Time-Resolved X-ray Emission Spectroscopy and Synthetic High-Spin Model Complexes Resolve Ambiguities in Excited-State Assignments of Transition-Metal Chromophores: A Case Study of Fe-Amido Complexes.

To fully harness the potential of abundant metal coordination complex photosensitizers, a detailed understanding of the molecular properties that dictate and control the electronic excited-state population dynamics initiated by light absorption is critical. In the absence of detectable luminescence, optical transient absorption (TA) spectroscopy is the most widely employed method for interpreting electron redistribution in such excited states, particularly for those with a charge-transfer character. The assignment of excited-state TA spectral features often relies on spectroelectrochemical measurements, where the transient absorption spectrum generated by a metal-to-ligand charge-transfer (MLCT) electronic excited state, for instance, can be approximated using steady-state spectra generated by electrochemical ligand reduction and metal oxidation and accounting for the loss of absorptions by the electronic ground state. However, the reliability of this approach can be clouded when multiple electronic configurations have similar optical signatures. Using a case study of Fe(II) complexes supported by benzannulated diarylamido ligands, we highlight an example of such an ambiguity and show how time-resolved X-ray emission spectroscopy (XES) measurements can reliably assign excited states from the perspective of the metal, particularly in conjunction with accurate synthetic models of ligand-field electronic excited states, leading to a reinterpretation of the long-lived excited state as a ligand-field metal-centered quintet state. A detailed analysis of the XES data on the long-lived excited state is presented, along with a discussion of the ultrafast dynamics following the photoexcitation of low-spin Fe(II)-Namido complexes using a high-spin ground-state analogue as a spectral model for the 5T2 excited state.

The Liquid Jet Endstation for Hard X-ray Scattering and Spectroscopy at the Linac Coherent Light Source.

The ability to study chemical dynamics on ultrafast timescales has greatly advanced with the introduction of X-ray free electron lasers (XFELs) providing short pulses of intense X-rays tailored to probe atomic structure and electronic configuration. Fully exploiting the full potential of XFELs requires specialized experimental endstations along with the development of techniques and methods to successfully carry out experiments. The liquid jet endstation (LJE) at the Linac Coherent Light Source (LCLS) has been developed to study photochemistry and biochemistry in solution systems using a combination of X-ray solution scattering (XSS), X-ray absorption spectroscopy (XAS), and X-ray emission spectroscopy (XES). The pump-probe setup utilizes an optical laser to excite the sample, which is subsequently probed by a hard X-ray pulse to resolve structural and electronic dynamics at their intrinsic femtosecond timescales. The LJE ensures reliable sample delivery to the X-ray interaction point via various liquid jets, enabling rapid replenishment of thin samples with millimolar concentrations and low sample volumes at the 120 Hz repetition rate of the LCLS beam. This paper provides a detailed description of the LJE design and of the techniques it enables, with an emphasis on the diagnostics required for real-time monitoring of the liquid jet and on the spatiotemporal overlap methods used to optimize the signal. Additionally, various scientific examples are discussed, highlighting the versatility of the LJE.

Terahertz-Driven Local Dipolar Correlation in a Quantum Paraelectric.

Light-induced ferroelectricity in quantum paraelectrics is a new avenue of achieving dynamic stabilization of hidden orders in quantum materials. In this Letter, we explore the possibility of driving a transient ferroelectric phase in the quantum paraelectric KTaO_{3} via intense terahertz excitation of the soft mode. We observe a long-lived relaxation in the terahertz-driven second harmonic generation (SHG) signal that lasts up to 20 ps at 10 K, which may be attributed to light-induced ferroelectricity. Through analyzing the terahertz-induced coherent soft-mode oscillation and finding its hardening with fluence well described by a single-well potential, we demonstrate that intense terahertz pulses up to 500 kV/cm cannot drive a global ferroelectric phase in KTaO_{3}. Instead, we find the unusual long-lived relaxation of the SHG signal comes from a terahertz-driven moderate dipolar correlation between the defect-induced local polar structures. We discuss the impact of our findings on current investigations of the terahertz-induced ferroelectric phase in quantum paraelectrics.

