Infrared degenerate four-wave mixing and resonance-enhanced stimulated Raman scattering in molecular gases and free jets

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H₂), methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O) in a cell under equilibrium conditions and cooled in free jet expansions. For methane at room temperature the detection limit was 2×10¹² molecules per cm³ and quantum state, enabling the detection of trace species with a spatial resolution of 1 mm²×30 mm. In an attempt to study transitions in the ν₁+ν₃ and 2ν₂+ν₃ combination bands of CO₂ or N₂O, it was not possible to observe any DFWM signal. Instead a surprisingly strong, backward- and forward-directed emission was found which could not be attributed to the DFWM process. The signal arising from this emission 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 ν₁+ν₃→ν₁ and 2ν₂+ν₃→2ν₂, respectively. Our investigations lead to the conclusion that the emission can be explained by stimulated Raman scattering, resonantly enhanced by transitions to the combination levels ν₁+ν₃ and 2ν₂+ν₃. This process seems to suppress the generation of DFWM signals.