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Quantitative UV laser absorption spectroscopy is implemented to measure the strength of the (6s^{2})^{1}S_{0}→(6s6p)^{3}P_{1} intercombination transition of Hg atoms near the wavelength of 253.7 nm and to retrieve the lifetime of the excited state. Tunable, single-mode, coherent radiation in the deep UV is produced through a double-stage second-harmonic generation process, starting from the output radiation of an external-cavity diode laser at 1014.8 nm. This latter is referenced to an optical frequency comb synthesizer. The measurement strategy exploits the relationship between the strength of an optical transition and the spontaneous emission Einstein coefficient. The complete uncertainty budget leads to an overall relative uncertainty of 0.6%, with a statistical contribution of 0.28 ns, which is the lowest ever obtained for the (6s6p)^{3}P_{1} lifetime. The new value, amounting to 125.12±0.79ns, compares well with most of the past determinations within two times the quoted uncertainties. Despite this, it does not agree with the weighted average of the 12 prior values, thus suggesting that a critical reassessment of the uncertainty of some of the past measurements is probably necessary.

Doppler-free saturated-absorption Lamb dips are observed for weak vibration-rotation transitions of C _2 H _2 between 7167 and 7217 cm ^−1 , using a frequency-comb assisted cavity ring-down spectrometer based on the use of a pair of phase-locked diode lasers. We measured the absolute center frequency of sixteen lines belonging to the 2 ${\nu }_{3}+{\nu }_{5}^{1}$ band, targeting ortho and para states of the molecule. Line pairs of the P and Q branches were selected so as to form a ‘V’-scheme, sharing the lower energy level. Such a choice made it possible to determine the rotational energy separations of the excited vibrational state for J -values from 11 to 20. Line-center frequencies are determined with an overall uncertainty between 3 and 13 kHz. This is over three orders of magnitude more accurate than previous experimental studies in the spectral region around the wavelength of 1.4 μ m. The retrieved energy separations provide a stringent test of the so-called MARVEL method recently applied to acetylene.

We report on precision spectroscopy of the 6s^{2} ^{1}S_{0}→6s6p ^{3}P_{1} intercombination line of mercury in the deep ultraviolet, by means of a frequency-comb referenced, wavelength-modulated, saturated absorption technique. This method allowed us to perform sub-Doppler investigations with an absolute frequency axis at 254 nm, while ensuring a relatively high signal-to-noise ratio. The absolute line center frequencies of the ^{200}Hg and ^{202}Hg bosonic isotopes were measured with a global uncertainty of 8 and 15 kHz (namely, 6.8×10^{-12} and 1.3×10^{-11}, in relative terms), respectively, the statistical and systematic components being significantly reduced as compared to past determinations. This remarkable result was achieved also thanks to an in-depth study of the ac Stark effect. Furthermore, we found the most accurate ^{200}Hg-^{202}Hg isotope shift ever obtained before, namely, 5 295 57 0±15_{stat}±8_{syst}  kHz.

Lamb dips of twenty lines in the P, Q, and R branches of the ν 1 + ν 3 + ν 4 1 vibrational band of 12 C 2 H 2 , in the spectral window of 7125-7230 cm -1 , have been measured using an upgraded comb-calibrated frequency-stabilized cavity ring-down spectrometer, designed for extensive sub-Doppler measurements. Due to the large number of carefully executed Lamb-dip experiments, and to the extrapolation of absolute frequencies to zero pressure in each case, the combined average uncertainty of the measured line-center positions is 15 kHz (5 × 10 -7 cm -1 ) with a 2-σ confidence level. Selection of the twenty lines was based on the theory of spectroscopic networks (SN), ensuring that a large number of transitions, measured previously by precision-spectroscopy investigations, could be connected to the para and ortho principal components of the SN of 12 C 2 H 2 . The assembled SN contains 331 highly precise transitions, 119 and 121 of which are in the ortho and para principal components, respectively, while the rest remain in floating components. The para - and ortho - 12 C 2 H 2 energy-level lists, determined during the present study, contain 82 and 80 entries, respectively, with an accuracy similar to that of the lines. Based on the newly assembled lists of para - and ortho - 12 C 2 H 2 empirical energy levels, a line list, called TenkHz, has been generated. The TenkHz line list contains 282 entries in the spectral range of 5898.97-7258.87 cm -1 ; thus far, only 149 of them have been measured directly via precision spectroscopy. The TenkHz line list includes 35 intense lines that are missing in the HITRAN2020 database.

