It remains an open question as to whether polycyclic aromatic hydrocarbons (PAHs) can be efficiently formed in the ISM by bottom-up mechanisms involving growth from small aromatic precursors like benzene. However, the lack of dipole moment renders benzene invisible in the radio regime making estimation of the abundance of benzene in the ISM difficult. The recent detection of benzonitrile in the Taurus Molecular Cloud (TMC)-1 has caused excitement in the astrochemical community as it is the first aromatic molecule detected in the interstellar medium (ISM) using radio astronomy. Benzonitrile is thought to form via the neutral-neutral reaction between the CN radical and benzene, and therefore may serve as a chemical proxy to determine the abundance of benzene. The abundances of aromatic species in ISM environments are not well understood, in part due to a lack of experimental kinetics data. Both rate constants and product-branching ratios for the reactions of aromatic molecules must be measured at low temperature in order to input these reactions into astrochemical models and accurately predict abundances. Benzene and toluene are two of the aromatic species detected in the atmosphere of Titan and their reactions with the CN radical have been studied down to 105 K by Trevitt et al. Here, we have extended this study down to 15 K to approach dense cloud conditions and have measured the rate constants of the reactions of benzene and toluene with the CN radical using the well-established CRESU technique (Cinétique de Réaction en Ecoulement Supersonique Uniforme, or Reaction Kinetics in Uniform Supersonic Flow) combined with the Pulsed Laser Photolysis-Laser-Induced Fluorescence method. I will also discuss our recent progress in combining chirped-pulse micro/mm-wave spectroscopy with the CRESU method and how we plan to employ this technique to measure product branching ratios for reactions of the CN radical with aromatics at low temperatures.

Time-resolved nonlinear spectroscopy techniques, such as transient absorption spectroscopy and 2D spectroscopy, are routinely used to study ultrafast dynamics. Due to the limited sensitivity of these techniques, they are most commonly applied to optically thick samples, such as solid and liquid solutions. Using a frequency comb laser and optical cavities, we present a technique for performing ultrafast optical spectroscopy with high sensitivity, enabling work in dilute gas-phase molecules and clusters. Resonantly enhancing the probe pulses, we have demonstrated transient absorption measurements with a detection limit of ∆OD = 2×10⁻¹⁰\text (1×10⁻⁹/√{Hz})M. A. R. Reber, Y. Chen, T. K. Allison, Optica 3, 311 (2016) Resonantly enhancing the pump pulses allows us to produce a high excitation fraction at a high repetition rate, so that signals can be recorded from samples with OD as low as 10⁻⁸, or column densities < 10¹⁰ molecules/cm². This sensitivity enables ultrafast spectroscopy in dilute molecular beams, where cold isolated designer molecules, radicals, and clusters can be produced that do not exist in solution. In this talk, I will discuss the basic principles of cavity-enhanced ultrafast spectroscopy, initial one-color demonstration experiments, and the development of widely tunable cavity-enhanced ultrafast spectrometersY. Chen, M. C. Silfies et al., Appl. Phys. B. 125, 81 (2019); M. C. Silfies et al. arXiv:2001.10680 (2020)perating in the ultraviolet, visible, and infrared. Finally, I will discuss progress on the development of cavity-enhanced 2DIR spectroscopy for the study of hydrogen-bonded clustersT. K. Allison, J. Phys. B. 50, 044004 (2017)

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Footnotes:

M. A. R. Reber, Y. Chen, T. K. Allison, Optica 3, 311 (2016). Y. Chen, M. C. Silfies et al., Appl. Phys. B. 125, 81 (2019); M. C. Silfies et al. arXiv:2001.10680 (2020)o T. K. Allison, J. Phys. B. 50, 044004 (2017).

