Core-level spectroscopies enable element specific measurements of electronic and vibrational dynamics on the femtosecond timescale. Advancements in the generation of XUV light through high harmonic generation (HHG) has opened the door to doing such measurements in-house; we are currently in the commissioning phase of the X-FAST instrument at the University of Wisconsin - Madison. Here, we describe the instrument and show the initial capabilities and studies carried out using the X-FAST instrument. The instrument is comprised of two paths, an optical pump in air and an XUV probe in vacuum. We used this system to carry out two preliminary experiments: a study of the kinetics and dynamics of transition-metal perovskite oxides, and a collection of static transient absorption spectra of a metallic shape-memory alloy, Ni₂MnGa.

We present a table-top instrument capable of ultrafast time-resolved x-ray absorption spectroscopy (TR-XAS) and emission spectroscopy (TR-XES). A laser plasma source provides better than 6 ps time resolution, while superconducting transition-edge sensor (TES) microcalorimeters are used for efficient, broadband, high-energy resolution x-ray detection. With TR-XAS, we have studied the photoreduction of ferrioxalate, supporting a framework in which the reduction of the central ion is complete within 100 ps and contradicting a theory in which the photoreduction occurs on much longer timescales. With TR-XES, we have studied spin cross-over in photoexcited iron tris-bipyridine and accurately measured the lifetime of the quintet state from simultaneous observation of Kα and Kβ features. The x-ray source and detector are currently being upgrading to enable new capabilities and probe chemical dynamics with increased time and energy resolution. Upgrades to the laser plasma source will result in a 10-40x improvement in the x-ray flux and time resolution on the order of 100s of fs. The energy resolving TES microcalorimeter will also be upgraded to achieve a 2x improvement in the energy resolution. The end result will be a laboratory tool capable of ultrafast time-resolved x-ray spectroscopy, with time resolution previously achievable only at free electron lasers.

Electron transfer induced by the optical pumping of systems leads to the formation of transient species that can be observed by time-resolved optical spectroscopy. However traditional linear spectroscopy cannot detect the formation of transient magnetic species formed by spin-polarized pump excitation of electrons. These transient magnetic species can be long-lived and have a wide variety of applications in spintronics and photocatalysis. In this talk, we utilize time-resolved XUV magnetic circular dichroism spectroscopy to detect transient magnetic states in Yttrium Iron Garnet (Y₃Fe₅O₁₂, YIG). This method provides direct observation of electron dynamics at the surfaces with element and spin state resolution. YIG is a ferrimagnetic semiconductor with two different lattices of tetrahedral and octahedral Fe(III) atoms with opposite spins. The measurements on YIG show lattice-dependent electron dynamics upon photoexcitation. When excited above the bandgap, a charge transfer occurs from oxygen to iron atoms. Depending on the lattice of iron where the electron is excited, the dynamics of these charge transfer electron varies. The electrons in the octahedral iron form a small polaron at the surface. The tetrahedral electrons are more mobile and diffuse from surface to bulk forming a natural spin filter. This filtering of spin depending on the lattice causes the surface to be spin-polarized. This spin-polarized electron localizes in the oxygen and octahedral iron atoms on the surface due to exchange interaction and can be observed with XUV-MCD Spectroscopy.

Cluster research offers doorways to establish connections between the macroscopic features of solutions- and condensed phase materials and the properties of their microscopic constituents. Apart from studying the solvation of diverse molecules and the properties of solvated electrons, clusters have also been employed to explore the mechanisms of intra-cluster charge transfer processes. In electron attachment processes, upon photoexcitation of the cluster or ion-molecule complex, the excess electron in ion can be transferred to the molecule. Numerous previous studies have shown that I⁻ is a proficient intra-cluster donor. Here, a new apparatus for ultrafast photofragmentation studies is described for investigating the charge transfer process and subsequently detecting the fragments from the photoexcitation of iodide-molecule clusters. Ions and ion-clusters are produced by perpendicularly oriented pulsed gas jet and continuous electron beam, accelerated and separated by a time-of-flight mass spectrometer, and then size-selected (iodide-molecule clusters) by a mass filter. The iodide molecule clusters are focused into the interaction region and are excited by UV laser pulse propagating perpendicularly. A linear-plus-quadratic (LPQ) reflectron capable of operating over a large voltage range is used to reverse the trajectories of the anions and mass-resolve the anion fragments and parent anion. The instrument can detect both charged and neutral fragments simultaneously. To test the performance of the instrument, CF₃ I.I⁻ is photoexcited by 266 nm UV laser pulse. Anion fragments I⁻ and CF₃ I⁻ are detected and identified with neutral fragments. Detection of CF₃ I⁻ provides evidence of the charge transfer inside the CF₃ I.I⁻ cluster when it is excited by 4.66 eV photons and implies the existence of charge-transfer excited state of CF₃ I.I⁻.

Molecular ions and radicals are important in astrochemical processes, but difficult to study in the lab. This talk will present benchmarking experiments on the production of protonated species of astrochemical relevance in the interstellar medium. We will be presenting on two apparatus. The first of these is a hollow cathode based on the designs of Amano and coworkers. Here, the precursor species and hydrogen are flowed through a plasma discharge to produce protonated species. In the other, hydrogen is converted to H₃⁺ and then combined with the precursor gas. In both of these experiments, we use a long path length to increase absorbance, increasing the signal-to-noise ratio of the spectra.