Enabling high repetition rate nonlinear THz science with a kilowatt-class sub-100 fs laser source.

Manipulating the atomic and electronic structure of matter with strong terahertz (THz) fields while probing the response with ultrafast pulses at x-ray free electron lasers (FELs) has offered unique insights into a multitude of physical phenomena in solid state and atomic physics. Recent upgrades of x-ray FEL facilities are pushing to much higher repetition rates, enabling unprecedented signal-to-noise ratio for pump probe experiments. This requires the development of suitable THz pump sources that are able to deliver intense pulses at compatible repetition rates. Here we present a high-power laser-driven THz source based on optical rectification in LiNbO3 using tilted pulse front pumping. Our source is driven by a kilowatt-level Yb:YAG amplifier system operating at 100 kHz repetition rate and employing nonlinear spectral broadening and recompression to achieve sub-100 fs pulses with pulse energies up to 7 mJ that are necessary for high THz conversion efficiency and peak field strength. We demonstrate a maximum of 144 mW average THz power (1.44 µJ pulse energy), consisting of single-cycle pulses centered at 0.6 THz with a peak electric field strength exceeding 150 kV/cm. These high field pulses open up a range of possibilities for nonlinear time-resolved THz experiments at unprecedented rates.

Reorientation-induced Stokes shifts caused by directional interactions in electronic spectroscopy: Fast dynamics of poly(methyl methacrylate).

Dynamic Stokes shift measurements report on structural relaxation, driven by a dipole created in a chromophore by its excitation from the ground electronic state to the S1 state. Here, we demonstrate that it is also possible to have an additional contribution from orientational relaxation of the Stokes shift chromophore. This effect, called reorientation-induced Stokes shift (RISS), can be observed when the reorientation of the chromophore and the solvent structural relaxation occur on similar time scales. Through a vector interaction, the electronic transition of the chromophore couples to its environment. The orientational diffusive motions of the chromophores will have a slight bias toward reducing the transition energy (red shift) as do the solvent structural diffusive motions. RISS is manifested in the polarization-dependence of the fluorescence Stokes shift using coumarin 153 (C153) in poly(methyl methacrylate) (PMMA). A similar phenomenon, reorientation-induced spectral diffusion (RISD), has been observed and theoretically explicated in the context of two dimensional infrared (2D IR) experiments. Here, we generalize the existing RISD theory to include properties of electronic transitions that generally are not present in vibrational transitions. Expressions are derived that permit determination of the structural dynamics by accounting for the RISS contributions. Using these generalized equations, the structural dynamics of the medium can be measured for any system in which the directional interaction is well represented by a first order Stark effect and RISS or RISD is observed. The theoretical results are applied to the PMMA data, and the structural dynamics are obtained and discussed.

Water Dynamics in Polyacrylamide Hydrogels.

Polymeric hydrogels have wide applications including electrophoresis, biocompatible materials, water superadsorbents, and contact lenses. The properties of hydrogels involve the poorly characterized molecular dynamics of water and solutes trapped within the three-dimensional cross-linked polymer networks. Here we apply ultrafast two-dimensional infrared (2D IR) vibrational echo and polarization-selective pump-probe (PSPP) spectroscopies to investigate the ultrafast molecular dynamics of water and a small molecular anion solute, selenocyanate (SeCN-), in polyacrylamide hydrogels. For all mass concentrations of polymer studied (5% and above), the hydrogen-bonding network reorganization (spectral diffusion) dynamics and reorientation dynamics reported by both water and SeCN- solvated by water are significantly slower than in bulk water. As the polymer mass concentration increases, molecular dynamics in the hydrogels slow further. The magnitudes of the slowing, measured with both water and SeCN-, are similar. However, the entire hydrogen-bonding network of water molecules appears to slow down as a single ensemble, without a difference between the core water population and the interface water population at the polymer-water surface. In contrast, the dissolved SeCN- do exhibit two-component dynamics, where the major component is assigned to the anions fully solvated in the confined water nanopools. The slower component has a small amplitude which is correlated with the polymer mass concentration and is assigned to adsorbed anions strongly interacting with the polymer fiber networks.

