Knowledge Bank

A difficulty in using molecular lasers for extending four-level double resonance experiments from the microwave to the infrared region is that we have to rely on rare coincidences between the laser line and the molecular absorption. In certain cases this difficulty can be overcome by using the recently developed microwave-infrared two-photon technique which makes the laser in effect frequency tunable. We report an experiment conducted on the four-level vibration-rotation energy scheme of $\mathrm{<mrow\; data-mjx-texclass="ORD">15</mrow>}NH3$. Two-photon transitions were used for both the alteration of molecular energy level populations, and the observation of the collisionally transferred effects of this pumping”. The experiments revealed an interesting characteristic of infrared double resonance which did not exist in the microwave experiments. In infrared double resonance we can select molecules with certain velocity components both for the pumping and for the monitoring. It is then possible to obtain information on the change of velocity simultaneously with that on the change of rotational state due to collisions. It was observed that a hole” burned in the Doppler profile of the pumped transition was transferred to the signal transition by collision. This indicates that collisionally-induced transitions between inversion levels of $NH3$ can occur without significant changes in the velocity.