RELAXATION IN EXCITED VIBRATIONAL LEVELS OF NH3 BY INFRARED TRIPLE-RESONANCE SPECTROSCOPY

A two step optical pumping technique in combination with transient diode laser absorption is described which provides a powerful tool for preparing selected vibrationally excited states beyond single photon excitation and state-and time-resolved analysis of collisional energy transfer processes. Two TE CO2 lasers operating on the 10P(32) and 9P(24) transitions are used to pump the aQ(5,3) and sR(4,3) of the nu-2 <-- 0 and 2-nu-2 <-- nu-2 bands of NH3. The 10P(32) line also pumps two-photon transitions directly. This technique relies on the coupling of the intermediate states by rapid rotational energy transfer (DELTA-J = 1 collisions) in the nu-2 = 1 region. Time resolved transient 3-nu-2 <-- 2-nu-2 absorption measurements probing 2-nu-2 levels coupled to the initial state by collisions, provide the possibility of state resolved collisional energy transfer studies in an energy region which is difficult to access with other excitation techniques.