Microwave Spectrum of HCOOH in the vCH=1 Vibrational State Observed by Laser-Microwave Double and Triple Resonance

Abstract

Application of laser-microwave double and triple resonance techniques to microwave spectroscopy of HCOOH in the CH stretching vibrational state is reported. The 3.39 µm output from a He–Ne Laser is Zeeman-tuned to saturate a rotation-vibration transition of HCOOH. The microwave transition from the upper state of the infrared transition is stimulated with a klystron and the resulting increase of the infrared absorption is detected. A technique of triple resonance is employed to observe microwave transitions which do not share a common level with the infrared transition. From the observed nineteen pure rotational transitions the rotational constants in the excited state are determined. Accuracy of the constants is limited by the effect of interactions between the vibrational states. The double and triple resonance effects for microwave spectroscopy of excited molecules are discussed by a rate equation approximation.