Evidence for speed-dependent effects in NH3 self-broadened spectra: towards a new determination of the Boltzmann constant

TL;DR

This study analyzes ammonia spectral lines to identify speed-dependent effects, providing precise lineshape parameters that enhance the potential for a highly accurate optical measurement of the Boltzmann constant.

Contribution

It provides the first experimental evidence of speed-dependent broadening and shifting in NH3 spectra, improving lineshape models for fundamental constant measurements.

Findings

Evidence of speed-dependent effects in NH3 spectra

Accurate lineshape parameters for the nu2 saQ(6,3) line

Potential for a high-precision optical 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 nu2 band around 10 $\mu$m, 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 NH3. Accurate lineshape parameters of the nu2 saQ(6,3) line are obtained. Our current project aiming at measuring the Boltzmann constant, kB, by laser spectroscopy will straight away benefit from such knowledge. We anticipate that a first optical determination of kB with a competitive uncertainty of a few ppm is now reachable.