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Spectroscopic parameters (line intensity factor, pressure self-broadening, and shifting coefficients) of C2H2 at 1.4 μm were accurately measured using a dual-laser approach, based upon the technique of optical phase locking. This generated an absolute frequency scale underneath the absorption spectra. A pair of extended-cavity diode laserswas used. One of them, the probe laser, is forced to maintain a precise frequency offset from a reference laser, which is an optical frequency standard based on noise-immune cavity-enhanced optical heterodyne molecular spectroscopy. Laser-gas interaction takes place inside an isothermal multipass cell that is stabilized at the temperature of the triple point of water. The fidelity in the observation of the shape associated to the Pe(14) line of the 2ν3 + ν5 band allowed us to measure the spectroscopic parameters, with a global uncertainty for the line strength of 0.22%.

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 m. The method consists in the highly accurate observation of the shape of the 44;1 ! 44;0 line of the H2 18O 1 þ3 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 106 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.

A laser absorption technique, based upon a pair of offset-frequency-locked extended-cavity diode lasers, was employed to perform line-shape measurements for H218O vibration-rotation transitions at 1.38 μm, with an extremely high spectral fidelity. This important feature was mostly ensured by the highly accurate, absolute, and repeatable frequency axis characterizing each acquired spectrum. We present the outcomes of a specific study of the speed dependence of collision parameters influencing the shape of the H218O absorption lines. More particularly, the performance of the quadratic approximation, either used alone or considered in conjunction with the Dicke narrowing effect, was investigated and compared to that of the confluent hypergeometric dependence. Hence, experimental evidences of the partial failure of the quadratic approximation are given and discussed. Finally, we investigate the possible influence of the choice of the speed-dependent line-shape model on the retrieved collisional parameters.

The FAMU experiment aims to measure for the first time the hyperfine splitting of the muonic hydrogen ground state. From this measurement the proton Zemach radius can be derived and this will shed light on the determination of the proton charge radius. In this paper, we describe the scientific goal, the method and the detailed preparatory work. This includes the outcome of preliminary measurements, subsequent refined simulations and the evaluation of the expected results. The experimental setup being built for the measurement of the hyperfine splitting to be performed at the RAL laboratory muon facility is also described.

The first measurement of the temperature dependence of the muon transfer rate from muonic hydrogen to oxygen was performed by the FAMU collaboration in 2016. The results provide evidence that the transfer rate rises with the temperature in the range 104-300 K. This paper presents the results of the experiment done in 2018 to extend the measurements towards lower (70 K) and higher (336 K) temperatures. The 2018 results confirm the temperature dependence of Λ p O observed in 2016 and sets firm ground for comparison with the theoretical predictions.

We report on the development of a new primary thermometer based upon high-precision spectroscopy of mercury vapors in the deep-ultraviolet region for the practical realization of the new kelvin. The line profile of the (6s 2)1 S 0 → (6s6p)3 P 1 intercombination transition of the 200Hg bosonic isotope is observed with a high spectral fidelity using a coherent radiation source at 253.7 nm. This latter consists of a near-IR external cavity diode laser followed by a double-stage second-harmonic generation apparatus. Metrology grade UV spectroscopy is demonstrated by locking the diode laser to a self-referenced optical frequency comb synthesizer.

We present high-accuracy measurements of the spectroscopic parameters of eight nitrous oxide transitions, mainly belonging to the P branch of the 3 ν 1 + 2 ν 2 vibrational band, in the spectral region around 4982 cm − 1 . We acquired absorption spectra in pure N2O samples, at room-temperature, by means of a diode laser based spectrometer in which the laser emission width was effectively narrowed by exploiting the optical feedback from a V-shaped high-finesse cavity. The spectrometer was assisted by a self-referenced optical frequency comb synthesizer for frequency calibration purposes. Light absorption from N2O molecules occurred inside a multiple reflections cell, which was placed on the transmission from the cavity. The successful application of a global fitting approach to a manifold of spectra, involving the symmetric uncorrelated version of quadratic speed-dependent hard collision profile, allowed us to determine line intensities with an overall relative uncertainty of 0.38%. We found a satisfactory agreement with data provided by the HITRAN and NOSD-1000 databases. Self-broadening coefficients, speed dependence components of the collisional line broadening, and Dicke narrowing parameters were also determined.

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 m. Lasergas 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 CO2 samples at different gas pressures, in coincidence with three lines of the R-branch of the nu1 + 2nu2 + nu3 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.

The expression of the Doppler width of a spectral line, valid for a gaseous sample at thermodynamic equilibrium, represents a powerful tool to link the thermodynamic temperature to an optical frequency. This is the basis of a relatively new method of primary gas thermometry, known as Doppler broadening thermometry. Implemented at the Second University of Naples on H218O molecules at the temperature of the triple point of water, this method has recently allowed to determine the Boltzmann constant with a global uncertainty of 24 parts over 106. Even though this is the best result ever obtained by using an optical method, its uncertainty is still far from the requirement for the new definition of the unit kelvin. To this end, Doppler broadening thermometry should approach the accuracy of 1 part per million. In this paper, we will report on our recent efforts to further develop and optimize Doppler broadening thermometry at 1.39 μm, using acetylene as a molecular target. Main progresses and current limitations will be highlighted.

Typically eliminated in any experiment of time and frequency metrology, the Doppler broadening effect can be regarded as a gift of nature for the purpose of measuring the thermodynamic temperature of a gaseous sample. Nevertheless, Doppler-width retrieval from highly-accurate absorption spectra is surely not an easy task as it requires an adequate knowledge of the lineshape function, accounting for the different mechanisms that contribute to the overall linewidth. Semiclassical theories provide several possibilities, more or less accurate in reproducing the observed profiles. Here, the influence of the choice of the lineshape model in Doppler-width thermometry is investigated in the physical situation of self-colliding O molecules. A large number of absorption profiles were simulated, using the uncorrelated version of the speed-dependent Galatry profile and setting different values for the gas pressure, the signal-to-noise ratio and the Dicke-narrowing parameter. Spectral analysis was performed by means o...