We demonstrate a novel platform for cryogenic measurements, recently launched in our laboratory, that is characterized by high stability and high uniformity of temperature, as well as a low level of mechanical noise and vibrations. It allowed us to extend, for the first time, the optical-cavity- enhanced spectroscopy technologies from room temperatures down to a deep cryogenic regime (to below 5 K). The unique feature of the system is a large high-vacuum cryogenic volume of nearly 9 litres and long axial dimension of 78 cm, which makes it well suited for operating optical-cavity-based spectrometers. In order to meet the strict requirements necessary to operate optical resonators, the system is equipped with multiple stages of heavy copper blocks and flexible thermal connectors that altogether not only serve as an excellent low-pass filter for temperature fluctuations (mainly originating from cryocooler operation cycle), but also well isolate the cavity from external noise and cryocooler vibrations. The system design ensures nearly perfect temperature uniformity across the experimental cryogenic chamber that is critical for ultra-accurate spectroscopic measurements at deep cryogenic conditions. Recently, we demonstrated [Nature Physics 22, 637 (2026)] that this system opens a way to new types of measurements that were not possible before, such as an accurate test of the quantum electrodynamics for molecules, realization of the primary SI standards for temperature, concentration, and pressure in the deep cryogenic regime, measurement of the H2 phase diagram, and determination of the ortho-para spin isomer conversion rate.

Cavity ring-down spectroscopy (CRDS) is widely used for sensitive optical absorption measurements, but its quantitative accuracy can be limited at the few-percent level by systematic distortions originating in the CRDS detection system, particularly at short ring-down times. These limitations can restrict demanding spectroscopic applications requiring sub-permille accuracy, including atmospheric sensing, tests of ab initio theory and fundamental physics, and quantum-based optical gas standards. Here we develop a methodology based on a system-level detection transfer function describing the complete CRDS detection-system response, including detector, electronics, and digitization stages, and use it to correct previously unaccounted-for instrumental distortions in retrieved absorption. We demonstrate the method on a CO absorption line measured down to very short ring-down times using multiple independent CRDS detection configurations, initially exhibiting line-area biases of up to 14

The absorption spectra of H2O in Ar gas with a mole fraction of 500 nmol/mol were obtained near 7181 cm-1. A reference gas (water vapor in Ar gas) was generated using a multi-gas trace-moisture generator (Multi-TMG), previously developed as a generation system of primary trace-moisture standards for multiple gas species. The values of the line-shape parameters and line intensity were determined with traceability to the International System of Units (SI) and were compared with previously reported values. The ratios of absorption coefficients of H2O in Ar and N2 gases near 7181 cm-1 containing the same amount of water vapor were also evaluated. These results provide important information for trace-moisture measurements in which the moisture concentration is determined only from the absorption coefficient at a specific frequency. Using the evaluated ratios, the measured values obtained in Ar gas can be converted to the correct moisture concentrations, enabling reliable trace-moisture measurements in Ar gas using a moisture analyzer calibrated only in N2 gas.

We study the impact of speed-dependent effects and velocity-changing collisions on the parameters of spectral line shapes in the SR subbranch of the a1 Delta g - X3 Sigma g-(0,0) band of 16O2 centered around 1.27 mu m. High-resolution absorption spectra with a high-signal-to-noise-ratio were acquired using the frequency-stabilized cavity ring-down spectrometer (FS-CRDS) for pure O2 and artificial air (O2 + N2 mixture) samples at room temperature. The measured spectra were fitted using the Voigt profile, the speed-dependent Voigt profile, the speed-dependent Nelkin-Ghatak profile, and the partially correlated speed-dependent Nelkin-Ghatak profile, assuming a quadratic speed dependence of the collisional broadening and shifting. Parameters of advanced line-shape models for the S13R14, S15R16, and S17R18 lines in the air-broadened case have been determined for the first time. The fitted profiles can be considered as simplified forms of the modified Hartmann-Tran profile (mHTP) recommended for the HITRAN database.