With increasing concerns over the concentration of carbon dioxide in the atmosphere, new materials are being explored to capture CO₂ for use as a feedstock. The suspected reaction sites of these materials deviates from the bulk phase, such as the quantum confinement of water droplets that happens within pores. The formic acid clusters’, (HCOOH)_n(H₂O)_m, minimum energy structures show an evolving cage structure with each additional molecule. The n,m = (5,0) cluster exhibits a much greater stability due to its rigid cage-like structure. This cage structure then encapsulates a water molecule to make an even more stable n,m = (5,1) cluster. The interaction of formic acid clusters with 200 fs linearly polarized laser pulses of 800 and 400 nm with intensities up to 1x10¹⁵ W/cm² was studied using time-of-flight mass spectrometry, verifying this trend in stability. An enhanced ionization is observed in clusters, leading to the production of triply charged carbon, oxygen, and CO ions that are not observed when only the formic acid molecule is present. Measurements of the kinetic energy release resulting from the Coulomb explosion of clusters are in excellent agreement with our simulations performed over the clusters observed in the mass spectra and suggest that almost no movement occurs during the ionization mechanism. Finally, ultrafast pump-probe spectroscopy was used to investigate how proton transfer dynamics and excited state lifetimes are influenced by the self-solvation of formic acid. These results highlight the role of microsolvation on the excited state dynamics of simple carboxyl groups, specifically formic acid, in producing or capturing carbon dioxide and will help to direct the design of the next generation of carbon capture materials.

The details of energy transfer between colliding atoms and molecules is essential in order to understand chemical processes such as those in Earth’s or other atmospheres, the Interstellar Medium (ISM) or for combustion chemistry processes in industrial and aeronautic applications. The most accurate and reliable data for these applications are mostly based on fully quantum mechanical calculations (classical treatments of the nuclei usually lead to inaccuracies, in particular when low temperature processes (common in the ISM) are of interest). To counter the curse-of-dimensionality (COD) problem that arises when cross-section calculations are performed on molecular systems with large masses (and thus dense state densities), two particularly promising methods have been developed recently: the Statistical Adiabatic Channel Model (SACM) and the Mixed Quantum-Classical Trajectory (MQCT) approach. We have also recently highlighted the effectiveness of the MultiConfiguration Time Dependent Hartree (MCTDH) approach to overcome the COD issue arising in the calculation of cross-sections of inelastic collisions. Indeed, since its inception, the MCTDH approach has allowed characterization, using a fully quantum dynamical approach and with an excellent accuracy, the spectroscopy and the dynamics for several challenging molecular systems and is thus known to push the boundaries of traditional quantum dynamical calculations. We will present our recent progress on this topic and discuss recent (and some preliminary) results on the collisions of H₂O with Ar, H₂ and H₂O using the MCTDH approach. We will also present the results of other collisions processes (HCOOCH₃+He, CH₃CH₂COH+He, N₂H⁺+H₂, …) which remain challenging with standard computation approaches. We will also discuss the limitations of the approach and possible routes for improvements.

In 1969 A. F. Andreev and I. M. Lifshitz radically changed the way we think about diffusion in cryocrystals by predicting that at sufficiently low temperatures the probability of exchange tunneling of neighboring particles in quantum crystals becomes noticeable such that impurities can move freely through the crystal as narrow-band quasiparticles.A. F. Andreev and I. M. Lifshitz, Sov. Phys. JETP. 29, 1107-1113 (1969).he term “quantum crystal” was introduced by de Boer in 1948 for substances in which the energy of the zero-point vibrations of the particles is comparable to the total energy of the crystal.J. de Boer, Physica 14, 139-148 (1948).he main idea put forth by Andreev and Liftshitz is that the rate of quantum diffusion should increase with falling temperatures and should show an inverse dependence on the concentration of impurities. As we will show, the hydrogen atom (H-atom) trapped in a parahydrogen crystal is an ideal candidate for quantum diffusion owing to its small mass and neutral charge. In 2013 our group published a communicationF. M. Mutunga, S. E. Follett, D. T. Anderson, J. Chem. Phys. 139, 151104-4 (2013).n the kinetics of the H + N₂O reaction in solid parahydrogen that showed an anomalous temperature dependence. In these studies we generate the H-atoms as byproducts of the in situ photodissociation of N₂O and monitor the subsequent reaction kinetics using rapid scan FTIR. Specifically, if we photolyze N₂O doped parahydrogen solids with 193 nm UV radiation at 4.3 K, we observe little to no reaction; however, if we then slowly reduce the temperature of the sample, we observe an abrupt onset to the reaction at temperatures below 2.4 K. In a number of studies conducted since this original work we have come to a better understanding of the effect of temperature on the reaction and will show data that the rate constant for the H + N₂O reaction shows an inverse dependence on the N₂O concentration. These findings support previous ESR measurements of H-atom quantum diffusion in solid parahydrogenT. Kumada et. al., J. Chem. Phys. 116, 1109-1119 (2002).nd more importantly illustrate how H-atom quantum diffusion impacts the kinetics of these anomalous low temperature, condensed phase reactions.

