PHYSICAL REVIEW APPLIED 11, 064060 (2019) Optical Determination of Thermodynamic Temperatures from a C 2 H 2 Line-Doublet in the Near Infrared Antonio Castrillo, Eugenio Fasci, Hemanth Dinesan, Stefania Gravina, Luigi Moretti, and Livio Gianfrani * Dipartimento di Matematica e Fisica, Università della Campania “Luigi Vanvitelli”, Viale Lincoln 5, 81100 Caserta, Italy (Received 2 April 2019; revised manuscript received 27 May 2019; published 25 June 2019) This paper reports the implementation of Doppler-broadening gas thermometry by use of line-shape analysis of a line-doublet. The two spectral components are vibration-rotation transitions belonging to a pair of acetylene combination bands at a wavelength of 1.39 μm. Characterized by an extraordinary spectral fidelity in combination with high resolution, the spectrometer is based on two phase-locked extended-cavity diode lasers, one of them being referenced to an optical-frequency-comb synthesizer. The global analysis of 1180 spectra, which are recorded as a function of the C 2 H 2 pressure at the constant temperature of the triple point of water, yields an optical determination of the thermodynamic tempera- ture with a combined relative uncertainty (type A plus type B) of 23 parts per million. Similar results are obtained at the melting point of gallium (approximately 303 K). Furthermore, we apply line-absorbance analysis to the acquired spectra, demonstrating a reduction by a factor of approximately 6 of the statistical uncertainty for the retrieved gas temperature. DOI: 10.1103/PhysRevApplied.11.064060 I. INTRODUCTION In November 2018, the 26th General Conference on Weights and Measures (CGPM) unanimously approved new definitions of four base units (kilogram, ampere, kelvin, and mole) in terms of fixed values of fundamen- tal physics constants (the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively). In particular, the new kelvin (the unit of thermodynamic temperature) is based on the fol- lowing value of the Boltzmann constant k B : 1.380649 × 10 -23 J/K. Reviews of experiments performed worldwide in the last 10 years and relevant for setting the k B value are provided in Refs. [1,2]. Very recently, research efforts of the international community working in the field of pri- mary gas thermometry have been devoted to the direct realization of the redefined kelvin [3]. In this respect, acoustic gas thermometry and dielectric-constant gas ther- mometry have been further developed to measure T - T 90 over a range of temperatures, T - T 90 being the difference between the thermodynamic temperature and that resulting from the International Temperature Scale of 1990 (ITS-90) [4–6]. Within this framework, Doppler-broadening ther- mometry (DBT) is widely recognized as a valuable tool for a cross-check with other methods, even though it has not reached yet the same level of accuracy as acoustic gas thermometry and dielectric-constant gas thermometry [7]. * livio.gianfrani@unicampania.it DBT consists of retrieving the thermodynamic temperature from the highly accurate observation and analysis of the shape of a given atomic or molecular line in a gas at ther- modynamic equilibrium. It exploits the physical relation between the thermodynamic temperature and the Doppler width of an optical absorption line, given by ν D = ν 0 c  2 ln 2 k B T M , (1) where ν D is the half width at half maximum, ν 0 is the line central frequency, c is the speed of light in a vac- uum, and M is the atomic or molecular mass. The history of DBT starts with the proposals of Bordé [8,9], fol- lowed by pioneering work on ammonia and carbon dioxide [10,11]. One of the most-accurate implementations of DBT involved an isolated vibration-rotation transition of a water isotopologue (H 18 2 O) at 1.39 μm and yielded an optical determination of the Boltzmann constant with a combined uncertainty of 24 parts per million (ppm) [12]. In this paper, we report the development and application of an optical apparatus acting as a primary thermome- ter based on Doppler-broadened precision spectroscopy of acetylene in the near infrared. This third-generation spec- trometer, following those reported in Refs. [11,12], gives a primary realization of the redefined kelvin. Acetylene has already been used as molecular target for the aims of DBT, as highlighted in Ref. [7]. In par- ticular, Hashemi et al. [13] probed the P(25) line of the 2331-7019/19/11(6)/064060(9) 064060-1 © 2019 American Physical Society