Knowledge Bank

The band center of the $\nu 51-\nu 41(\Pi u-\Pi g)$ difference band of acetylene-$d2$ falls at about $27.1cm-1$, and the P branch of this band passes through the microwave region. The $\ell -$ doublet lines of the P(14) and P(18) transitions in the 100 GHz region and the P(15) and P(17) lines in the 50 GHz region have been measured, and a set of spectroscopic constants for the band has been derived. The observed lines are unusually narrow and even at high pressures the line width is instrumentally limited. The Stark effect of the observed lines is quite slow, and even with an applied electric field of 9200 V/ cm, the lines are not completely modulated. A crude analysis of the Stark shifted lines yields an upper limit to the vibrational transition moment of 0.05 D.