In an effort to find alternative energy sources to fossil fuels, there is much interest in coupling water oxidation and CO₂ reduction reactions to create "solar fuels". Through water oxidation, the following reaction occurs: 2H₂O → 4H⁺ + 4e⁻ + O₂. The de facto representative of homogenous mononuclear water oxidation catalysts is [Ru^II(bpy)(tpy)(H₂O)]²⁺, or more simply [Ru^IIH₂O]²⁺. For this catalyst, the rate limiting step and reaction bottleneck is the O-O bond formation step, which proceeds through a water nucleophilic attack of the electrophilic Ru=O bond. The two intermediates that can potentially undergo this water nucleophilic attack to form the O-O bond are [Ru^IV=O]²⁺ and [Ru^V=O]³⁺. Kinetic evidence and our calculations suggest that the [Ru^IV=O]²⁺ intermediate is significantly less reactive with H₂O than the [Ru^V=O]³⁺ species, which may be due to the Ru=O bond being less electrophilic in the [Ru^IV=O]²⁺ species than the [Ru^V=O]³⁺ species. Experimentally, we form these highly reactive intermediates in an octopole ion trap by introducing a buffer gas mix of O₃/O₂ to [Ru^II]²⁺ or [Ru^III]³⁺, and then capture them by evaporative quenching of collision complexes. This has allowed us to isolate the [Ru^IV=O]²⁺ species and will allow us to later focus on isolating and probing [Ru^V=O]³⁺ using cryogenic ion IR predissociation spectroscopy. These predissociation spectra allow us to determine the Ru=O stretching frequencies of these two intermediates, which can be confirmed through isotopically labeled ¹⁸O substitution. In particular, the frequency of the Ru=O stretching mode is likely sensitive to the electronic structure of the Ru=O bond. Also, with the addition of our dual reaction traps, we will cluster water on [Ru^IV=O]²⁺ and [Ru^V=O]³⁺ in order to elucidate the water bonding orientation and arrangement, which will reveal if the water acts as a nucleophile, or if it is the Ru=O that is the electrophile in the water addition step.

Multidimensional nonlinear spectroscopic methods are powerful tools to investigate ultrafast vibrational dynamics and coherences. Several techniques that have recently been applied to excited electronic states require three or more ultrashort pulses to measure the nonlinear optical response. The additional beamsplitters, mirrors and optical delay stages in these techniques complicates their application in the extreme ultraviolet and X-ray regime, where reflectance is low except at grazing incidence. Optical Kerr effect spectroscopy is a traditional two-pulse technique to measure the 3^rd-order nonlinear optical susceptibility of molecules in their ground electronic state. Ultrafast transient polarization spectroscopy (UTPS) is an extension of this technique to excited electronic states, enabling coupled electronic and nuclear dynamics to be investigated by the 3^rd-order nonlinear optical susceptibility. In UTPS, a pump pulse excites a population, then a Kerr gate and probe pulse monitor the 3^rd-order nonlinear response of excited-state molecules, as a function of delay between each of the three pulses. Two-photon absorption of femtosecond pump pulses populate the S₁ excited-state of nitrobenzene. We were able to extract a dephasing time that oscillates as function of the time delay after excitation. This dephasing time and supporting theoretical calculation allows for distinguishing between an intersystem crossing conical intersection and an internal conversion conical intersection on the same excited-state potential energy surface.

Emission and excitation spectra of the coordination polymers containing trivalent lanthanides such as Sm³⁺, Eu³⁺, or Tb³⁺ as the coordination center with cyclohexanedicarboxylate (CHDC, C₈H₁₀O₄) ligands were recorded at ambient temperature. Time-resolved emission spectra yield the emission rising and decay curves that reveal the energy transfer mechanisms in these luminescent crystals. The temporal data were successfully analyzed by kinetics models, and the corresponding energy transfer rates were determined. Based upon our results, the energy transfer mechanisms and efficiency can be outlined. In addition, we recently observed a few blue-shift signals in the emission spectra of these coordination polymers. Interestingly, different trivalent lanthanides exhibit profoundly different blue-shift signals. The blue-shift signals possibly arise from some energy transfer processes between the trivalent lanthanide coordination center and the CHDC ligands. The assignments and formation mechanisms of these blue-shift signals are under study. Details of our recent progress will be presented.