We introduce a spectroscopic approach to primary gas thermometry, harnessing precise optical cavity resonance frequencies and ab initio molecular line intensity calculations. By utilizing CO (3-0) vibrational band lines and cavity mode dispersion spectroscopy, we achieve an uncertainty of 82 ppm (24 mK at 296 K) in line-intensity-ratio thermometry (LRT) - over an order of magnitude lower than any previously reported spectroscopic thermometry at gas pressures above 1.2 kPa. This method extends high-precision spectroscopic thermometry across a pressure range an order of magnitude larger than prior techniques, enabling a fully optical, non-contact, and molecule-selective primary amount-of-substance measurement. We further demonstrate sub-permille uncertainty in gas concentration measurements across pressures from 50 Pa to 20 kPa, significantly enhancing the precision and versatility of spectroscopic gas metrology.

Measuring low light absorption with combined uncertainty < 1 permille is crucial in a wide range of applications. Popular cavity ring-down spectroscopy can provide ultra-high precision, below 0.01 permille, but its accuracy is strongly dependent on the measurement capabilities of the detection system and typically is about 10 permille. Here, we exploit the optical frequency information carried by the ring-down cavity electromagnetic field, not explored in conventional CRDS, for high-fidelity spectroscopy. Instead of measuring only the decaying light intensity, we perform heterodyne detection of ring-downs followed by Fourier analysis to provide exact frequencies of optical cavity modes and a dispersive spectrum of a gas sample from them. This approach is insensitive to inaccuracies in light intensity measurements and eliminates the problem of detector band nonlinearity, the main cause of measurement error in traditional CRDS. Using the CO and HD line intensities as examples, we demonstrate the sub-permille accuracy of our method, confirmed by the best ab initio results, and the long-term repeatability of our dispersion measurements at 10^(-4) level. Such results have not been achieved in optical spectroscopy before. The high accuracy of the presented method indicates its potential in atmospheric studies, isotope ratio metrology, thermometry, and the establishment of primary gas standards.

We recently reported the quantity values of line intensity for H2O near 1.393 mu m in a manner traceable to the International System of Units (SI). This paper briefly describes how to determine the reliable values of the line intensity, mainly focusing on the SI-traceability.

Main text We present primary spectroscopic measurements of line intensities in the 3-0 vibrational band of 12 C 16 O. This international measurement campaign was organized under the auspices of the Consultative Committee for Amount of Substance (CCQM) and involved six laboratories carrying out independent measurements of more than forty rotation-vibration transitions. A total of three measurement techniques on samples of pure carbon monoxide were applied in this intercomparison, including Fourier transform spectroscopy, cavity ring-down spectroscopy, and cavity mode dispersion spectroscopy. Using advanced spectroscopic line shape models and thorough characterization of instrument response to analyze the measured spectra, artifact-free measurements of line intensities were obtained by each laboratory. Average intensities were weighted by the inverse of the combined variance determined from rigorous estimates of combined systematic and statistical uncertainties. Results for all lines from five of the laboratories were scattered about the weighted mean value by nominally one part per thousand. Comparison of the weighted integrated band intensity with quantum-chemical calculations differed by nominally one part per ten thousand, thus demonstrating excellent agreement between experiment and theory. These results serve as an experimental benchmark for assessing the uncertainty in theoretical calculations, and they reveal the benefits of coordinated experiments that leverage complementary and independent primary measurements from contributing laboratories. To reach the main text of this paper, click on Final Report . The final report has been peer-reviewed and approved for publication by the CCQM.

The redefinition of the International System of Units (SI) [1] separated unit definitions, based on a set of fundamental constants, from their realizations. Rotational line intensity ratio thermometry (LRT) [2], [3], also called rotational-state distribution thermometry, is one of optical realizations of kelvin that promises high accuracy. In contrast to Doppler broadening thermometry [4], LRT uses the integrated spectral line area which can be accurately retrieved even at pressures in which collisions significantly alter the line shape.

