Infrared third-harmonic generation in molecular gases

We report the first observation of third-harmonic generation (THG) in molecular gases. Using a CO
2
TEA laser and choosing molecules with vibrational transitions that contribute to a resonance enhancement of the third-order susceptibility tensor we have studied absolute conversion efficiencies, spectral response, and pressure dependences for THG in SF
6
, BCl
3
, and CO and compared the results to theoretical predictions.