Silicon is one of the most abundant elements in the Earth’s crust and is also prevalent in space in the form of gaseous molecules, interstellar dust, and ice grains. The recent radio astronomy detection of cyano-substituted benzene, e.g., benzonitrile [1] in TMC-1 has accelerated the search for substituted aromatic molecules in the interstellar medium. Considering the presence of silicon in the interstellar medium, silicon-containing aromatic molecules are likely targets of future astronomical searches. To aid these searches and understanding the silicon chemistry in space, laboratory data are crucial. In the laboratory, exotic neutral silicon-containing molecules can be produced and spectroscopically characterized by combining spectroscopic techniques with electrical discharge sources. In this work, using mass-selective IR-UV ion dip spectroscopy coupled with a molecular beam and the free electron laser FELIX (Nijmegen, The Netherlands), we have investigated the electrical discharge chemistry of phenyl silane in selected IR regions (500 – 3200 cm⁻¹) [2]. Such broad coverage provides significant information about the structure and chemical composition of the discharge products formed in the experiment. The aim is to investigate the variety of discharge products generated by the electrical discharge experiment of pure phenyl silane. Numerous fragments, products, and reaction intermediates are identified by mass-selective IR-UV spectroscopy combined with quantum-chemical calculations. [1] B.A. McGuire, A.M. Burkhardt, S. Kalenskii, C.N. Shingledecker, A.J. Remijan, E. Herbst, M.C. McCarthy, Detection of the aromatic molecule benzonitrile (c-C₆H₅CN) in the interstellar medium, Science. 359 (2018) 202–205. [2] A.K. Lemmens, D.B. Rap, J.M.M. Thunnissen, B. Willemsen, A.M. Rijs, Polycyclic aromatic hydrocarbon formation chemistry in a plasma jet revealed by IR-UV action spectroscopy, Nat. Commun. 11 (2020) 269.

Silicon is among the most abundant elements in interstellar environments, being only one order of magnitude less than C, N, and O. More than 15 gas-phase silicon bearing molecules have been already detected and a large fraction of silicon is considered to be locked up in interstellar dust grains, where also polycyclic aromatic hydrocarbons (PAHs) are a major constituent. Due to its abundance, silicon can represent an important player in the chemical evolution of the interstellar medium and, in particular, in many interstellar PAHs formation zone. Because of its chemical analogy with carbon, both are tetravalent atoms that form primarily covalent bonds, silicon could substitute either within or on the carbon skeleton of a PAH under the energetic conditions of the ISM, and silicon-substituted PAHs could represent promising candidates for future astronomical searches. In the laboratory, one way to study the make-up of silicon containing PAH-like molecules is by combining cutting-edge spectroscopic techniques with plasma sources. In this work, we used molecular beam mass-selective IR spectroscopy with the free-electron laser FELIX and quantum-chemical calculations to analyze the electrical discharge of phenylsilane and naphthalene. A variety of molecular species resulting from the combination of the precursors were identified via their mass and their IR spectra. The analysis of the latter could help elucidate the influence of Si on the IR spectra of PAHs.

The observation and quantification of reactive intermediates is crucial for the validation and development of kinetic models. However, very often the same reactivity that makes the role of such species important in reaction networks hinders our ability to detect them, due to their low concentrations and short lifetimes. As a result, many kinetic models rely on the formation of various intermediates which were never quantified, and sometimes not even established. One particularly important case is the pyrolysis of ethane (). As this reaction plays a central role in industrial processes such as production of ethylene () and combustion of ethane, the understanding of its underlying mechanism is crucial for process optimization, in order to reduce economic and environmental costs. Although ethyl () is a key intermediate towards the formation of ethylene, it has never been directly observed in this reaction. We report a double imaging photoelectron photoion coincidence (i²-PEPICO) spectroscopy study of the flash pyrolysis of ethane and its isotopologue using VUV radiation generated by the Swiss Light Source (SLS) and a resistively heated SiC microreactor reaching temperatures of up to 1700 K. This apparatus is especially appropriate for the observation of reactive radicals in thermal reactions – the short residence times before quenching of the intermediates, and the low ionization energies of radicals favor their observation. These properties allowed the observation of ethyl and its isotopologues, which were identified through their threshold photoelectron spectrum (TPES), by comparison to reported spectra and simulated ones. Together with observation of isotopic selectivity and comparison to fully coupled computational fluid dynamics and kinetics simulations, these results provide valuable mechanistic insights into the initiation steps of this reaction.

Polycyclic aromatic hydrocarbons (PAHs) are of great interest for its potential central role in astrochemistry. They are assumed to connect the chemistry between fullerenes and smaller carbon species, and it is assumed that they contain up to 20 % of the galactic carbon. To unravel the various aspects of their impact in astrochemistry, we perform a multi-spectroscopic approach covering large areas of the electromagnetic spectrum. The structures and intermolecular interactions of PAH-water complexes are investigated using high-resolution chirped-pulse rotational spectroscopy in our lab and infrared spectroscopy using the Free-Electron Laser (FEL) FELIX in Nijmegen in the gas phase. With this work, we aim at understanding interactions between PAHs themselves and water. Another motivation is to learn how an ice layer begins to form on an extended carbon surface. In another series of experiments, we perform plasma studies with PAHs and other astrochemically relevant molecules as precursors using an electric discharge. We observe a rich chemistry that we can then characterize spectroscopically using IR and rotational spectroscopy, which is highly complementary. To study PAH photophysics, we use the Hamburg XUV to soft-X-ray Free-Electron Laser FLASH and perform ultrafast XUV-UV/IR pump-probe experiments, with ionization and dissociation as competing pathways. We can extract life times of electronically excited neutral and cationic states and investigate characteristic fragmentation patterns depending on the nature of the PAH under investigation. In my presentation, I will provide an overview of our research activities on PAHs and discuss some of the latest results.

