Knowledge Bank

Experiments in which a vibrational transition is saturated with a laser source and the microwave rotational spectrum used to monitor the effect are described. In these experiments the pumped and monitored transitions may or may not share a common level. Many molecular systems have been investigated but to date positive effects have been obtained only with $CH3Br$. A decrease in the intensity of certain microwave transitions of methyl bromide was observed when the gas sample was exposed to the laser irradiation; this effect is completely reversible and the time constant is essentially that associated with the microwave detection system (approximately 0.1 second). The results will be discussed with respect to the definite selection rules for collisional transfer of rotational energy.