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Accurate molecular spectroscopy in the mid-infrared region allows precision measurements of fundamental constants. For instance, measuring the linewidth of an isolated Doppler-broadened absorption line of ammonia around 10 mu m enables a determination of the Boltzmann constant k(B). We report on our latest measurements. By fitting this lineshape to...
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Context 1
... principle of the DBT [6], illustrated in Figure 1, is to record the Doppler profile of an absorption line of a vapour in thermal equilibrium. In the Doppler regime, when all other broadening mecha- nisms are negligible, the recorded gaussian profile derives from the Maxwell-Boltzmann distribution of velocities along the laser beam axis. The Boltzmamn constant is then calculated from the e-fold half-Doppler width ∆ν D ...
Context 2
... experimental setup has been described in detail in [31,32]. It contains a CO 2 laser stabilized on a saturated absorption line of osmium tetroxyde. Thanks to the resulting unique spectral properties (below 10 Hz width, 0.1 Hz frequency instability for 100 s integration time [33]), an incomparable control of the frequency scale is reached, a crucial element for a careful reproduction of the line- shape. The intensity of the laser beam is stabilized before being sent to the absorption cell filled with gaseous ammonia. The absorption length of the cell can be adjusted from 37 cm in a single-pass configuration (SPC) to 3.5 m in a multipass configuration (MPC). By varying the pressure from 1 to 25 Pa (SPC) and 0.1 to 2.5 Pa (MPC), absorption amplitude can be varied between 20 to 98%. The laser frequency is tuned close to the ν 2 saQ(6,3) line of 14 NH 3 (of measured central frequency ν 0 = 28 953 693.9(1) MHz), and scanned via a tunable electro-optic modulator. The absorption cell is placed inside a thermostat at 273.15 K (see Section 3). In order to avoid pressure induced modification of the lineshape (see Section 4.1.1) while preserving a good signal-to-noise ratio, spectra are recorded at pressures around 1 Pa in MPC (the SPC is used when pressure effects are to be studied). Typical spectra are displayed on Figure 1. They are recorded in steps of 500 kHz with 30 ms of integration time per point, leading to a typical signal-to-noise ratio of 10 3 ...
Citations
... (a) The Doppler broadening technique principle. Reproduced from[66]. CC BY 4.0. ...
The past decade saw the emergence of new temperature sensors that have the potential to disrupt a century-old measurement infrastructure based on resistance thermometry. In this review we present an overview of emerging technologies that are either in the earliest stages of metrological assessment or in the earliest stages of commercial development and thus merit further consideration by the measurement community. The following emerging technologies are reviewed: Johnson noise thermometry, optical refractive-index gas thermometry, Doppler line broadening thermometry, optomechanical thermometry, fiber-coupled phosphor thermometry, fiber-optic thermometry based on Rayleigh, Brillouin and Raman scattering, fiber-Bragg-grating thermometry, Bragg-waveguide-grating thermometry, ring-resonator thermometry, and photonic-crystal-cavity thermometry. For each emerging technology, we explain the working principle, highlight the best known performance, list advantages and drawbacks of the new temperature sensor and present possibilities for future developments.
