Supersonic jets do get used in spectroscopy. Not only does the massive expansion from a high pressure area to a low pressure area cool the sample by adiabatic expansion, removing thermal broadening of spectroscopic lines, but the massive velocity in one direction also removes line broadening due to the Doppler effect. Basically, if you have a gas, then some molecules will travel away from the sensor and redshift slightly, others will travel towards the sensor and blueshift slightly, the result being that instead of nice clean lines representing your energy levels, you have large, wide and broad ones (Lorentzian or Gaussian, I don't remember) that often blur together. The supersonic jets stop this by running all the molecules at high velocity perpendicular to the sensor, so no red or blue shift is observed.