Designing earth-abundant photosensitizers to replace rare-earth complexes is of high importance for the efficient collection and utilization of solar energy. The primary obstacle to using first-row transition metal complexes is the ultrafast dissipation of collected energy due to low energy metal-centered states. Here, we study a series of Fe(II) complexes with different ligand frameworks in hopes to alter the intersection between various metal-centered states. Using femtosecond M-edge XANES, the involvement of triplet and quintet metal-centered states are identified. By decoupling the axial and equatorial stretching modes of an Fe(II) complex, we find that the lifetime of a triplet metal-centered state is significantly increased due to Jahn-Teller distortion in the excited state. This behavior is not observed in prototypical Fe(II) polypyridyl complexes, and provides a method to reduce energy losses in earth-abundant photosensitizers.

Photoinjection studies of two wide bandgap semiconductors, 4H-SiC and GaN, are conducted with fs infrared reflectivity measurements in order to characterize how the relative penetration depths of UV-pump/IR-probe light affects the modulation of the IR reflectivity spectrum. The infrared spectrum of these materials is dominated by the high reflectivity reststrahlen band region that occurs between the longitudinal optical and transverse optical phonons. The injection of free carriers shifts this metal-like region to higher frequencies via coupling of the longitudinal optical phonon to the free carrier plasma (LOPC effect). The result of this LOPC active tuning is strongly perturbed by the charge carrier spatial distribution, and is thus highly sensitive to the means of carrier generation. We probe the effects of charge carrier spatial distribution on the photomodulated reflectivity of two promising wide bandgap semiconductors, indirect bandgap 4H-SiC and direct bandgap GaN, by comparing the transient reflectivity following photoexcitation with light of short and long penetration depths relative to the IR probe depth. This work shows sensitivity of bulk electronic properties to the charge carrier distribution that is critical to understanding the contributions of these materials to complex nanophotonic devices.

In the past few years, the Suits group has successfully adapted the chirped-pulsed rotational spectroscopy technique to study reaction dynamics. A clear advantage of this method is its ability to simultaneously detect the appearance/disappearance of products/reactants of reactions within the frequency range of a single chirp. However, kinetic studies require a well-known, uniform density and temperature environment throughout the course of the reaction. Our Chirped Pulse Uniform Flow (CPUF) spectrometer achieves these conditions by the uniform supersonic flow produced from a Laval nozzle expansion following the pioneering CRESU technique developed in France. However, high densities in the uniform flow (10¹⁴ – 10¹⁷ molecules/cm³) can attenuate the molecular coherence through collisions and therefore limit the MW/mmW spectroscopic detection. To overcome this, we adapted an airfoil sampling technique in which the reaction takes place in the uniform supersonic flow and then expands into a cold, low-density sampling region that is optimal for MW/mmW spectroscopic detection. This airfoil sampling CPUF spectrometer, in the 70 – 90 GHz range, has been used to study the low-temperature kinetics of bimolecular reactions and preliminary results will be presented. Initiated by a pulsed photolysis laser at 193 nm, appearance of products/disappearance of reactants have been probed as a function of time with a 5 μs resolution. The data were fitted to pseudo first-order or second-order rate equations to determine the rate constants for chemical reactions.

Molecules excited by strong-field laser pulses evolve through coupled nuclear and electronic motion over ultrafast timescales. Even simple molecules demonstrate a variety of characteristic dynamics, including vibrations, fragmentation, and ionization. The final products and their relative yields depend sensitively on details of these coupled processes that occur within the first few femtoseconds of interacting with the laser beam. Here, ultrafast pump-probe spectroscopy is applied to study the fragmentation and ionization dynamics of several molecules. Photodissociation of CO₂ proceeds via multiple pathways depending on available energy, but commonly into CO and O. Multiphoton excitation from a 35 fs pump laser impulsively transfers CO₂ from the ground to 1st excited state, preparing a bending vibrational wavepacket that influences its dissociation. Using mass spectrometry, we report changes in the fragmentation pattern as a function of time delay between the pump and probe laser pulses that reflect the vibrational motion. At well-defined time delays the dissociation oscillates between observable CO⁺ and O₂⁺ fragments. The state relaxes to the ground state through a conical intersection. Similar control over the fragmentation pathway and ion yields will be demonstrated for the excited state motions of ethanol and formic acid.

