Speed-dependent effects in NH3self-broadened spectra: Towards the determination of the Boltzmann constant

Abstract

In this paper we present an accurate analysis of the shape of an isolated rovibrational ammonia line from the strong 2 band around 10 µm, recorded by laser absorption spectroscopy. Experimental spectra obtained under controlled temperature and pressure, are confronted to various models that take into account Dicke narrowing or speed-dependent effects. Our results show clear evidence for speed-dependent broadening and shifting, which had never been demonstrated so far in NH 3 . Accurate lineshape parameters of the ν 2 saQ(6,3) line are obtained. Our current project aiming at measuring the Boltzmann constant, k B , by laser spectroscopy will straight away benefit from such knowledge. We anticipate that a first optical determination of k B with a competitive uncertainty of a few ppm is now reachable.

Velocity-changing collisions in pure H2 and H2-Ar mixture

“…Clearly, the use of realistic description of collisions in line shape calculations is crucial for different applications such as the spectroscopic determination of the Boltzmann constant, 4,5,46,86 testing quantum treatment of molecules with high precision (in particular, line intensities) 49 or search for new fundamental physical effects. 87,88 Comparing the descriptions of the velocity-changing collisions given by various models (see Sec.…”

“…These results allow to remove the limitations on accuracy of physical quantities obtained from spectra analysis and therefore they are important for optical metrology based on molecular spectroscopy like Doppler-width thermometry and Boltzmann constant determination, 4,5,46 atmosphere monitoring, 47,48 or transition position and intensity determination for fundamental studies. 49 …”

Accurate determination of the Boltzmann constant by Doppler spectroscopy: Towards a new definition of the kelvin

“…It for instance, constituted 97% of the 144 ppm standard uncertainty published in [29]. The influence of the lineshape model on the spectroscopic determination of the Boltzmann constant has been raised by us and other groups working with O 2 and H 2 O [29,32,[40][41][42][43][44][45]. We recently carried out an extensive lineshape study at relatively high pressure in SPC, in order to enhance collisional effects and to determine the most suitable model for our data [40].…”

“…The uncertainty on k B due to collisional effects is then only limited by our knowledge on the associated parameters m and n. Simulated SDVP spectra corresponding to typical experimental conditions for k B determination have been fitted taking into account uncertainties on m and n (see above) in order to estimate the impact of these uncertainties on the Doppler width accuracy. We followed the line-absorbance based analysis method recently proposed in [46], which leads to an estimated uncertainty of 0.9 ppm (respectively 1.8 ppm) on ν D (respectively k B ), if spectra are recorded at ammonia pressures below 2 Pa. 3 The m and n parameters are related to the speed-dependence of the collisional broadening and shift: under the hypothesis that the molecular interaction potential V is proportional to some inverse power of the internuclear separation R, V ∼ 1/R q , where q is determined by the kind of the interaction, the dependence of the collisional width and shift versus the relative collisional speed v r are given by (v r ) ∼ v m r and D(v r ) ∼ v n r respectively [40]. …”

“…Typical lifetimes for rovibrational levels are of the order of 100 ms, translating into a minute natural width. Our fitting procedure [40] is such that presence of outgassing or partial pressure of impurities has rigourously no impact. The world record frequency stability and accuracy of our CO 2 laser (see Sect.…”