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.

Full-dimensional quantum scattering calculations are reported for rovibrational transitions in HD+HD collisions using a highly accurate interaction potential for the H2-H2 system. Several near-resonant rovibrational transitions are identified that conserve the overall rotational angular momentum and nearly conserve the internal energy of the collision partners. Key anisotropic terms that drive the rotational transitions and angular momentum partial waves that contribute to low-energy resonant features in the energy dependence of the cross-sections are identified. The computed results agree with total cross-sections reported in previous experimental results, including resonant features in the energy dependence of the cross-section. In particular, low-energy cross-sections show a strong resonant feature associated with an l = 3 partial wave in the incident channel. Rate coefficients for several inelastic rotational and rovibrational transitions are reported for temperatures ranging from 0.1 to 200 K, and they display a maximum between 1 and 10 K, reflecting the important contributions from the l = 3 shape resonance that occurs around 2.5 K.

Exotic helium atoms act as unique atomic traps for heavy, negatively charged particles, protecting them from nuclear annihilation and nuclear capture on timescales long enough to enable high-precision laser spectroscopy. Such measurements serve as stringent tests of three-body quantum electrodynamics and offer a direct route to determining fundamental particle masses. Motivated by upcoming spectroscopic efforts targeting pionic (π^- 4He^+) and kaonic (K^- 4He^+) helium, we present a rigorous theoretical evaluation of the collisional and density effects governing these systems. Using an ab initio potential energy surface and coupled-channel quantum scattering calculations, we study the collisional stability of the candidate metastable states against inelastic quenching in a cryogenic helium buffer gas. Furthermore, we provide theoretical reference values for the pressure broadening and pressure shift coefficients of the targeted transitions. These results establish an essential benchmark for future experiments, paving the way for refined determinations of the pion and kaon masses.

We present the first fully ab initio calculations of collision-induced broadening and shift of spectral lines in antiprotonic helium (p¯He+) perturbed by atomic helium. To overcome critical limitations of previous studies, we construct a new highly accurate potential energy surface (PES) that spans a wide range of p¯He+-He geometries relevant to all metastable states of the exotic helium atom. Rigorous quantum scattering calculations performed using the new PES yield scattering S-matrices from which we extract pressure broadening and shift coefficients for 50 transitions in antiprotonic helium-4 (p¯4He+). This data set provides the first rigorous benchmark for earlier semiclassical calculations and establishes a robust theoretical reference for high-precision spectroscopy of antiprotonic helium, which is used to test the fundamental charge, parity, and time reversal (CPT) symmetry. The results extend to temperatures relevant to nongaseous phases of helium, supporting a new class of precision measurements. This study introduces a methodological framework for future investigations of other exotic systems, such as pionic or kaonic helium atoms, enabling the development of reference data for high-precision spectroscopy of these species─an essential component for improving the determination of the pion and kaon masses.

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.

In the early days of laser spectroscopy Letokhov and Chebotayev proposed a scheme for measuring narrow spectral lines where the resolution is not restricted to Doppler effects because the molecules are entrained in a standing-wave light field. Now, such one-dimensional trapping in the intensity maxima of an intracavity field, slightly detuned from resonance, is experimentally demonstrated in the measurement of the very weak S(0) (2-0) quadrupole overtone transition in H_2 at 1189 nm. The trapping manifests as an extremely narrow absorption feature at the predicted zero-recoil position, a 70 kHz shift from the blue-recoil component observed in Lamb-dip spectroscopy. A quantitative analysis of the saturation and trapping conditions supports the findings.

We present the first experimental determination of room-temperature N2 pressure broadening, speed dependent broadening, and pressure shift coefficients of the three lowest rotational lines of HCN. The experimental results served to assess the accuracy of a low-cost yet accurate computational strategy, which relies on a simplified characterization of the HCN-N2 interaction potential, and employs a novel approximate method of solving the quantum scattering problem. Building on the validation of this computational approach, the dataset was extended to higher rotational transitions, up to J(HCN)=5-4. For these transitions, we provide the temperature dependence of the pressure broadening coefficient, its speed dependence parameter, and the Dicke narrowing parameter. This new dataset can support and refine the modeling of HCN in both the terrestrial and Titan's atmospheres. This work constitutes an important step towards populating spectroscopic databases with accurate HCN line-shape parameters.

The intensities of all rovibrational electric quadrupole absorption lines in 16O2(X3 Sigma g-), for which the vibrational quantum number is v <= 35 and the total angular momentum quantum number is J <= 40, are calculated in the intermediate coupling using anew ab initio quadrupole moment curve of the ground electronic state of O2. The calculated values agree with those available in the HITRAN database, which at present includes, for quadrupole transitions, only the 1-0 fundamental vibrational band of 16O2(X3 Sigma g-). We therefore recommend using the intensities of the vibrational overtones and hot bands reported herein updating the HITRAN database for O2 in the upcoming 2024 edition.

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.

