INFRARED DEGENERATE 4-WAVE-MIXING AND RESONANCE-ENHANCED STIMULATED RAMAN-SCATTERING IN MOLECULAR GASES AND FREE JETS

Infrared degenerate four-wave mixing spectroscopy (IR-DFWM) has been e mployed to study the molecules acetylene (C2H2), methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) in a cell under equilibrium co nditions and cooled in free jet expansions. For methane at room temper ature the detection limit was 2 x 10(12) molecules per cm(3) and quant um state, enabling the detection of trace species with a spatial resol ution of 1 mm(2) x 30 mm. In an attempt to study transitions in the nu (1) + nu(3) and 2 nu(2) + nu(3) combination bands of CO2 or N2O, it wa s not possible to observe any DFWM signal. Instead a surprisingly stro ng, backward- and forward-directed emission was found which could not be attributed to the DFWM process. The signal arising from this emissi on was more than 2 orders of magnitude stronger than the DFWM signals obtained for other molecules. The frequencies of the emitted radiation were found to correlate with the transitions nu(1) + nu(3) --> nu(1) and 2 nu(2) + nu(3) --> 2 nu(2), respectively. Our investigations lead to the conclusion that the emission can be explained by stimulated Ra man scattering, resonantly enhanced by transitions to the combination levels nu(1) + nu(3) and 2 nu(2) + Vg. This process seems to suppress the generation of DFWM signals.