Knowledge Bank

The $\nu 1,eju$ fundamental band of $C5H6$ has been recorded with sub-Doppler resolution under collisionless low-temperature molecular-beam conditions using bolometric detection of molecules excited by tunable microwave sidebands of a Lamb-dip-stabilized $CO2$ laser. The sidebands cover about $40\%$ of the spectrum between laser lines in which 117 transitions for J.K $\le 11$ have been observed. The overall welghted standard deviation of 124 kHz yields improved ground and excited state rotational constants with nearly microwave accuracy. Our measured ground state rotational constant, $Bo=0.18977389(3)$ $cm-1$, is compared with several disparate values reported previously.