Carbon dioxide in an ionic liquid: Structural and rotational dynamics.

Ionic liquids (ILs), which have widely tunable structural motifs and intermolecular interactions with solutes, have been proposed as possible carbon capture media. To inform the choice of an optimal ionic liquid system, it can be useful to understand the details of dynamics and interactions on fundamental time scales (femtoseconds to picoseconds) of dissolved gases, particularly carbon dioxide (CO2), within the complex solvation structures present in these uniquely organized materials. The rotational and local structural fluctuation dynamics of CO2 in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf2) were investigated by using ultrafast infrared spectroscopy to interrogate the CO2 asymmetric stretch. Polarization-selective pump probe measurements yielded the orientational correlation function of the CO2 vibrational transition dipole. It was found that reorientation of the carbon dioxide occurs on 3 time scales: 0.91 ± 0.03, 8.3 ± 0.1, 54 ± 1 ps. The initial two are attributed to restricted wobbling motions originating from a gating of CO2 motions by the IL cations and anions. The final (slowest) decay corresponds to complete orientational randomization. Two-dimensional infrared vibrational echo (2D IR) spectroscopy provided information on structural rearrangements, which cause spectral diffusion, through the time dependence of the 2D line shape. Analysis of the time-dependent 2D IR spectra yields the frequency-frequency correlation function (FFCF). Polarization-selective 2D IR experiments conducted on the CO2 asymmetric stretch in the parallel- and perpendicular-pumped geometries yield significantly different FFCFs due to a phenomenon known as reorientation-induced spectral diffusion (RISD), revealing strong vector interactions with the liquid structures that evolve slowly on the (independently measured) rotation time scales. To separate the RISD contribution to the FFCF from the structural spectral diffusion contribution, the previously developed first order Stark effect RISD model is reformulated to describe the second order (quadratic) Stark effect--the first order Stark effect vanishes because CO2 does not have a permanent dipole moment. Through this analysis, we characterize the structural fluctuations of CO2 in the ionic liquid solvation environment, which separate into magnitude-only and combined magnitude and directional correlations of the liquid's time dependent electric field. This new methodology will enable highly incisive comparisons between CO2 dynamics in a variety of ionic liquid systems.

Separation of experimental 2D IR frequency-frequency correlation functions into structural and reorientation-induced contributions.

A vibrational transition frequency can couple to its environment through a directional vector interaction. In such cases, reorientation of the vibrational transition dipole (molecular orientational relaxation) and its frequency fluctuations can be strongly coupled. It was recently shown [Kramer et al., J. Chem. Phys. 142, 184505 (2015)] that differing frequency-frequency correlation function (FFCF) decays, due to reorientation-induced spectral diffusion (RISD), are observed with different two-dimensional infrared polarization configurations when such strong coupling is present. The FFC functional forms were derived for the situation in which all spectral diffusion is due to reorientational motion. We extend the previous theory to include vibrational frequency evolution (spectral diffusion) caused by structural fluctuations of the medium. Model systems with diffusive reorientation and several regimes of structural spectral diffusion rates are analyzed for first order Stark effect interactions. Additionally, the transition dipole reorientational motion in complex environments is frequently not completely diffusive. Several periods of restricted angular motion (wobbling-in-a-cone) may precede the final diffusive orientational randomization. The polarization-weighted FFCF decays are presented in this case of restricted transition dipole wobbling. With these extensions to the polarization-dependent FFCF expressions, the structural spectral diffusion dynamics of methanol in the room temperature ionic liquid 1-hexyl-3-methylimidazolium hexafluorophosphate can be separated quantitatively from RISD using the experimental center line slope data. In addition, prior results on the spectral diffusion of water, methanol, and ethanol in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide are re-examined to elucidate the influence of reorientation on the data, which were interpreted in terms of structural fluctuations.

Dynamics of water, methanol, and ethanol in a room temperature ionic liquid.