The intensities of all rovibrational electric quadrupole absorption lines in ^16O_2(X^3Σ^-_g), for which the vibrational quantum number is v ≤ 35 and the total angular momentum quantum number is J ≤ 40, are calculated in the intermediate coupling using a new ab initio quadrupole moment curve of the ground electronic state of O_2. The calculated values agree with those available in the HITRAN database, which at present includes only the 1-0 fundamental vibrational band of ^16O_2(X^3Σ^-_g). We therefore recommend using the intensities of the vibrational overtones and hot bands reported here in updating the HITRAN database for O_2 in the upcoming 2024 edition.

Context. The collisional (de-)excitation of H 2 by He plays an important role in the thermal balance and chemistry of various astro-physical environments, making accurate rate coefficients essential for interpreting observations of the interstellar medium. Aims. Our goal is to utilize a state-of-the-art potential energy surface (PES) to provide comprehensive state-to-state rate coefficients for He-induced transitions among rovibrational levels of H 2 . Methods. We performed quantum scattering calculations for the H 2 -He system. Thus, we were able to provide state-to-state rate coefficients for 1059 transitions between rovibrational levels of H 2 , with internal energies up to ≃15 000 cm −1 , for temperatures ranging from 20 to 8000 K. Results. Our results demonstrate a good agreement with previous calculations for pure rotational transitions between low-lying rotational levels. However, we do find significant discrepancies for rovibrational processes involving highly-excited rotational and vibrational states. We attribute these differences to two key factors: (1) the broader range of intramolecular distances covered by ab initio points and (2) the superior accuracy of the PES, resulting from the utilization of the state-of-the-art quantum chemistry methods, compared to previous lower-level calculations. Conclusions. Radiative transfer calculations performed with the new collisional data indicate that the population of rotational levels in excited vibrational states experiences significant modifications, highlighting the critical need for this updated dataset in models of high-temperature astrophysical environments.

We present a rigorous quantum scattering study of the effects of hyperfine and Zeeman interactions on cold Li - H_2 collisions in the presence of an external magnetic field using a recent ab initio potential energy surface. We find that the low-field-seeking states of H_2 predominantly undergo elastic collisions: the ratio of elastic-to-inelastic collisions exceeds 100 for collision energies below 1.5 K. Furthermore, we demonstrate that most inelastic collisions conserve the space-fixed projection of the nuclear spin. We show that the anisotropic hyperfine interaction between the nuclear spin of H_2 and the electron spin of Li can have a significant effect on inelastic scattering in the ultracold regime, as it mediates two processes: the electron spin relaxation in lithium, and the nuclear spin - electron spin exchange. Given the predominance of elastic collisions and the propensity of inelastic collisions to retain H_2 in its low-field-seeking states, our results open up the possibility of sympathetic cooling of molecular hydrogen by atomic lithium, paving the way for future exploration of ultracold collisions and high-precision spectroscopy of H_2 molecules.

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.

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.

Accurate spectroscopy of molecular hydrogen isotopologues is used for testing quantum electrodynamics and searching for physics beyond the standard model. Recent measurements of energies of rovibrational resonances in the ground electronic state have reached a level of uncertainty lower than the magnitude of the hyperfine splitting. The underlying hyperfine components of the resonance clearly perturb sub-Doppler saturation spectra. The extent to which hyperfine structure influences the Doppler-limited spectra is not fully understood, as there are two contradicting experimental works that show either a 350 kHz shift or lack of any deviation from the central frequency of the resonance in the HD molecule. Here, we address this problem theoretically. Using the spherical tensor approach, we prove that the barycenter of all hyperfine-resolved spectra corresponds to the unperturbed transition frequency (the first moment of the hyperfine-resolved spectra vanishes). This property is directly transferred to Doppler-limited spectra: we show that there is no detectable shift due to the hyperfine structure unless the ratio of the Doppler width to the root-mean-square hyperfine splitting is less than 50.

Information about molecular collisions is encoded in the shapes of collision-perturbed molecular resonances. This connection between molecular interactions and line shapes is most clearly seen in simple systems, such as the molecular hydrogen perturbed by a noble gas atom. We study the H2-Ar system by means of highly accurate absorption spectroscopy and ab initio calculations. On the one hand, we use the cavity-ring-down-spectroscopy technique to record the shapes of the S(1) 3-0 line of molecular hydrogen perturbed by argon. On the other hand, we simulate the shapes of this line using ab initio quantum-scattering calculations performed on our accurate H2-Ar potential energy surface (PES). In order to validate the PES and the methodology of quantum-scattering calculations separately from the model of velocity-changing collisions, we measured the spectra in experimental conditions in which the influence of the latter is relatively minor. In these conditions, our theoretical collision-perturbed line shapes reproduce the raw experimental spectra at the percent level. However, the collisional shift, δ0, differs from the experimental value by 20%. Compared to other line-shape parameters, collisional shift displays much higher sensitivity to various technical aspects of the computational methodology. We identify the contributors to this large error and find the inaccuracies of the PES to be the dominant factor. With regard to the quantum scattering methodology, we demonstrate that treating the centrifugal distortion in a simple, approximate manner is sufficient to obtain the percent-level accuracy of collisional spectra.