Knowledge Bank

Recently several experiments have been reported in which infrared pumping of molecules with $CO2$ or $N2O$ lasers is used for infrared-microwave double $resonance1$ or for far infrared laser $emission2$. The limiting feature of such experiments is that we must rely on rare coincidences between the laser frequency and the molecular absorption frequency. In this paper we report the observation of an infrared-microwave two photon absorption in which frequency tunable microwave radiation is “added” to the laser radiation by using the absorbing molecules as non-linear frequency mixing elements. We therefore make the laser in effect tunable near the original laser frequency. An absorption coefficient of the order of $0.1cm-1$ is achieved for the $v2$[asQ (4,4)] line of $\mathrm{<mrow\; data-mjx-texclass="ORD">15</mrow>}NH3$ which is off-resonant from the P (15) line of the $N2O$ laser by about 330 MHz by applying microwave power of the order of 20 watts/$cm2$. A similar technique is being applied for other transitions. This technique will be useful for high resolution infrared spectroscopy and for infrared pumping of molecules.