Suppression of nonresonant background in coherent picosecond active Raman spectroscopy of molecular gases

Picosecond active Raman spectra were obtained for gaseous ammonia using frequency-tunable pulses. The totally symmetric vibration of the ammonia molecule at 3334 cm^–1 was excited. It was found experimentally that a delay of a test pulse made it possible to suppress a signal due to a nonresonant susceptibility, whose presence restricted potential applications of active Raman spectroscopy.