Light-matter interactions involving molecular oxygen (O2) span numerous decades in the frequency of electromagnetic radiation and are important to many thermophysical and thermochemical mechanisms, ranging from atmospheric remote sensing of greenhouse gases, aerosols, pollutants, temperature and pressure, visible and infrared radiative exchange in the upper atmosphere, ozone formation and decomposition, and the search for life beyond Earth, among many other examples. Here, we highlight advances in the quantitative spectroscopy of O2, for which updated band-specific, line-by-line parameters have been provided in the HITRAN2024 spectroscopic database. Theoretical results are presented for electric quadrupole transition intensities in the ground state of O 2 16 , and the Noxon band in the near-infrared region has been included in HITRAN for the first time. Particular focus is placed on the 1.27 μm, A- and B-bands of O2, in which intensities, line-shape (including beyond-Voigt parameterizations), and position parameters with improved accuracy and/or extended spectral coverage are presented. Corrections to the Schumann-Runge bands are also reported. The paper closes with recommendations and an outlook on key challenges in advancing our understanding of the spectroscopy of O2.

Spectrometers based on high-finesse optical cavities have proven to be powerful tools for applied and fundamental studies. Extending this technology to the deep cryogenic regime is beneficial in many ways: Doppler broadening is reduced, peak absorption is enhanced, the Boltzmann distribution of rotational states is narrowed, all unwanted molecular species disturbing the spectra are frozen out, and dense spectra of complex polyatomic molecules become easier to assign. We demonstrate a cavity-enhanced spectrometer fully operating in the deep cryogenic regime down to 4 K. We solved several technological challenges that allowed us to uniformly cool not only the sample but also the entire cavity, including the mirrors and cavity length actuator, which ensures the thermodynamic equilibrium of a gas sample. Our technology well isolates the cavity from external noise and cryocooler vibrations. This instrument enables a variety of fundamental and practical applications. We demonstrate a few examples based on accurate spectroscopy of cryogenic hydrogen molecules: accurate test of the quantum electrodynamics for molecules; realization of the primary SI standards for temperature, concentration and pressure in the deep cryogenic regime; measurement of the H_2 phase diagram; and determination of the ortho-para spin isomer conversion rate.

We present an experimental study of carbon dioxide absorption in the transparency window near 700 nm, where different calculations predicted several extremely weak bands. The measurements were done using the optical frequency comb-assisted cavity ring-down spectrometer reaching sensitivity (noise-equivalent absorption) of 3.1⋅10−12 cm−1. None of 21 lines, predicted by the most comprehensive calculations, were detected in five separate spectral regions near 700 nm. Our estimated detection limit ranges between 3 and 21 times lower than the expected peak absorption in the conditions of the experiment.

The high-precision measurement of trace water vapor (trace moisture) in gases is important in technologyintensive industries. We have been developing measuring instruments for trace moisture using cavity ring-down spectroscopy (CRDS). In CRDS, an optical cavity consisting of highly reflective mirrors is used as a sample cell to extend the effective optical path length. We measured the ring-down time in dry N-2 gas and acquired the absorption spectra of H2O near 7180 cm(-1). When we acquired the absorption spectra in the dry N-2 gas, periodic structure which was not related to the water absorption line was observed. This structure was caused by fringe noise, which occurred when the reflected light from an optical component surface incident on the cavity. In our measuring system, to stabilize the resonant frequencies of the cavity, the length of the cavity was adjusted using a piezoelectric actuator (PZT) mounted at one end of the cavity. To accurately measure trace moisture, the sample cell was sealed using windows to prevent moisture from entering the cell from the outside, and the PZT was placed outside the sample cell to avoid the effects of water desorption from the element. To adjust the position of the mirror, the mirror and a window were connected and their position was moved using the PZT. In this structure, because the distance between the mirror and the window was small and they were fixed parallel to each other, it was difficult to prevent the reflected light from returning into the cavity, which tended to cause fringe noise. In this study, to eliminate the fringe noise, the mirror and the window were separated so that they could be adjusted independently. In addition, the window was wedged so that the light reflections on both sides were not collinear. The mirror and the PZT were directly connected and placed inside the cell. The gas flow was designed so that the gas was discharged without stagnation to prevent desorbed moisture from entering the measurement space. This improvement has made it possible to measure the trace moisture more precisely.