At Berkeley lab we are developing a versatile mass spectrometer to probe spectroscopic and time-resolved chemical dynamics involving non-covalent interactions. We are interested in the dynamic interactions of water weakly bound to hydrophobic molecular substrates (water in confinement).¹ Despite the cold conditions used to produce such complexes, they still retain large amounts of flexibility. Related are solvent-molecule rearrangements. Solvation can have a strong influence on chemical properties, and of particular interest is solvent dynamics as response to a stimulus, such as photoexcitation.² \ Within the framework of astrochemistry, dynamics in larger molecules or their van der Waals clusters are largely unexplored. Processes that occur upon (V)UV radiation such as dehydrogenation, fragmentation, and ionization will be explored with the new laser system. Moreover, the tunability and high repetition rate of the UV/VIS pulses allows for fast electronic spectroscopy of reaction products for identification but also to spectroscopically characterize possible candidates for the diffuse interstellar bands. \ The system will use 35% of the output from a 300 fs Yb fiber laser for generating the 115 nm ninth harmonic by a phase matched process of the third harmonic in a Xenon gas cell, running at up to 750 kHz.³ The remaining 65% is used to pump an OPA that covers a tuning range of 210 nm to 16 μm. The resulting wide spectral range allows for direct ionization as well as electronic and vibrational spectroscopy of gas phase systems. The combination with short laser pulses allows for the characterization of fast structural and/or electronic changes upon excitation, ideal for tracking the dynamics of non-covalent systems. 1. Molina, E. R., et al. (2022), PCCP, 24(38), 23106-23118. 2. Miyazaki, M., et al. (2018), PCCP, 20(5), 3079-3091. 3. Peli, S., et al. (2019), arXiv preprint arXiv:1911.05590.

The Vacuum Ultraviolet (VUV) photodissociation of CO₂ via the dominant O(¹D) channel near 155 nm were studied using Velocity Map Imaging (VMI) technique. Speed dependent vector correlations (the correlations among the transition dipole moment μ of the parent molecule, the recoil velocity vector v and rotational angular momentum vector j of the photofragments) were extracted. This was made possible via the application of a set of equations first developed by Grubb et al. And in combination with a Monte Carlo style simulation program and necessary approximations, full vector correlation information were extracted from the anisotropic angular distributions of the images. Our results indicated a picture of photodissociation dynamics mainly via the excited 2¹A’ (A) state. The transition dipole moment lies in the bent molecular plane. It was not parallel, but pointing away from the dissociating C-O bond. In addition, speed dependent μ -v correlation showed a clear trend, with higher rotationally excited CO correlating to larger angle between μ and the recoil direction. Such clear trend was successfully explained with a mathematical model (first developed by Reisler and coworkers) considering non-axial recoil effect. Although 1¹A” (B) state was also energetically available, we do not believe it was involved in this case.

In 1997 Eppink and Parker introduced velocity map imaging (VMI) based on the ion imaging technique developed by Chandler and Houston. This method has been as a high-resolution tool to study the detailed dynamics of chemical or physical processes. This approach can measure the momenta and angular distributions of charged particles (ions/electrons) released in these processes and provide detailed information about the potential energy surfaces. Over the years, this technique has been evolving; now it can do direct three-dimenssional measurements with multi-hit capability. In this work, we report further improvement to the 3D-VMI setup. With the conventional digitizers employed previously replaced by a low-cost USB oscilloscope, VMI has become more cost-effective, simpler and even portable. The performance of this new VMI apparatus was characterized in a study of laser desorption ionization of 2,5-dihydroxybenzoic acid.

Over the last several decades, the development of time- and frequency-domain nuclear resonance spectroscopies that take advantage of the tunability and pulsed nature of synchrotron radiation at storage ring facilities has greatly expanded both the portfolio of routinely accessible nuclei and the range of material properties that may be probed via the Mössbauer effect. Continuing in this tradition, we recently developed a method to probe the photophysical dynamics of solid-state materials using a pump-probe implementation of time-domain Mössbauer spectroscopy that we refer to as time-resolved nuclear forward scattering (TRNFS). After introducing this novel technique and discussing its challenges, I will present our measurements of tin(II) oxide to showcase the power and unique opportunities of TRNFS. In particular, I will demonstrate how variable repetition rate TRNFS may be used to distinguish between electronic and thermal contributions to laser-induced difference spectra and how small contributions to the quantum beating signal originating from the photoexcited material may be clearly identified using reference scatterers.

Synchrotron insertion devices and x-ray free electron lasers deliver pulse trains of x-rays with up-to MHz to sub-GHz repetition rate. These x-rays can ablate samples in a single shot, necessitating material replacement in a short time before the next pulse arrives. Typically, this is done with a dilute solution in a high-speed jet or with a sputtering source, yet many applications make these solutions unworkable. We present the development of a fast, solid-target sample delivery system for high repetition rate x-ray sources, to alleviate these issues. Tests of the system at the LCLS nano-focusing beamline and future applications in various user experiments are outlined.