Observation of Population Inversion by Optical Adiabatic Rapid Passage

I report the observation of population inversion by optical adiabatic rapid passage. These observations, on an ${\mathrm{NH}}_3$ infrared transition with all the relevant parameters known, agree with theoretical expectations. The pressure dependence of $T_1$, obtained by this technique, makes possible the first estimate of the collision-induced rotational lifetime of ${\mathrm{NH}}_3$ in an excited band, which is significantly longer than the rotational lifetime in the ground vibrational band.