  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.
