Speed-dependent Voigt lineshape parameter database from dual frequency comb measurements up to 1305 K. Part I: Pure H2O absorption, 6801-7188 cm-1

The W2024 database of the water isotopologue H 2 16 O

The rovibrational spectrum of the water molecule is the crown jewel of high-resolution molecular spectroscopy. While its significance in numerous scientific and engineering applications and the challenges behind its interpretation have been well known, the extensive experimental analysis performed for this molecule, from the microwave to the ultraviolet, is admirable. To determine empirical energy levels forH 2 16 O , this study utilizes an improved version of the MARVEL (Measured Active Rotational-Vibrational Energy Levels) scheme, which now takes into account multiplet constraints and first-principles energy-level splittings. This analysis delivers 19027 empirical energy values, with individual uncertainties and confidence intervals, utilizing 309 290 transition wavenumbers collected from 189 (mostly experimental) data sources. Relying on these empirical, as well as some computed, energies and first-principles intensities, an extensive composite line list, named CW2024, has been assembled. The CW2024 dataset is compared to lines in the canonical HITRAN 2020 spectroscopic database, providing guidance for future experimental investigations.

A Network Approach for the Accurate Characterization of Water Lines Observable in Astronomical Masers and Extragalactic Environments

The water molecule, crucial to the chemical composition and dynamics of the universe, is typically identified in its gas phase via radio and submillimeter transitions, with frequencies up to a few THz. To understand the physicochemical behavior of astronomical objects, accurate transition frequencies are required for these lines. From a set of 26 new and 564 previous Lamb dip measurements, utilizing our ultrasensitive laser-based spectrometers in the near-infrared region, ultrahigh-precision spectroscopic networks were set up for H2 16O and H2 18O, augmented with 40 extremely accurate frequencies taken from the literature. Based on kHz-accuracy paths of these networks, considerably improved line-center frequencies have been obtained for 35 observed or predicted maser lines of H2 16O, as well as for 14 transitions of astronomical significance of H2 18O. These reference frequencies, attached with 5-25 kHz uncertainties, may help future studies in various fields of astrochemistry and astrophysics, in particular when precise information is demanded about Doppler-velocity components, including the gas flows of galactic cores, the kinematics of planetary nebulae, or the motion in exoplanetary atmospheres.

Integer-locking condition for stable dual-comb interferometry in situations with fluctuating frequency-comb repetition rates

To make dual-comb interferometry usable in a wide range of applications, it is important to achieve reproducible measurement results even in non-ideal environments that affect the repetition-rate stability. Here, we consider dual-comb interferometry based on a pair of fully referenced optical frequency combs (OFCs) and investigate the impact of fluctuations in the OFC repetition frequencies on the peak position of the center burst in the interferogram. We identify a phase-locking scheme that minimizes the impact of these fluctuations through choosing a special combination of phase-locked frequencies, and the resulting type of operating condition is termed integer-locking condition. Under the integer-locking condition, the number of sampling points in each interferogram remains constant regardless of repetition-rate variations, and this enables more stable phase-resolved measurements in non-ideal environments. We demonstrate the application of this approach using absolute path-length measurements and discuss the accuracy limit imposed by the integer-locking condition. Our findings offer a strategy for robust dual-comb interferometry outside metrology laboratories.

Interferogram-based determination of the absolute mode numbers of optical frequency combs in dual-comb spectroscopy

Dual-comb spectroscopy (DCS), which uses two optical frequency combs (OFCs), requires an accurate knowledge of the mode number of each comb line to determine spectral features. We demonstrate a fast evaluation method of the absolute mode numbers of both OFCs used in DCS system. By measuring the interval between the peaks in the time-domain interferogram, it is possible to accurately determine the ratio of one OFC repetition frequency (f_rep) to the difference between the f_rep values of the two OFCs (Δf_rep). The absolute mode numbers can then be straightforwardly calculated using this ratio. This method is applicable to a broad range of Δf_rep values down to several Hz without any additional instruments. For instance, the minimum required measurement time is estimated to be about 1 s for Δf_rep ≈ 5.6 Hz and f_rep ≈ 60 MHz. The optical frequencies of the absorption lines of acetylene gas obtained by DCS with our method of mode number determination shows good agreement with the data from the HITRAN database.

Speed-dependent Voigt lineshape parameter database from dual frequency comb measurements at temperatures up to 1305 K. Part II: Argon-broadened H2O absorption, 6801-7188 cm-1

We report argon-broadened water vapor transition parameters and their temperature dependence based on measured spectra spanning 6801-7188 cm-1 from a broad-bandwidth, high-resolution dual frequency comb spectrometer. The 25 collected spectra of 2% water vapor in argon ranged from 296 K to 1305 K with total pressure spanning 100 Torr to 600 Torr. A multispectrum fitting routine was used in conjunction with a quadratic speed-dependent Voigt profile to extract broadening and shift parameters, and a power-law temperature-dependence exponent for both. The measurements represent the first broad bandwidth, argon-broadened water vapor absorption study, and are an important step toward a foreign-gas-perturbed, high-temperature database developed using advanced lineshape profiles.