Line intensities for carbon dioxide are measured with a novel spectroscopic approach, assisted by an optical frequency comb synthesizer for frequency calibration purposes. The main feature of the spectrometer consists in the exploitation of optical feedback from a V-shaped high-finesse optical resonator to effectively narrow a distributed feedback diode laser at the wavelength of 2 μm. Laser-gas interaction takes place inside an isothermal cell, which is placed on the transmission from the cavity. High quality, self-calibrated, absorption spectra are observed in pure CO 2 samples at different gas pressures, in coincidence with three lines of the R-branch of the ν 1 + 2ν 2 + ν 3 band. Line intensities are determined using a global fitting approach in which a manifold of spectra are simultaneously analyzed across the range of pressures between 5 and 100 Torr, sharing a restricted number of unknown parameters. Various sources of uncertainty have been identified and carefully quantified, thus leading to an overall uncertainty ranging between 0.17% and 0.23%. The measured values are in a very good agreement with recent ab initio predictions.

We report on absolute measurements of saturated-absorption line-center frequencies of room-temperature trifluoromethane using a quantum cascade laser at 8.6 μm and the frequency modulation spectroscopy method. Absolute calibration of the laser frequency is obtained by direct comparison with a mid-infrared optical frequency comb synthesizer referenced to a radio-frequency Rb standard. Several sub-Doppler transitions falling in the υ 5 vibrational band are investigated at around 1158.9  cm -1 with a fractional frequency precision of 8.6·10 -12 at 1-s integration time, limited by the Rb-clock stability. The demonstrated frequency uncertainty of 6.6·10 -11 is mainly limited by the reproducibility of the frequency measurements.

We report a high-precision spectroscopic study of room-temperature trifluoromethane around 8.6 μm, using a CW quantum cascade laser phase-locked to a mid-infrared optical frequency comb. This latter is generated by a nonlinear down-conversion process starting from a dual-branch Er:fiber laser and is stabilized against a GPS-disciplined rubidium clock. By tuning the comb repetition frequency, several transitions falling in the υ5 vibrational band are recorded with a frequency resolution of 20 kHz. Due to the very dense spectra, a special multiple-line fitting code, involving a Voigt profile, is developed for data analysis. The combination of the adopted experimental approach and survey procedure leads to fractional accuracy levels in the determination of line center frequencies, down to 2 × 10(-10). Line intensity factors, pressure broadening, and shifting parameters are also provided.

We developed a high-precision spectroscopic system at 8.6 μm based on direct heterodyne detection and phase-locking of a room-temperature quantum-cascade-laser against an harmonic, 250-MHz mid-IR frequency comb obtained by difference-frequency generation. The ∼30  dB signal-to-noise ratio of the detected beat-note together with the achieved closed-loop locking bandwidth of ∼500  kHz allows for a residual integrated phase noise of 0.78 rad (1 Hz-5 MHz), for an ultimate resolution of ∼21  kHz, limited by the measured linewidth of the mid-IR comb. The system was used to perform absolute measurement of line-center frequencies for the rotational components of the ν2 vibrational band of N2O, with a relative precision of 3×10(-10).

Frequency fluctuations of an optical frequency standard at 1.39 µm have been measured by means of a highly-sensitive optical frequency discriminator based on the fringe-side transmission of a high finesse optical resonator. Built on a Zerodur spacer, the optical resonator exhibits a finesse of 5500 and a cavity-mode width of about 120 kHz. The optical frequency standard consists of an extended-cavity diode laser that is tightly stabilized against the center of a sub-Doppler H(2) (18)O line, this latter being detected by means of noise-immune cavity-enhanced optical heterodyne molecular spectroscopy. The emission linewidth has been carefully determined from the frequency-noise power spectral density by using a rather simple approximation, known as β-line approach, as well as the exact method based on the autocorrelation function of the laser light field. It turns out that the linewidth of the optical frequency standard amounts to about 7 kHz (full width at half maximum) for an observation time of 1 ms. Compared to the free-running laser, the measured width corresponds to a line narrowing by a factor of ~220.

We report on a narrow-linewidth distributed-feedback quantum cascade laser at 8.6 μm that is optical-feedback locked to a high-finesse V-shaped cavity. The spectral purity of the quantum cascade laser is fully characterized using a high-sensitivity optical frequency discriminator, leading to a 1 ms linewidth of less than 4 kHz and a minimum laser frequency noise spectral density as low as 0.01 Hz2/Hz for Fourier frequencies larger than 100 kHz. The cumulative standard deviation of the laser intensity is better than 0.1% over an integration bandwidth from 2 Hz to 100 MHz.

We implemented an optical frequency standard based on noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) at 1.39 μm. The emission frequency of an extended-cavity diode laser was actively stabilized against the center of the 4(4,1)→4(4,0) transition of the H(2)(18)O ν1+ν3 band, under optical saturation conditions. The nonlinear regime of laser-gas interaction was reached by using an optical cavity with a finesse of about 8700. By filling it with an 18O-enriched water sample at a pressure of a few Pa, the Lamb dip could be observed with a full width at half-maximum of about 2 MHz. Absolute frequency stabilization was obtained by locking the cavity resonance to the center of the sub-Doppler signal, which was provided by the NICE-OHMS technique under the dispersion regime of operation. An Allan deviation analysis demonstrated a relative frequency stability of ∼5×10(-13) for an integration time of 1 s. For longer integration times, the flicker frequency noise floor set the stability at the level of 4×10(-14).

