Determining spectroscopically line parameters of atomic or molecular resonances such as the width or the central frequency calls for some prudence. Well-known from the spectroscopy community, the rule of thumb formulated by Townes and Schawlow states that “The necessary ratio of bandwidth to sweep rate for good portrayal of a line cannot be specified uniquely, but is often of the order of 20” . To go beyond this qualitative advice, we thoroughly modelled the influence of the detection chain with which a spectroscopic signal is recorded and precisely determined the lineshape deformation associated to absorption spectroscopy. Our study predicts artificial frequency shifts, extra broadenings and line asymmetries. This work was conducted in collaboration with F. Rohart (Laboratoire de Physique des Lasers, Atomes et Molécules, Lille) et L. Gianfrani’s group (Molecules and Precision Measurement Laboratory, Naples). It was motivated by efforts carried out in both Naples and Villetaneuse to measure the Boltzmann constant using laser spectroscopy. Indeed, given the targeted part per million uncertainty, determining a linear absorption spectrum Doppler broadening at the same level of accuracy is indispensable.
This work just appeared in Physical Review A and was selected by the editors of Physical Review A to be an Editors' Suggestion.
 Townes CH, Schawlow AL. Microwave Spectroscopy. McGraw-Hill, New York, 1955.