transition-metal complex Fe(CO)5 in solution, that the photo-induced removal of CO generates the 16-electron Fe(CO)4 species, a homogeneous catalyst with an electron deficiency at the Fe centre, in a hitherto unreported excited singlet state that either converts to the triplet ground state or combines with a CO or solvent molecule to regenerate a penta-coordinated Fe species on a sub-picosecond timescale. This finding, which resolves the debate about the relative importance of different spin channels in the photochemistry of Fe(CO)5 (refs 4, 16,17,18,19 and 20), was made possible by the ability of femtosecond X-ray spectroscopy to probe frontier-orbital interactions with atom specificity. We expect the method to be broadly applicable in the chemical sciences, and to complement approaches that probe structural dynamics in ultrafast processes.

Ultrafast Dynamics of Hydrogen Bond Exchange in Aqueous Ionic Solutions

The structural and dynamical properties of aqueous ionic solutions influence a wide range of natural and biological processes. In these solutions, water has the opportunity to form hydrogen bonds with other water molecules and anions. Knowing the time scale with which these configurations interconvert represents a key factor to understanding the influence of molecular scale heterogeneity on chemical events in aqueous ionic solutions. We have used ultrafast IR spectroscopy and Car-Parrinello molecular dynamics (CPMD) simulations to investigate the hydrogen bond (H-bond) structural dynamics in aqueous 6 M sodium perchlorate (NaClO4) solution. We have measured the H-bond exchange dynamics between spectrally distinct water-water and water-anion H-bond configurations with 2DIR spectroscopy and the orientational relaxation dynamics of water molecules in different H-bond configurations with polarization-selective IR pump-probe experiments. The experimental H-bond exchange time correlates strongly with the experimental orientational relaxation time of water molecules. This agrees with prior observations in water and aqueous halide solutions, and has been interpreted within the context of an orientational jump model for the H-bond exchange. The CPMD simulations performed on aqueous 6 M NaClO4 solution clearly demonstrate that water molecules organize into two radially and angularly distinct structural subshells within the first solvation shell of the perchlorate anion, with one subshell possessing the majority of the water molecules that donate H-bonds to perchlorate anions and the other subshell possessing predominantly water molecules that donate two H-bonds to other water molecules. Due to the high ionic concentration used in the simulations, essentially all water molecules reside in the first ionic solvation shells. The CPMD simulations also demonstrate that the molecular exchange between these two structurally distinct subshells proceeds more slowly than the H-bond exchange between the two spectrally distinct H-bond configurations. We interpret this to indicate that orientational motions predominantly dictate the rate of H-bond exchange, while translational diffusion must occur to complete the molecular exchange between the two structurally distinct subshells around the perchlorate anions. The 2DIR measurements observe the H-bond exchange between the two spectrally distinct H-bond configurations, but the lifetime of the hydroxyl stretch precludes the observation of the slower molecular exchange. Our 2DIR experiments and CPMD simulations demonstrate that orientational motions predominantly equilibrate water molecules within their local solvation subshells, but the full molecular equilibration within the first solvation shell around the perchlorate anion necessitates translational motion.

Efficient Multiple Exciton Generation Observed in Colloidal PbSe Quantum Dots with Temporally and Spectrally Resolved Intraband Excitation

We have spectrally resolved the intraband transient absorption of photogenerated excitons to quantify the exciton population dynamics in colloidal PbSe quantum dots (QDs). These measurements demonstrate that the spectral distribution, as well as the amplitude, of the transient spectrum depends on the number of excitons excited in a QD. To accurately quantify the average number of excitons per QD, the transient spectrum must be spectrally integrated. With spectral integration, we observe efficient multiple exciton generation in colloidal PbSe QDs.

Orientational relaxation and vibrational excitation transfer in methanol–carbon tetrachloride solutions

Time and polarization resolved ultrafast infrared vibrational spectroscopy of the hydroxyl stretch of methanol dissolved in carbon tetrachloride has been utilized to investigate orientational relaxation and vibrational excitation transfer. The anisotropy decay of the deuterated hydroxyl stretch of methanol-d was measured in two solutions: Isotopically mixed 0.8 mol % methanol-d 23 mol % methanol-h in CCl4 and isotopically pure methanol-d at 26 mol % in CCl4. The anisotropy decay in the isotopically mixed methanol solution is a biexponential characterized by 1.7±0.7 ps and 17±3 ps time constants, with 40±10% of the decay occurring with the slower time constant. The biexponential anisotropy decay has been analyzed with a restricted orientational diffusion model that involves fast orientational diffusion within a cone of semi-angle θc, followed by slower, full orientational relaxation. The fast orientational relaxation occurs within a cone semi-angle of θc=45°±5°, with a diffusion coefficient of Dc−1=13±5 ps...

Ultrafast X-ray Auger probing of photoexcited molecular dynamics

Molecules can efficiently and selectively convert light energy into other degrees of freedom. Disentangling the underlying ultrafast motion of electrons and nuclei of the photoexcited molecule presents a challenge to current spectroscopic approaches. Here we explore the photoexcited dynamics of molecules by an interaction with an ultrafast X-ray pulse creating a highly localized core hole that decays via Auger emission. We discover that the Auger spectrum as a function of photoexcitation--X-ray-probe delay contains valuable information about the nuclear and electronic degrees of freedom from an element-specific point of view. For the nucleobase thymine, the oxygen Auger spectrum shifts towards high kinetic energies, resulting from a particular C-O bond stretch in the ππ* photoexcited state. A subsequent shift of the Auger spectrum towards lower kinetic energies displays the electronic relaxation of the initial photoexcited state within 200 fs. Ab-initio simulations reinforce our interpretation and indicate an electronic decay to the nπ* state.

Hydrogen bond breaking probed with multidimensional stimulated vibrational echo correlation spectroscopy

H bond population dynamics are extricated with exceptional detail using ultrafast (<50 fs) IR multidimensional stimulated vibrational echo correlation spectroscopy with full phase information and frequency resolved IR pump-probe expts. performed on the hydroxyl stretch of MeOH-OD oligomers in CCl4. H bond breaking makes it possible to acquire data for times much greater than the hydroxyl stretch vibrational lifetime. The correlation spectra and detailed calcns. demonstrate that vibrational relaxation leads to H bond breaking for oligomers that have hydroxyl stretch frequencies on the low energy (red) side of the hydroxyl stretch spectrum, the spectral region that is assocd. with the strongest H bonds. Frequency resolved pump-probe data support the conclusions drawn from the correlation spectra. Using a global fit to the pump-probe spectra, in conjunction with assignments made possible through the correlation spectra, the residual ground state and photoproduct of H bond breaking were prepd. near their thermal equil. distribution. The spectrum of the H bond breaking photoproduct and the residual ground state approach the steady-state temp. difference spectrum on the tens of picoseconds time scale, indicating the system thermalizes on this time scale. [on SciFinder(R)]

Ultrafast heterodyne detected infrared multidimensional vibrational stimulated echo studies of hydrogen bond dynamics

Multidimensional vibrational stimulated echo correlation spectra with full phase information are presented for the broad hydroxyl stretch band of methanol-OD oligomers in CCl4 using ultrashort ( 1 ps) shows that there is frequency correlation between the initially excited hydroxyl stretch and the frequency shifted hydroxyl stretch formed by hydrogen bond breaking.