The dynamics of a series of small molecule probes with increasing alkyl chain length: water, methanol, and ethanol, diluted to low concentration in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, was investigated with 2D infrared vibrational echo (2D IR) spectroscopy and polarization resolved pump-probe (PP) experiments on the deuterated hydroxyl (O-D) stretching mode of each of the solutes. The long timescale spectral diffusion observed by 2D IR, capturing complete loss of vibrational frequency correlation through structural fluctuation of the medium, shows a clear but not dramatic slowing as the probe alkyl chain length is increased: 23 ps for water, 28 ps for methanol, and 34 ps for ethanol. Although in each case, only a single population of hydroxyl oscillators contributes to the infrared line shapes, the isotropic pump-probe decays (normally caused by population relaxation) are markedly nonexponential at short times. The early time features correspond to the timescales of the fast spectral diffusion measured with 2D IR. These fast isotropic pump-probe decays are produced by unequal pumping of the OD absorption band to a nonequilibrium frequency dependent population distribution caused by significant non-Condon effects. Orientational correlation functions for these three systems, obtained from pump-probe anisotropy decays, display several periods of restricted angular motion (wobbling-in-a-cone) followed by complete orientational randomization. The cone half-angles, which characterize the angular potential, become larger as the experimental frequency moves to the blue. These results indicate weakening of the angular potential with decreasing hydrogen bond strength. The slowest components of the orientational anisotropy decays are frequency-independent and correspond to the complete orientational randomization of the solute molecule. These components slow appreciably with increasing chain length: 25 ps for water, 42 ps for methanol, and 88 ps for ethanol. The shape and volume of the probe, therefore, impact reorientation far more severely than they do spectral diffusion at long times, though these two processes occur on similar timescales at earlier times.

Observation and theory of reorientation-induced spectral diffusion in polarization-selective 2D IR spectroscopy.

In nearly all applications of ultrafast multidimensional infrared spectroscopy, the spectral degrees of freedom (e.g., transition frequency) and the orientation of the transition dipole are assumed to be decoupled. We present experimental results which confirm that frequency fluctuations can be caused by rotational motion and observed under appropriate conditions. A theory of the frequency-frequency correlation function (FFCF) observable under various polarization conditions is introduced, and model calculations are found to reproduce the qualitative trends in FFCF rates. The FFCF determined with polarization-selective two-dimensional infrared (2D IR) spectroscopy is a direct reporter of the frequency-rotational coupling. For the solute methanol in a room temperature ionic liquid, the FFCF of the hydroxyl (O-D) stretch decays due to spectral diffusion with different rates depending on the polarization of the excitation pulses. The 2D IR vibrational echo pulse sequence consists of three excitation pulses that generate the vibrational echo, a fourth pulse. A faster FFCF decay is observed when the first two excitation pulses are polarized perpendicular to the third pulse and the echo, 〈XXY Y〉, than in the standard all parallel configuration, 〈XXXX〉, in which all four pulses have the same polarization. The 2D IR experiment with polarizations 〈XY XY〉 ("polarization grating" configuration) gives a FFCF that decays even more slowly than in the 〈XXXX〉 configuration. Polarization-selective 2D IR spectra of bulk water do not exhibit polarization-dependent FFCF decays; spectral diffusion is effectively decoupled from reorientation in the water system.

Bulk-like and Interfacial Water Dynamics in Nafion Fuel Cell Membranes Investigated with Ultrafast Nonlinear IR Spectroscopy.

The water confined in the hydrophilic domains of Nafion fuel cell membranes is central to its primary function of ion transport. Water dynamics are intimately linked to proton transfer and are sensitive to the structural features and length scales of confinement. Here, ultrafast polarization-selective pump-probe and two-dimensional infrared vibrational echo (2D IR) experiments were performed on fully hydrated Nafion membranes with sodium counterions to explicate the water dynamics. Like aerosol-OT reverse micelles (AOT RMs), the water dynamics in Nafion are attributed to bulk-like core water in the central region of the hydrophilic domains and much slower interfacial water. Population and orientational dynamics of water in Nafion are slowed by polymer confinement. Comparison of the observed dynamics to those of AOT RMs helps identify local interactions between water and sulfonate anions at the interface and among water molecules in the core. This comparison also demonstrates that the well-known spherical cluster morphology of Nafion is not appropriate. Spectral diffusion of the interfacial water, which arises from structural dynamics, was obtained from the 2D IR experiments taking the core water to have dynamics similar to bulk water. Like the orientational dynamics, spectral diffusion was found to be much slower at the interface compared to bulk water. Together, the dynamics indicate slow reorganization of weakly hydrogen-bonded water molecules at the interface of Nafion. These results provide insights into proton transport mechanisms in fuel cell membranes, and more generally, water dynamics near the interface of confining systems.

