Two-Photon Adiabatic Inversion

This Letter reports the first experimental observation of population inversion of a two-photon transition in N${\mathrm{H}}_3$ using the adiabatic rapid-passage technique. The population inversion was demonstrated by the observation of stimulated two-photon gain and emission. By use of this technique, the two-photon population relaxation time $T_1$ was measured.