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Footnotes:

A. F. Andreev and I. M. Lifshitz, Sov. Phys. JETP. 29, 1107-1113 (1969).T J. de Boer, Physica 14, 139-148 (1948).T F. M. Mutunga, S. E. Follett, D. T. Anderson, J. Chem. Phys. 139, 151104-4 (2013).o T. Kumada et. al., J. Chem. Phys. 116, 1109-1119 (2002).a

As a long-lived greenhouse gas, methane plays a major role in the chemistry of the Earth's atmosphere. Most atmospheric methane is removed by oxidations by OH (84%) and O(¹D) (8%). The study of kinetic isotope effects (KIEs) on the reaction rate constants involving the transfer between hydrogen atoms is of importance but challenging experimentally because they are quite small. In this work, the ^DKIE for the singly substituted methane (CH₃D and CH₄) with O(¹D) radicals have been measured using frequency-stabilized cavity ring down spectroscopy (FS-CRDS), where the isotopic composition is determined based on a dual near-infrared DFB laser system. The high sensitivity of the FS-CRDS system, with a detection limit of 6.0×10⁻¹² cm⁻¹, enables the KIE to be measured with low depletion to minimize possible secondary effects. In this experiment, the O(¹D) radicals are produced by the 193 nm photolysis of N₂O. The preliminary result of ^DKIE is 1.052(39).

Recently, an electron imaging apparatus that utilizes angular streaking was developed to directly determine the in-situ absolute CEP of ultrashort pulses without theory inputs [Debrah et al. Opt. Lett. 44, 14, 3582-3585, (2019)]. Here we demonstrate a different approach based on ion imaging. By measuring the angular distribution of the methyl cation arising from dissociative double ionization of methyl iodide, we observed a strong correlation between the CEPs of the laser pulses and the preferred ejection angle of methyl cation. Unlike electron imaging, complex effects due to Coulomb deflection and laser vector potentials are effectively suppressed. This development will improve the measurement of absolute CEP, which is important for strong field physics/chemistry and attosecond spectroscopy.

Determining the mechanism by which Cytochrome P450s (P450s) can catalyze oxidation reactions of substrates with differing specificity and regioselectivity is crucial for aiding drug development and understanding drug metabolism. Cytochrome P450cam (P450cam), a model P450, catalyzes the hydroxylation of d-camphor to 5-exo-hydroxycamphor with high specificity and regioselectivity, it can also act upon camphor-like analogs at the expense of regioselectivity. Previous studies have suggested conformational dynamics may play a role in the recognition and hydroxylation of substrates with varying degrees of regioselectivity. To investigate the role of dynamics in regioselectivity, we characterized P450cam when bound to a camphor and norcamphor, substrates acted upon with 100% and 45% regioselectivity respectively. 2D IR spectroscopy was paired site-specific labeling and used to measure protein side-chain dynamics with high spatial and temporal resolution. Cyanophenylalanine was used as a vibrational probe and incorporated in five distinct locations of P450cam, three sites in the active site and two progressively distal from the active site. The results suggest different parts of the protein active site are preferentially involved in substrate binding and contributions from inhomogeneous broadening are more significant for substrates acted upon with high regioselectivity.

As the most abundant atmospheric hydrocarbon and potent greenhouse gas, methane plays a major role in the chemistry of Earth’s atmosphere. The study of kinetic isotope effects (KIEs) for H–D and ¹²C–¹³C substitutions on the reaction rates for methane is of importance for modelers, but is challenging because the effects are small, and producing methane oxidants results in a variety of secondary chemistry. In this work, the single-substitution methane KIEs were produced using the flash photolysis of different O(¹D) precursors: N₂O and O₃, and the methane isotopic compositions were determined via frequency-stabilized cavity ring-down spectroscopy using a dual wavelength near-IR DBF laser system. Despite the different chemistry of the oxidative samples, the KIEs for both methods were found to be consistent with the literature values at room temperature of 1.060 for H–D and 1.013 for ¹²C–¹³C substitutions. Further, the expected lack of a temperature-dependence of the KIE over the range 170 – 300 K was seen regardless of O(¹D) precursor.