Ionic Liquid versus Li(+) Aqueous Solutions: Water Dynamics near Bistriflimide Anions.

The ultrafast dynamics of concentrated aqueous solutions of the salt lithium bistriflimide and ionic liquid (IL) 1-ethyl-3-methylimidazolium bistriflimide was studied using two-dimensional infrared (2D IR) vibrational echo and polarization-selective IR pump-probe techniques to monitor water's hydroxyl stretch. Two distinct populations of hydroxyl groups, with differing vibrational lifetimes, are detected in solution: those engaged in hydrogen bonding with other water molecules and those engaged in hydrogen bonding with the bistriflimide anion. Water molecules with the same hydrogen bond partner exhibit similar vibrational lifetimes in the two solutions. The reorientation dynamics of the anion-associated waters is also similar in form in the two solutions, showing a restricted wobbling-in-a-cone motion followed by a slower diffusive orientational randomization. However, the wobbling motions are much more angularly restricted in the IL solution. Spectral diffusion dynamics, which tracks the structural fluctuations of water's hydrogen bonds, is very different in the two solutions. Water in the IL solution experiences much faster fluctuations overall and shows a greater extent of motional narrowing, resulting in a larger homogeneously broadened component in the spectral line, compared to those in the aqueous lithium salt. Thus, even when the hydroxyls of water associate with the same anion in solution, the cation identity and extent of ionic ordering (i.e., salt solution vs IL) can play an important role in determining the structural fluctuations experienced by a small hydrogen-bonded solute.

Water Dynamics in 1-Alkyl-3-methylimidazolium Tetrafluoroborate Ionic Liquids.

The effects of water concentration and varying alkyl chain length on the dynamics of water in 1-alkyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) were characterized using two-dimensional infrared (2D IR) vibrational echo spectroscopy and polarization-selective IR pump-probe experiments to study the water hydroxyl (OD) stretching mode of dilute HOD in H2O. Three imidazolium cation alkyl chain lengths, ethyl (Emim+), butyl (Bmim+), and decyl (Dmim+), were investigated. Both Bmim+ and Dmim+ cations have sufficiently long chains that the liquids exhibit polar-apolar segregation, whereas the Emim+ IL has no significant apolar aggregation. Although the OD absorption spectra are independent of the chain length, the measured reorientation and spectral diffusion dynamics are chain length dependent and tend to slow when the alkyl chain is long enough for polar-apolar segregation. As the water concentration is increased, a water-associated water population forms, absorbing in a new spectral region red-shifted from the isolated, anion-associated, water population. Furthermore, the anion-associated water dynamics are accelerated. At sufficiently high water concentrations, water in all of the RTILs experiences similar dynamics, the solvent structures having been fluidized by the addition of water. The water concentration at which the dilute water dynamics changes to fluidized dynamics depends on the alkyl chain length, which determines the extent and ordering of the apolar regions. Increases in both water concentration and alkyl chain length serve to modify the ordering of the RTIL, but with opposite and competing effects on the dissolved water dynamics.

Coupling of Carbon Dioxide Stretch and Bend Vibrations Reveals Thermal Population Dynamics in an Ionic Liquid.

