Knowledge Bank

Infrared spectra of the C$2$D$2$ dimer in the monomer \nub{3} region ($\sim$2439 $\backslash wn$) are observed by direct absorption using a rapid-scan tuneable diode laser spectrometer to probe a pulsed supersonic slit-jet expansion. We analyze the perpendicular $K$ = 1-0 and 0-1 subbands of the vibrational mode involving the C$2$D$2$ monomer located at the top of the T-shaped dimer, but miss the parallel band involving the stem monomer vibration due to limited laser coverage. The results are consistent with previous work on acetylene dimers, but perturbations are much less evident than in the analogous infrared spectrum of C$2$H$2$. \textbf{89}, 6028 (1988).} As expected, the tunneling splitting in the excited state (8 MHz) is much smaller than in the ground state (424 MHz). \textbf{150}, 576 (1991).} In the same region, we observe the H-bonded isomer of the C$2$D$2$-C$2$H$2$ dimer. This has not previously been observed, even though microwave spectra of almost every other conceivable deuterated isotopologue are known. In addition to these acetylene dimers, our spectra also contain bands arising from impurities in the gas mixture which we assign to the C$2$D$2$-nitrogen and C$2$D$2$-water complexes