Water plays a vital role in many biological processes like enzyme activity, protein folding-refolding and denaturation. It is essential to know the time scales characteristic of both, the local protein rearrangements and water dynamics within the solvation shell to understand the protein-water interactions. Urea is a chaotropic agent and a well-known denaturant for proteins. The molecular picture of the interaction of urea with the water hydrogen bond network and thereby, the chemical denaturation of the proteins is still ambiguous. Time-resolved Optical Kerr effect (OKE) spectroscopy is a powerful spectroscopic technique to study the hydrogen-bonded structure and dynamics of complex aqueous systems, in the picosecond time scales. In this study, we have investigated the mechanism behind urea denaturation of three proteins of different hydrophobicities- lysozyme, BSA and trypsin. The OKE data reveals the effect that different concentrations of urea have on the aqueous protein solutions. The spectral density (SD) obtained contains the α relaxation at the lowest frequencies corresponding to the orientational diffusion of the molecules, linked by the stretched β relaxation to the intermolecular librational modes at terahertz frequencies. The shape of the SDs resembles that of urea solutions; the addition of protein brings down the contribution from the α relaxation. At lower urea concentrations, this change is even more apparent. Preliminary analysis of the SDs shows the β relaxation timescales of water changes on the addition of urea and the subtracted spectra for the urea denatured lysozyme shows two distinct β processes characteristic of water and water-urea dynamics. A detailed analysis of the changes in the line shapes of the reduced spectral densities (RSDs) is required to elucidate the effect urea has on the water hydrogen bond network and to map out the structural changes occurring in three different proteins on the addition of urea.

The structure and dynamics of molecular liquids and their binary mixtures have been of interest for a long time. Azeotropes is a particular class of liquid mixture that boils at a constant temperature at a specific composition. Understanding of azeotropic systems at the molecular level is limited. In this study, we try to investigate this azeotropic non-ideality, using ultrafast Optical Kerr Effect (OKE) spectroscopy, broadband (1-10 THz) THz-Time domain spectroscopy (THz-TDS) along with temperature-dependent Nuclear Magnetic Resonance spectroscopy (NMR) of the azeotropic as well as mixtures of different compositions of benzene and methanol. Vibrational and NMR spectroscopic studies show that the formation of the methanol-benzene azeotrope weakens the hydrogen bond network. Intermolecular forces between the benzene molecules are also reduced significantly. Methanol disrupts the stacking in benzene which is also evident from the depression in the boiling points. Ultrafast OKE spectroscopy is a powerful tool to probe both, the collective orientational diffusion and the intermolecular dynamics in liquids. The spectral density (SD) obtained by the Fourier deconvolution of the OKE time transients gives us information about the structural relaxation occurring in the liquid at the terahertz and sub terahertz frequencies. With the increase of methanol in the mixtures, the α relaxation timescales become faster. An azeotropic composition mixture was prepared at room temperature without further distillation. The spectral densities of this mixture and the azeotrope at frequencies pertaining to collective intermolecular dynamics were quite different from each other. All of benzene’s entire rotational dynamics show up as molecular reorientation having contributions throughout the OKE spectra. The centre-of-mass translational part is only visible when it affects the interaction induced (I-I) part of the polarizabilityRyu, S.; Stratt, R. M. J. Phys. Chem. B 2004, 108, 6782. The addition of methanol decreases this contribution of the translations to the I-I term for benzene. A detailed analysis of the similarity of the azeotrope spectral density to benzene and how they differ from the other composition mixtures can give us useful insights into the structural fluctuations happening in the picosecond timescales in the benzene-methanol mixtures.

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Footnotes:

Ryu, S.; Stratt, R. M. J. Phys. Chem. B 2004, 108, 6782..