Cavity-ring-down Doppler-broadening primary thermometry

Abstract

A step forward in Doppler-broadening thermometry is demonstrated using a comb-assisted cavity-ring-down spectroscopic approach applied to an isolated near-infrared line of carbon dioxide at thermodynamic equilibrium. Specifically, the line-shape of the P e (12) line of the (30012) ← (00001) band of CO 2 at 1.578 µm is accurately measured and its Doppler width extracted from a refined multispectrum fitting procedure accounting for the speed dependence of the relaxation rates, which were found to play a role even at the very low pressures explored, from 1 to 7 Pa. The thermodynamic gas temperature is retrieved with relative uncertainties of 8 × 10 −6 (type A) and 11 × 10 −6 (type B), which ranks the system at the first place among optical methods. Thanks to a measurement time of only ≈5 h, the technique represents a promising pathway toward the optical determination of the thermodynamic temperature with a global uncertainty at the 10 −6 level.DOI: 10.1103/PhysRevA.97.012512The forthcoming redefinition of the unit kelvin [1], in 2018, in terms of a fixed value of the Boltzmann constant, prompts the interest for primary thermometers that are capable to operate over a relatively large part of the temperature scale with very high accuracy. Among primary methods, important advancements [2] have been obtained over the past decade on dielectric constant gas thermometry and Johnson noise thermometry [3,4]. After a first successful experiment of Doppler-broadening thermometry (DBT) [5], followed by significant improvements [6,7], the international community of fundamental metrology recognized the importance of an optical method that links the thermodynamic temperature to an optical frequency, as an independent confirmation of other primary approaches.DBT consists of retrieving the Doppler width ν D from the highly accurate observation of the shape of a given atomic or molecular line, in a laser-based absorption-spectroscopy experiment under a linear regime of radiation-matter interaction [8]. Once ν D is measured, if the central frequency (ν 0 ) and the atomic or molecular mass (M) are known, the inversion of the well-known equationallows one to determine the thermal energy and, consequently, either the gas temperature (T ) or the Boltzmann constant (k B ). So far, the most accurate implementation of DBT has been performed on a rovibrational transition of a water isotopologue at 1.39 µm. Using a dual-laser spectrometer and adopting a very sophisticated spectral analysis procedure, the Boltzmann constant could be determined with a combined uncertainty of 24 parts per million (ppm) [6,9,10].The history of DBT [11] shows that the major hurdle for a low-uncertainty determination of the Doppler width and hence of the thermal energy is the choice of the line-shape model adopted for the spectral analysis. Since a fully ab initio lineshape calculation is prohibitively complex for self-colliding molecules, a model suitable for being implemented into a fitting routine requires approximations and simplifications. At the same time, th...

Linestrength ratio spectroscopy as a new primary thermometer for redefined Kelvin dissemination

“…This accuracy makes LRT comparable even with the most precise PTs as those based in AGT. Even more, if compared with the other optical-based thermometry, the DBT [38], an accuracy improvement of a factor of 15 for T;300 K is prospected. Discrepancies under mK level of T-T 90 in 9-700 K range can be tested by this accurate LRT thermometry, and in particular, the T range of meteorological interest [27].…”

“…Freezing points of CO, CO 2 and H 2 O are also indicated. 5 A constant relative uncertainty of about 9ppm was considered in all the range of applicability corresponding to that reported for CO 2 at 300 K [38]. Different values could be achieved with different sample gases at other T's.…”

“…To evaluate it, we have simulated a Voigt profile of an absorption transition with a Doppler width contribution, ω much larger than the collisional one and with an area A. This is the usual situation in optical spectroscopy-based thermometric techniques: LRT and Doppler Broadening Thermometry, DBT [38,39]. A random white noise of the 1% was added to the spectral profile to simulate a real experimental spectrum of the transition.…”

Recent advances in collisional effects on spectra of molecular gases and their practical consequences

“…Nevertheless, large systematic deviations (of about 800 ppm) remained, mostly ascribed to weak hidden lines overlapping with the selected transition. Very recently, a comb-assisted highly sensitive CRDS spectrometer, operating at 1.578 m, has been developed [671] using pure CO 2 as a molecular gas target since, as discussed in [162], this molecule turns out to be an excellent choice for DBT measurements. It was subsequently shown that the combination of high sensitivity, absolute frequency calibration and extremely dense sampling of the absorption profiles leads to a statistical uncertainty of 8 ppm over a measurement time of only 5 h. Thermodymanic temperature determinations have been demonstrated by using this approach, with the spectroscopically unprecedented combined uncertainty of 14 ppm [671].…”

Quantitative modeling of complex molecular response in coherent cavity-enhanced dual-comb spectroscopy

“…Optical frequency combs (OFCs) [1][2][3][4] enable increasingly precise applications of atomic and molecular spectroscopy [5], including primary thermometry [6,7], optical radiocarbon dating [8], sub-Doppler and Doppler-free spectroscopy [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26], ultrafast and multidimensional spectroscopy [27][28][29][30], survey spectroscopy of molecular ions and cold molecules [31][32][33], and time-resolved spectroscopy for fundamental chemical kinetics [34][35][36][37]. Applied spectroscopies [38] also make novel use of optical frequency combs for actively monitoring greenhouse gas fluxes [39][40][41], methane for open-path detection [42] and unambiguous source attribution [43] and reactive atmospheric species [44], elucidating the chemical composition of combustion and open flames…”