Infrared-microwave two-photon spectroscopy: Theν2band of NH3

A spectroscopic method is developed in which the frequency of microwave radiation is added to or subtracted from that of infrared radiation by using two-photon processes in molecules. By utilizing many lines of the C${\mathrm{O}}_2$ and ${\mathrm{N}}_2$O lasers, this method enables us to perform systematic high-resolution spectroscopy in the 10-μm region. Both straightforward two-photon absorption spectroscopy and two-photon Lamb-dip spectroscopy can be used. We describe the theory of this method and its application to the observation of the ${\nu }_2$ band of ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ ${\mathrm{H}}_3$ and ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ ${\mathrm{H}}_3$.