The population relaxation of carbon dioxide dissolved in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf2) was investigated using polarization-selective ultrafast infrared pump-probe spectroscopy and two-dimensional infrared (2D IR) spectroscopy. Due to the coupling of the bend with the asymmetric stretch, excitation of the asymmetric stretch of a molecule with a thermally populated bend leads to an additional peak, a hot band, which is red-shifted from the main asymmetric absorption band by the combination band shift. This hot band peak exchanges population with the main peak through the gain and loss of bend excitation quanta. The isotropic pump-probe signal originating from the unexcited bend state displays a fast, relatively small amplitude, initial growth followed by a longer time scale exponential decay. The signal is analyzed over its full time range using a kinetic model to determine both the vibrational lifetime (the final decay) and rate constant for the loss of the bend energy. This bend relaxation manifests as the fast initial growth of the stretch/no bend signal because the hot band (stretch with bend) is "over pumped" relative to the ground state equilibrium. The nonequilibrium pumping occurs because the hot band has a larger transition dipole moment than the stretch/no bend peak. The system is then prepared, utilizing an acousto-optic mid-infrared pulse shaper to cut a hole in the excitation pulse spectrum, such that the hot band is not pumped. The isotropic pump-probe signal from the stretch/no bend state is altered because the initial excited state population ratio has changed. Instead of a growth due to relaxation of bend quanta, a fast initial decay is observed because of thermal excitation of the bend. Fitting this curve gives the rate constant for thermal excitation of the bend and the lifetime, which agree with those determined in the pump-probe experiments without frequency-selective pumping.

Structural and Rotational Dynamics of Carbon Dioxide in 1-Alkyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquids: The Effect of Chain Length.

Ionic liquids (ILs) have been proposed as possible carbon dioxide (CO2) capture media; thus, it is useful to understand the dynamics of both the dissolved gas and its IL environment as well as how altering an IL affects these dynamics. With increasing alkyl chain length, it is well-established that ILs obtain a mesoscopic structural feature assigned to polar-apolar segregation, and the change in structure with chain length affects the dynamics. Here, the dynamics of CO2 in a series of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ILs, in which the alkyl group is ethyl, butyl, hexyl, or decyl, were investigated using ultrafast infrared spectroscopy by measuring the reorientation and spectral diffusion of carbon dioxide in the ILs. It was found that reorientation of the carbon dioxide occurs on three time scales, which correspond to two different time scales of restricted wobbling-in-a-cone motions and a long-time complete diffusive reorientation. Complete reorientation slows with increasing chain length but less than the increases in viscosity of the bulk liquids. Spectral diffusion, measured with two-dimensional IR spectroscopy, is caused by a combination of the liquids' structural fluctuations and reorientation of the CO2. The data were analyzed using a recent theory that takes into account both contributions to spectral diffusion and extracts the structural spectral diffusion. Different components of the structural fluctuations have distinct dependences on the alkyl chain length. All of the dynamics are fast compared to the complete orientational randomization of the bulk ILs, as measured with optical heterodyne-detected optical Kerr effect measurements. The results indicate a hierarchy of constraint releases in the liquids that give rise to increasingly slower dynamics.

Dynamics of dihydrogen bonding in aqueous solutions of sodium borohydride.

Dihydrogen bonding occurs between protonic and hydridic hydrogens which are bound to the corresponding electron withdrawing or donating groups. This type of interaction can lead to novel reactivity and dynamic behavior. This paper examines the dynamics experienced by both borohydride and its dihydrogen-bound water solvent using 2D-IR vibrational echo and IR pump-probe spectroscopies, as well as FT-IR linear absorption experiments. Experiments are conducted on the triply degenerate B-H stretching mode and the O-D stretch of dilute HOD in the water solvent. While the B-H stretch absorption is well separated from the broad absorption band of the OD of HOD in the bulk of the water solution, the absorption of the ODs hydrogen bonded to BHs overlaps substantially with the absorption of ODs in the bulk H2O solution. A subtraction technique is used to separate out the anion-associated OD dynamics from that of the bulk solution. It is found that both the water and borohydride undergo similar spectral diffusion dynamics, and these are very similar to those of HOD in bulk water. Because the B-H stretch is triply degenerate, the IR pump-probe anisotropy decays very rapidly, but the decay is not caused by the physical reorientation of the BH4⁻ anions. Spectral diffusion occurs on a time scale longer than the anisotropy decay, demonstrating that spectral diffusion is not yet complete even when the transition dipole has completely randomized. To prevent chemical decomposition of the BH4⁻, 1 M NaOH was added to stabilize the system. 2D-IR experiments on the OD stretch of HOD in the NaOH/water liquid (no borohydride) show that the NaOH has a negligible effect on the bulk water dynamics.