Knowledge Bank

The two-photon Lamb dip technique previously applied to ammonia using infrared and microwave $radiations1$ is now extended to the radio-frequency region. If a rotational level of a symmetric top molecule with $K\ne 0$ has double parity, a radiofrequency photon can be ``added” directly to an infrared laser photon using the non-linearlity of the molecular transition process. The two-photon Lamb dips observed are listed below. [FIGURE] The observation gives the difference between the laser lines and molecular absorptions with an accuracy of about 0.5 MHz. Precision molecular constants which supplement recent laser-Stark spectroscopy on $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}CH3F$ are obtained. M. R\”omheld was the holder of a German Academic Exchange Service Fellowship.