![]() Concurrently, the majority of wavenumber entries of line-by-line databases is based on less precise (mainly Doppler-limited) experiments, typically accurate to 10 −3 cm −1. To serve the large community of users of spectroscopic results, the data have been deposited in a number of validated, annotated, and regularly updated spectroscopic information systems, such as the HITRAN (high-resolution transmission molecular absorption) database 1. Similar content being viewed by othersĬomprehensive spectroscopic information about small gas-phase molecules is indispensable for the characterization of various natural and artificial environments. Based on the limited number of observed transitions, 1219 calibration-quality lines are obtained in a wide wavenumber interval, which can be used to improve spectroscopic databases and applied to frequency metrology, astrophysics, atmospheric sensing, and combustion chemistry. These measurements, augmented with 28 extremely-accurate literature lines to ensure overall connectivity, allow the precise determination of the lowest ortho-H 2 16O energy, now set at 23.794 361 22(25) cm −1, and 160 energy levels with similarly high accuracy. As a proof of concept, 156 carefully-selected near-infrared transitions are detected for H 2 16O, a benchmark system of molecular spectroscopy, at kHz accuracy. Network theory, based on the generalized Ritz principle, offers a powerful tool for the intelligent design and validation of such precision-spectroscopy experiments and the subsequent derivation of accurate energy differences. Frequency combs and cavity-enhanced optical techniques have revolutionized molecular spectroscopy: their combination allows recording saturated Doppler-free lines with ultrahigh precision.
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