The effective atmospheric absorption of the earth atmosphere within its 10 μm transparency window is crucial for radiative transfer calculations. The contribution of the water continuum in this region is still under discussion. The proposal of an updated value at 1184 cm⁻¹should increase the accuracy of the semi-empirical MTCKD model, which is incorporated in radiative transfer codes. For this purpose, we used a newly developed spectrometer combining a quantum cascade laser (QCL) with a high finesse linear cavity, working around 8.45 μm. This setup takes benefit of optical feedback from an optical cavity and constitutes a Optical-Feedback-Cavity Ring Down Spectrometer (OF-CRDS). We will present the behavior of the laser in such a configuration and demonstrate the sharpening of the QCL emission profile under optical feedback from a TEM₀₀ resonant mode of a linear cavity. Absorption spectra are measured using the CRDS method, and reach a sensitivity of 10⁻⁸ cm⁻¹per ring down event. Long-term performance will be presented. The spectral resolution, governed by the free spectral range of the optical resonator, is about 300 MHz. The spectra are obtained by adjusting the temperature of the QCL. Some already known water absorption lines were identified in the spectra, from 1183 cm⁻¹and 1186 cm⁻¹, and used for calibration. The broadband absorption of the water self-continuum is evaluated and will be compared to the literature. Acknowledgements: ANR FULLDIBS for financial support

We developed a type of toroidal multi-pass cell with multilayer patterns based on the off-axis model. The effective path length of the original toroidal multi-pass cell is extended several roundtrips in comparison with the single-layer pattern, since the inner surface of the toroidal multi-pass cell is more efficiently utilized. The light pattern has been achieved by using the simple ring surface, which is easy to fabricate. The exact analytical equations for the design of the toroidal multi-pass cell were derived based on analytical vector calculations. A homemade electronic system was designed for implementing a DFB diode laser controller, an analog lock-in amplifier, data acquisition, and communication. Calibration-free scanned wavelength modulation spectroscopy was employed to determine the concentration of the gas and reduce the random fluctuations from electronical noise and mechanical vibration. The measurement of CH4 in ambient air was demonstrated using a DFB laser at 1.653 μm. The rise time and fall time for renewing the gas mixture are approximately 16 and 14 s, respectively. Vibration and temperature tests have been carried out for verifying the performance of the spectrometer, and standard deviations of 0.38 ppm and 0.11 ppm for 20 ppm CH4 at different vibration frequencies and temperatures, respectively, have been determined. According to the Allan deviation analysis, the minimum detection limit for CH4 can reach 22 ppb at an integration time of 57.8 s.

In this work, we propose a novel approach for spectroscopic analysis in Cavity Ring-Down Comb Vernier Spectroscopy (CRDCVS) by exploiting FPGA architecture and an Arm-based embedded Linux system. This approach takes advantage of the high-speed data processing and acquisition capabilities of FPGA. In the Vernier configuration, a mode-resolved comb spectrum should align non-equidistant comb peaks in different scans and fitting the ring down time for each comb line. Especially a significant challenge in spectroscopic analysis is presented when the sample accumulation is required for improved sensitivity or temporal resolution. To address this challenge, we developed a machine-learning framework to predict comb line appearance in the operating platform. This enables ring-down-time accumulation for different comb peaks in CRDCVS. We demonstrated the proof-of-principle performance of the developed system with CRDCS measurements of toluene using a 3.3um chip-scale Interband Cascade Laser (ICL) comb. We will discuss the overall performance and the potential to advance the field of CRDCVS by improving measurement accuracy and reliability.

Gas phase O(¹D) insertion reactions are a useful laboratory tool for the spectroscopic study of transient species that are believed to exist in extreme environments, such as star-forming regions of the interstellar medium. One of the most common and efficient methods of O(¹D) production is the photolysis of ozone (O₃) at 248 nm. However, previous studies in our group have revealed that numerous side products are generated because of the high reactivity of O₃ and O₂(¹∆). Here we present modelling that demonstrates the feasibility of N₂O as an alternative to O₃ as an O(¹D) precursor. Using the Framework for 0-Dimensional Atmospheric Modelling (F0AM), we show that the number of unwanted side products should be reduced owing to the absence of O₃ and O₂(¹∆). Additionally, we present initial laboratory experiments utilizing N₂O as an O(¹D) precursor to produce CH₃OH from CH₄ via O(¹D) insertion.

Kinetics of O$_{2}$ vibrational excitation is studied during the O atom recombination in an O$_{2}$-Ar mixture partially dissociated by a burst of ns discharge pulses in a heated flow reactor at T=400-800 K and P=200-600 Torr. Time-resolved vibrational level populations of molecular oxygen in the ground electronic state, O$_{2}$(v=8-13,17-20), are measured by ps Laser Induced Fluorescence on the O$_{2}$ Schumann Runge bands, with absolute calibration by NO LIF in a NO-N$_{2}$ mixture with a known composition, at quenching-free conditions. O atom number density in the same mixture is measured by ps Two-Photon absorption LIF (TALIF). The discharge generates a diffuse volumetric plasma, without well-pronounced filaments. The results indicate a rapid initial decay of the O$_{2}$(X,v=8-20) molecules generated by electron impact in the discharge, on ~20 $\mu$s time scale, due to the V-V exchange and V-T relaxation. This decay is followed by a much slower decay, on the time scale much longer compared to the characteristic time for V-V relaxation, ~1 ms. This indicates an additional process of O$_{2}$(v) generation by chemical reactions initiated by the O atom recombination and possibly ozone chemistry. Comparison of the experimental data with the master equation kinetic modeling predictions is used to infer the state-specific rates of chemical reactions generating vibrationally excited O$_{2}$.