Abstract
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.
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