SPECIFICALLY, THIS TECHNIQUE ELIMINATES THE DOPPLER BROADENING CAUSED BY THE RANDOM MOTION OF MOLECULES IN THE GAS PHASE. A TWO-PHOTON SPECTROMETER WAS ASSEMBLED AND IT WAS DEMONSTRATED FOR THE FIRST TIME THAT THE METHOD IS APPLICABLE TO THE DETECTION OF COMMONLY OCCURRING MOLECULES. THE PARAMETERS REQUIRED TO DETERMINE THE TWO-PHOTON DETECTION LIMITS WERE MEASURED USING AN APPROPRIATE LASER SYSTEM. IT WAS SHOWN THAT WITH THE NEW METHOD TYPICAL MOLECULES SUCH AS BENZENE AND NITRIC OXIDE SHOULD BE DETECTABLE WITH HIGH SENSITIVITY EVEN IN THE PRESENCE OF OTHER MOLECULES. HOWEVER, DETECTION SENSITIVITY FOR EXPLOSIVE VAPORS WAS FOUND TO BE INADEQUATE BECAUSE THE MOLECULES DO NOT FLUORESCE EFFICIENTLY. IT WAS SHOWN THAT FOR REPRESENTATIVE EXPLOSIVE MOLECULES THE OPTICAL ABSORPTION BANDS REMAIN BROAD EVEN WHEN THE DOPPLER BROADENING IS ELIMINATED. (AUTHOR ABSTRACT)...PLO