Abstract:The kinetics of the OH + CO reaction, fundamental to both atmospheric and combustion chemistry, are complex due to the formation of the HOCO intermediate. Despite extensive studies on this reaction, HOCO has not been observed at thermal reaction conditions. Exploiting the sensitive, broadband, and high-resolution capabilities of time-resolved cavityenhanced direct frequency comb spectroscopy, we observe OD + CO reaction kinetics with the detection of stabilized trans-DOCO, the deuterated analogue of trans-HOCO, and its yield. By simultaneously measuring the time-dependent concentrations of both trans-DOCO and OD species, we observe unambiguous low-pressure termolecular dependence on the reaction rate coefficients for both N2 and CO bath gases. These results confirm the HOCO formation mechanism and quantify its yield. One Sentence Summary:We detect trans-DOCO and OD in the reaction of OD + CO using cavity-enhanced direct frequency comb spectroscopy and determine the kinetics and trans-DOCO branching yield in the low pressure regime. Main Text:2 The apparent simplicity of gas phase bimolecular reaction kinetics of free radicals often belies the complexity of the underlying dynamics. Reactions occur on multidimensional potential energy surfaces that can possess multiple pre-reactive complexes, bound intermediate complexes and multiple transition states. As a result, effective bimolecular rate coefficients often exhibit complex temperature and pressure dependence. The importance of free radical reactions in processes such as combustion and air pollution chemistry has motivated efforts to determine these rate constants both experimentally and theoretically. Quantitative ab initio modeling of kinetics remains a major contemporary challenge (1), requiring accurate quantum chemical calculations of energies, frequencies and anharmonicities, master equation modeling, energy transfer dynamics, and, when necessary, calculation of tunneling and non-statistical behavior.Experimentally, detection of the transient intermediates, which is the key to unraveling the dynamics, is often quite challenging. has been extensively studied over the last four decades because of its central role in atmospheric and combustion chemistry (2); it has come to serve as a benchmark for state-of-the-art studies of chemical kinetics of complex bimolecular reactions (3,4). In Earth's atmosphere, the hydroxyl radical OH is critical as the primary daytime oxidant (5, 6). CO, a byproduct of fossil fuel burning, acts through reaction 1 as an important global sink for OH radicals and is the dominant OH loss process in the free troposphere. In fossil fuel combustion, OH + CO is the final step that oxidizes CO to CO2 and is responsible for a large amount of heat released.The rate of reaction 1 is pressure dependent and exhibits an anomalous temperature dependence, which led Smith and Zellner (7) to propose that the reaction proceeds through a highly energized, strongly bound intermediate, HOCO, the hydrocarboxyl radical. Formation of H + CO2 produc...

Substrate-transferred crystalline coatings have recently emerged as a groundbreaking new concept in optical interference coatings. Building upon our initial demonstration of this technology, we have now realized significant improvements in the limiting optical performance of these novel single-crystal GaAs/Al x Ga 1-x As multilayers. In the near-infrared (NIR), for coating center wavelengths spanning 1064 to 1560 nm, we have reduced the excess optical losses (scatter + absorption) to levels as low as 3 parts per million, enabling the realization of a cavity finesse exceeding 3×10 5 at the telecom-relevant wavelength range near 1550 nm. Moreover, we demonstrate the direct measurement of sub-ppm optical absorption at 1064 nm. Concurrently, we investigate the mid-IR (MIR) properties of these coatings and observe exceptional performance for first attempts in this important wavelength region. Specifically, we verify excess losses at the hundred ppm level for wavelengths of 3300 and 3700 nm. Taken together, our NIR optical losses are now fully competitive with ion beam sputtered multilayer coatings, while our first prototype MIR optics have already reached state-of-the-art performance levels for reflectors covering this portion of the fingerprint region for optical gas sensing. Mirrors fabricated with our crystalline coating technique exhibit the lowest mechanical loss, and thus the lowest Brownian noise, the highest thermal conductivity, and, potentially, the widest spectral coverage of any "supermirror" technology in a single material platform. Looking ahead, we see a bright future for crystalline coatings in applications requiring the ultimate levels of optical, thermal, and optomechanical performance.

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