Dispersion spectroscopy based on frequency measurement of the high finesse optical cavity mode shifts induced by the presence of a gas medium with absorption resonances is an attractive alternative to standard absorptive spectroscopy. The main advantage of such pure frequency-based dispersive spectroscopy technique is the unique combination of sensitivity and accuracy. High sensitivity is ensured by high finesse optical cavities providing extremely long effective optical paths. On the other hand, high accuracy is provided by frequency measurement cavity mode shift. This makes this technique free from systematic errors coming from the non-linearity of light intensity measurements, which is one of the limiting factors in typical absorptive spectroscopy. We review several realizations of dispersive spectroscopy in high finesse optical cavities from Doppler-free to broadband. These techniques are especially useful for studying weak molecular transitions applicable in testing fundamental physics, gas metrology, spectroscopic thermometry, and a new generation of spectroscopic databases.

We present the results of the spectral line-shape study of the first measurement of the extremely weak (7–0) band of the 12C16O molecule. Measurements were done with a highly sensitive cavity ring-down spectrometer. Collisional narrowing, analyzed in terms of speed-dependent effects, was observed for the first time for transitions with line intensities below 2⋅10−29 cm/molecule at 296 K. We provide a full set of line-shape parameters of the speed-dependent and regular Voigt profile analysis for 14 transitions from P and R branches. Experimental verification of a strong vibrational dependence of the pressure shifting described by the Hartmann model (Hartmann, 2009) is extended up to the sixth overtone highly sensitive to the model parameter.

The hydrogen deuteride (HD) molecule is an important deuterium tracer in astrophysical studies. The atmospheres of gas giants are dominated by molecular hydrogen, and simultaneous observation of H$_2$ and HD lines provides reliable information on the D/H ratios on these planets. The reference spectroscopic parameters play a crucial role in such studies. Under thermodynamic conditions encountered in these atmospheres, the spectroscopic studies of HD require not only the knowledge of line intensities and positions but also accurate reference data on pressure-induced line shapes and shifts. Our aim is to provide accurate collision-induced line-shape parameters for HD lines that cover any thermodynamic conditions relevant to the atmospheres of giant planets, i.e., any relevant temperature, pressure, and perturbing gas (the H$_2$/He mixture) composition. We perform quantum-scattering calculations on a new highly accurate ab initio potential energy surface, and we use scattering S-matrices obtained this way to determine the collision-induced line-shape parameters. We use the cavity ring-down spectroscopy for validation of our theoretical methodology. We report accurate collision-induced line-shape parameters for the pure rotational R(0), R(1), and R(2) lines, the most relevant HD lines for the investigations of atmospheres of the giant planets. Besides the basic Voigt-profile collisional parameters (i.e. the broadening and shift parameters), we also report their speed dependences and the complex Dicke parameter, which can influence the effective width and height of the HD lines up to almost a factor of 2 for giant planet conditions. The sub-percent-level accuracy, reached in this work, considerably improves the previously available data. All the reported parameters are consistent with the HITRAN database format, hence allowing for the use of HAPI for generating the beyond-Voigt spectra of HD.

The absorption spectra of trace water vapor in N2 gas with a mole fraction of 500 nmol/mol in the pressure range of 20 kPa to 140 kPa were obtained near 7181 cm−1 using wavelength-meter-controlled cavity ring-down spectroscopy (WMC-CRDS). The reference gas (water vapor in N2 gas) was produced using a generation system of primary measurement standards for trace moisture in gases, where the amount of water vapor was measured using a gravimetric method. The obtained spectra of H2O were analyzed using a multispectrum fitting technique with a speed-dependent asymmetric Voigt profile (SDAVP). The values of the line intensity were determined with traceability to the International System of Units (SI). We could achieve the expanded uncertainty below 2 % for the line intensities of the strong lines.

Intensities of 14 lines in the sixth overtone (7-0) band of carbon monoxide (12C16O) are measured in the visible range between 14 300 and 14 500 cm-1 using a frequency-stabilized cavity ring-down spectrometer. This is the first observation of such a high and weak overtone spectrum of the CO molecule. A theoretical model is constructed and tested based on the use of a high accuracy ab initio dipole moment curve and a semi-empirical potential energy curve. Accurate studies of high overtone transitions provide a challenge to both experiment and theory as the lines are very weak: below 2 × 10-29 cm molecule-1 at 296 K. Agreement between theory and experiment within the experimental uncertainty of a few percent is obtained. However, this agreement is only achieved after issues with the stability of the Davidson correction to the multi-reference configuration interaction calculations are addressed.