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We present an experimental and theoretical investigation of the reaction of vibrationally excited CN ( v = 1) with isomers of butadiene at low temperature. The experiments were conducted using the newly built apparatus, UF-CRDS, which couples near-infrared cw-cavity ring-down spectroscopy with a pulsed Laval flow. The well-matched hydrodynamic time and long ring-down time decays allow measurement of the kinetics of the reactions within a single trace of a ring-down decay, termed Simultaneous Kinetics and Ring-down (SKaR). The pulsed experiments were carried out using a Laval nozzle designed for the 70 K uniform flow with nitrogen as the carrier gas. The measured bimolecular rates for the reactions of CN ( v = 1) with 1,3-butadiene and 1,2-butadiene are (3.96 ± 0.28) × 10 −10 and (3.06 ± 0.35) × 10 −10 cm 3 per molecule per s, respectively. The reaction rate measured for CN ( v = 1) with the 1,3-butadiene isomer is in good agreement with the rate previously reported for the reaction with ground state CN ( v = 0) under similar conditions. We report the rate of the reaction of CN ( v = 1) with the 1,2-butadiene isomer here for the first time. The experimental results were interpreted with the aid of variable reaction-coordinate transition-state theory calculations to determine rates and branching of the addition channels based on a high-level multireference treatment of the potential energy surface. H-abstraction reaction rates were also theoretically determined. For the 1,2-butadiene system, theoretical estimates are then combined with literature values for the energy-dependent product yields from the initial adducts to predict overall temperature-dependent product branching. H loss giving 2-cyano-1,3-butadiene + H is the main product channel, exclusive of abstraction, at all energies, but methyl loss forming 1-cyano-prop-3-yne is 15% at low temperature growing to 35% at 500 K. Abstraction forming HCN and various radicals is important at 500 K and above. The astrochemical implications of these results are discussed. 

Chirped-Pulse Fourier-Transform millimeter wave (CP-FTmmW) spectroscopy is a powerful method that enables detection of quantum state specific reactants and products in mixtures. We have successfully coupled this technique with a pulsed uniform Laval flow system to study photodissociation and reactions at low temperature, which we refer to as CPUF (“Chirped-Pulse/Uniform flow”). Detection by CPUF requires monitoring the free induction decay (FID) of the rotational coherence. However, the high collision frequency in high-density uniform supersonic flows can interfere with the FID and attenuate the signal. One way to overcome this is to sample the flow, but this can cause interference from shocks in the sampling region. This led us to develop an extended Laval nozzle that creates a uniform flow within the nozzle itself, after which the gas undergoes a shock-free secondary expansion to cold, low pressure conditions ideal for CP-FTmmW detection. Impact pressure measurements, commonly used to characterize Laval flows, cannot be used to monitor the flow within the nozzle. Therefore, we implemented a REMPI (resonance-enhanced multiphoton ionization) detection scheme that allows the interrogation of the conditions of the flow directly inside the extended nozzle, confirming the fluid dynamics simulations of the flow environment. We describe the development of the new 20 K extended flow, along with its characterization using REMPI and computational fluid dynamics. Finally, we demonstrate its application to the first low temperature measurement of the reaction kinetics of HCO with O2 and obtain a rate coefficient at 20 K of 6.66 ± 0.47 × 10−11 cm3 molec−1 s−1. 

State-to-state rotational energy transfer in collisions of ground ro-vibrational state 13 CO molecules with N 2 molecules has been studied using the crossed molecular beam method under kinematically equivalent conditions used for 13 CO + CO rotationally inelastic scattering described in a previously published report (Sun et al. , Science , 2020, 369 , 307–309). The collisionally excited 13 CO molecule products are detected by the same (1 + 1′ + 1′′) VUV (Vacuum Ultra-Violet) resonance enhanced multiphoton ionization scheme coupled with velocity map ion imaging. We present differential cross sections and scattering angle resolved rotational angular momentum alignment moments extracted from experimentally measured 13 CO + N 2 scattering images and compare them with theoretical predictions from quasi-classical trajectories (QCT) on a newly calculated 13 CO–N 2 potential energy surface (PES). Good agreement between experiment and theory is found, which confirms the accuracy of the 13 CO–N 2 potential energy surface for the 1460 cm −1 collision energy studied by experiment. Experimental results for 13 CO + N 2 are compared with those for 13 CO + CO collisions. The angle-resolved product rotational angular momentum alignment moments for the two scattering systems are very similar, which indicates that the collision induced alignment dynamics observed for both systems are dominated by a hard-shell nature. However, compared to the 13 CO + CO measurements, the primary rainbow maximum in the DCSs for 13 CO + N 2 is peaked consistently at more backward scattering angles and the secondary maximum becomes much less obvious, implying that the 13 CO–N 2 PES is less anisotropic. In addition, a forward scattering component with high rotational excitation seen for 13 CO + CO does not appear for 13 CO–N 2 in the experiment and is not predicted by QCT theory. Some of these differences in collision dynamics behaviour can be predicted by a comparison between the properties of the PESs for the two systems. More specific behaviour is also predicted from analysis of the dependence on the relative collision geometry of 13 CO + N 2 trajectories compared to 13 CO + CO trajectories, which shows the special ‘do-si-do’ pathway invoked for 13 CO + CO is not effective for 13 CO + N 2 collisions. 

Abstract Direct D-H exchange in radicals is investigated in a quasi-uniform flow employing chirped-pulse millimeter-wave spectroscopy. Inspired by the H-atom catalyzed isomerization of C₃H₂reported in our previous study, D-atom reactions with the propargyl (C₃H₃) radical and its photoproducts were investigated. We observed very efficient D-atom enrichment in the photoproducts through an analogous process of D addition/H elimination to C₃H₂isomers occurring at 40 K or below. Cyclic C₃HD is the only deuterated isomer observed, consistent with the expected addition/elimination yielding the lowest energy product. The other expected addition/elimination product, deuterated propargyl, is not directly detected, although its presence is inferred by the observations in the latter part of the flow. There, in the high-density region of the flow, we observed both isotopomers of singly deuterated propyne attributed to stabilization of the H+C₃H₂D or D+C₃H₃adducts. The implications of these observations for the deuterium fractionation of hydrocarbon radicals in astrochemical environments is discussed with the support of a monodeuterated chemical kinetic model.