“…Among them, highresolution spectroscopic techniques, such as visible absorption, [3] diode-laser infrared absorption, [4,5] Fourier transform infrared (FTIR) absorption spectroscopy, [6,7] Fourier transform microwave (FTMW) measurements, [8] resonance enhanced multiphoton ionization (REMPI), [9,10] cavity ring down spectroscopy, [11] or nonlinear Raman spectroscopies (stimulated Raman Spectroscopy (SRS), CARS, etc), [12,13] have been widely used to measure vibrationrotation spectra of molecules cooled in the expansion and for the analysis of phenomena naturally occurring, or externally induced, in free jets and supersonic beams, like nonequilibrium between the molecular degrees of freedom, [9,10] nucleation and clustering of the gas, [11,12] photodissociation and photon-induced reactions, [14,15] or interactions of molecules with fields. [16] In the spectra obtained from jets with techniques that have a great field depth, like infrared absorption, the measured intensity and the profile of the spectral lines are the result of an integration along the line of sight, containing information about the density, temperature, and velocity of the observed particles within the jet. An inhomogeneous broadening of the lines is thus produced when the expansion is optically probed in a direction perpendicular to the jet axis, where different groups of molecules contribute to the lineshape with a specific Doppler shift and a specific weight given by their number density.…”