We report on a new implementation of Doppler broadening thermometry based on precision absorption spectroscopy by means of a pair of offset-frequency locked extended-cavity diode lasers at 1.39  μm. The method consists in the highly accurate observation of the shape of the 4(4,1)→4(4,0) line of the H2(18)O ν1+ν3 band, in a water vapor sample at thermodynamic equilibrium. A sophisticated and extremely refined spectral analysis procedure is adopted for the retrieval of the Doppler width as a function of the gas pressure, taking into account the Dicke narrowing effect, the speed dependence of relaxation rates, and the physical correlation between velocity-changing and dephasing collisions. A spectroscopic determination of the Boltzmann constant with a combined (type A and type B) uncertainty of 24 parts over 10(6) is reported. This is the best result obtained so far by means of an optical method. Our determination is in agreement with the recommended CODATA value.

Complementary tests of the partially correlated speed-dependent Keilson-Storer (pCSDKS) model for the shape of isolated transition of pure water vapor [N. H. Ngo et al., J. Chem. Phys. 136, 154310 (2012)] are made using new measurements. The latter have been recorded using a high sensitivity cavity ring down spectrometer, for seven self-broadened H(2)O lines in the 1.6 μm region at room temperature and for pressures from 0.5 to 15 Torr. Furthermore, the H(2) (18)O spectra of [M. D. De Vizia et al., Phys. Rev. A 83, 052506 (2011)] in the 1.38 μm region, measured at 273.15 K and for pressures from 0.3 to 3.75 Torr have also been used for comparison with the model. Recall that the pCSDKS model takes into account the collision-induced velocity changes, the speed dependences of the broadening and shifting coefficients as well as the partial correlation between velocity and rotational-state changes. All parameters of the model have been fixed at values previously determined, except for a scaling factor applied to the input speed-dependent line broadening. Comparisons between predictions and experiments have been made by looking at the results obtained when fitting the calculated and measured spectra by Voigt profiles. The good agreement obtained for all considered lines, at different temperature and pressure conditions, confirms the consistency and the robustness of the model. Limiting cases of the model have been then derived, showing the influence of different contributions to the line shape.

We describe a continuous-wave diode laser spectrometer for water-vapour precision spectroscopy at 1.38 μm. The spectrometer is based upon the use of a simple scheme for offset-frequency locking of a pair of extended-cavity diode lasers that allows to achieve unprecedented accuracy and reproducibility levels in measuring molecular absorption. When locked to the master laser with an offset frequency of 1.5 GHz, the slave laser exhibits residual frequency fluctuations of 1 kHz over a time interval of 25 minutes, for a 1-s integration time. The slave laser could be continuously tuned up to 3 GHz, the scan showing relative deviations from linearity below the 10{-6} level. Simultaneously, a capture range of the order of 1 GHz was obtained. Quantitative spectroscopy was also demonstrated by accurately determining relevant spectroscopic parameters for the 22,1→22,0line of the H2(18)O v1+v3 band at 1384.6008 nm.

We report the frequency stabilization of a cw extended-cavity diode laser against saturated absorption lines of the H(2)O17 isotopologue of water vapor at around 1.384 microm. The saturation of rotovibrational transitions is achieved by filling a high-finesse optical resonator with H(2)O17 at low pressure and by locking the laser frequency to the resonator by using the Pound-Drever-Hall technique. Absolute frequency stabilization is obtained, locking the cavity resonance to the center of the sub-Doppler line by means of the wavelength modulation method. A relative frequency stability of sigma(y)(tau)=10(-13)(0.1tau(-2)+0.9)(1/2) is demonstrated for integration times in the range 4 ms

An intensity-stabilized diode laser absorption spectrometer was developed and used to perform a highly accurate study of the line shape of CO(2) absorption lines, in the spectral region around 5000 cm(-1), belonging to the nu(1) + 2nu(2)(0) + nu(3) combination band, at a temperature of 296.00 K. Standard and complex semiclassical models, including Dicke narrowing and speed-dependent broadening effects, were applied, tested, and compared in the pressure range between 0.7 and 4 kPa, in order to single out the model best reproducing the absorption profile and, hence, the physical situation of self-colliding CO(2) molecules. Line intensity factors and self-broadening coefficients were determined. The 1-sigma overall accuracy of our determinations is at a level of 0.1%, which is, to our knowledge, the highest ever reached.