The cooling of molecules during the adiabatic expansion of supersonic
seeded molecular beams is reviewed and illustrated by the example of
NO -molecules. The reduction of the Doppler width by collimation of the
beam and the cooling to low rotational temperatures brings a significant
simplification of the complex NO -absorption spectrum and allows its
assignment. The measured rotational temperature is T = 10°K. Some
examples from our work illustrate this promising technique of high resolution
laser spectroscopy in cold beams