Doppler-free two-photon absorption of NH3using a CO2and diode laser

A successful two-photon spectroscopy experiment using combined C${\mathrm{O}}_2$ and diode lasers operating in the 10-μm region is reported. Beams from the two lasers were passed in nearly opposite directions through a sample cell while the intensity of the transmitted diode laser beam was monitored as a function of its frequency. Doppler-free two-photon absorption signals were observed for the rovibrational transitions of N${\mathrm{H}}_3$, $T(v_2,J,K)=(2^{-},7,7)\leftarrow (0^{-},7,7) \mathrm{and} (2^{-},1,1)\leftarrow (0^{-},1,1)$.