Knowledge Bank

IF-MW double and triple resonance technique was applied to microwave spectroscopy of propynal in the $V2=1$ vibrational state (CH stretching vibration of aldehyde group). The $3.51\mu m$ He-Xe laser was tuned over $\pm 0.1cm-1$ by a Zeeman effect and used as an infrared radiation source. A waveguide cell of one meter in length was placed within the laser cavity. The effect of microwave pumping was detected as a decrease of the laser output. Eleven microwave transitions of HCCCHO were newly observed in the excited state by triple resonance. The observed spectrum was analyzed including the eleven transitions previously observed by double resonance. A least squares fit of the thirteen low-J transitions $(J\le 5)$ revealed that the rotational constants of the $K-1=1$ states were slightly different from those of the $K-1=0$ states. A higher-order rotational resonance was proposed for explanation of the effect. In DCCCHO eight microwave transitions were observed in the excited state by double and triple resonance. The low-J transitions were found to be perturbed more drastically than those of HCCCHO. In addition to the transitions in the $v2=1$ state, several unassigned lines were observed with a very good S/N ratio. Further investigation on DCCCHO is in progress.