Alcohols are common additive in fuels due to their high-octane content that allows higher compression ratio to be achieved. While other alcohols such as ethanol and the butanols have been extensively studied, n-propanol and isopropanol have received far less attention in the literature. A possible reason for this is that resolving the chemistry of the propanols and their radical pyrolytic products requires overcoming significant experimental and theoretical challenges, especially at the high temperatures involved for combustion. In order to help resolve the chemistry of these alcohols for combustion conditions, we studied the pyrolysis of both n-propanol and isopropanol under varying pyrolytic conditions using experimental and theoretical approaches in tandem. The decomposition pathways of n-propanol and isopropanol were experimentally probed using a silicon carbide microreactor, which is resistively heated to temperatures ranging over 300 - 1500 K. Speciation products were detected using a table-top line-tunable VUV light source in combination with a Photoionization Mass Spectrometer, enabling selective ionization followed by detection of ionization masses. Owing to the availability of multiple spectra lines, this tool allows us to selectively ionize acetaldehyde and vinyl alcohol at their corresponding ionization energies to separate the contribution of the two species with equal mass-to-charge ratio in isopropanol. To further confirm experiment, we employed different levels of electronic structure theory-Density Functional and Coupled Cluster theory- in combination of various basis sets to accurately compute the potential energy surfaces and thermal chemical properties of the pyrolytic products and the associated transition states. The potential energy surface calculations, along with the associated predicted ro-vibrational frequencies, rotors, etc., were employed to calculate rate constants and branching ratios for reaction to confirm the experimentally observed mechanisms for n-propanol and isopropanol.

Called “the miracle material”(D.N. Payne, SPIE 2020), silica is the one of the new generation of materials used for optical fiber, as nanoparticles for medical purposes in cancer treatment and for mask repair and production, “the holy grail” of the electronics industry. We have used Si(OEt)₄ or TEOS to deposit silica structure on a surface using the method of Focused Electron Beam Induced Deposition (FEBID), whilst also undertaking a thorough analysis of TEOS electron induced fragmentation using velocity map imaging. Experiments are combined with theoretical calculations of electron interactions with TEOS and models of the FEBID process. The electron beam energy range we used was 0 - 25eV. The present research is part of an ITN wider research project, ELENA (www.elena.hi.is) exploring the dynamics of FEBID and Extreme Ultraviolet Lithography and their development as a methodology for building nanostructures. ELENA is an Initial Training Network funded by the European Union Horizon 2020 program Grant number 722149.

The CPUF (Chirped Pulse in Uniform Flow) technique has been used previously in pulsed CRESU (Reaction Kinetics in Uniform Supersonic Flow) conditions to observe chemical reactions at low temperatures. We have adapted the technique to the continuous CRESU flows available in Rennes, and have observed the products of photolysis and chemical reactions using a chirped pulse Fourier transform millimeter wave spectrometer. We have characterized the flow conditions suitable for observing products of reactions and provide limits to the performance of these systems. In particular, pressure broadening is found to dominate these measurements, so steps had to be taken to resolve as much of the free induction decays as possible. We also observe the products of chemical reactions, particularly of CN radicals with hydrocarbons. The behavior of these products in CRESU environments as well as the results from these studies will be given and the application to observing the branching ratios of chemical reactions will be discussed.

Extreme ultraviolet (XUV) absorption spectroscopy, paired with few-femtosecond temporal resolution, offers a unique capacity to studying the driving mechanics of fundamental chemical systems such as ionization, fragmentation, and relaxation. The generation of XUV light using high harmonic generation in a table-top setting provides the ability to observe transitions from localized core shells to delocalized valence shells to glean information about their larger molecular systems (e.g. oxidation state, spin state, magnetic quantum number, and local bonding environment). Organic halides (R-F, Cl, Br, I) are both naturally occurring and synthetic molecules used in widely in industries such as drug delivery and crop production. The use of heavier halogens (bromine and iodine) make them ideal molecules for studying electronic and vibrational dynamics of small organics using XUV absorption spectroscopy as their core-valence transitions are accessible with XUV spectrometer. This presentation will cover both experimental and computational results of few-femtosecond dynamics in organic halides and how these results inform larger processes.