Rotational spectroscopy of vibrational excited molecules by infrared-microwave double resonance

A description is given of a novel method of microwave spectroscopy of molecules in vibrationally excited states by a technique of infrared-microwave double resonance. Rotational transitions undetectable by any conventional method can easily be observed by this technique if an infrared laser can be tuned to a vibrational transition of the molecule. Experimental results on methane, formic acid, and propynal are presented.