In Molecular Laser Isotope Separation processes, the use of high resolution spectroscopy tools is common to determine the rotational and vibrational temperatures. In both versions of the MLIS processes (dissociation or SILEX version), the jet cooled gas tends to rapidly form van der Waals complexes with the rare gas used as carrier gas. These phenomena are important for isotopic selection as they initiate the condensation phenomena. In case of SF6, small heteroclusters (SF6-rare gas) are investigated with rovibrational laser jet-cooled spectroscopy in the v3 region (10 µm for SF6), for some rare gas atom (RG). High resolution spectra are measured for both the perpendicular and parallel bands for Ne, Ar, Kr and Xe heteroclusters. It is shown that the reduced vibrational red-shifts are independent of the RG atom, leading us to a good understanding of their conformation /1/. In the case of UF6, the unusual wavelength of the v3 band brings us /2/ to develop a DFB laser at 16µm. The InAs/AlSb quantum cascade laser (QCL-DFB) technology permits IES to demonstrate room temperature operation in continuous regime for the first time above 16 μm with a linewidth of a few MHz. Absorbance measurements with these lasers are now demonstrated on isolated NH3 lines in the 16µm range, at low pressure. Thermal induced frequency modulation (2f/1f) is used to improve signal-to-noise ratio. In the case of online isotopic ratios measurements, this laser will be used also to follow the UF6 molecule. For that purpose, it will be coupled to an integrated pulsed supersonic jet.