Knowledge Bank

Well resolved spectra of CO-hydrogen complexes have been obtained in the region of the CO fundamental band using a long path absorption cell and a Bomem DA3. 002 spectrometer. Typical experimental conditions are: path = 84 m; temperature = 50 K; CO pressure = 0.2 Torr; and hydrogen pressure = 5 Torr. Spectral lines of the complexes appear as relatively weak fine structure between the strong, broad (saturated) lines of $12C16O$ and are accompanied by a few sharp lines from the other CO monomer isotopes. It can be safely assumed that hydrogen is almost freely rotating in the complex, and spectra have been recorded using $para-H2$ (which is entirely in its $J=0$ rotational state at this temperature), normal $H2$ (25% J=0, 75% J=1), and ortho-$D2$ (100% J=D). As expected, the $CO-H2$ spectrum is quite different for the para- and normal $H2$ samples. The CO constituent of the complex is, however, not free to rotate, and in terms of its motion the complex lies between the free internal rotation and semirigid molecule limits. By analogy with spectra of CO-Ar and CO-Ne recorded under similar conditions, it is possible to assign some of the lines in the $CO-H2$ and $CO-D2$ spectra for the $J=0$ hydrogens; all of these species behave to some extent as T-shaped triatomics. However, the present challenge is to assign the spectra more completely, and it appears that some input from theoretical calculations using a realistic intermolecular potential will be necessary to accomplish this. Ultimately these results should help to define the potential much better than it is currently known, and they will also yield useful predictions for the as yet unobserved millimeterwave spectrum of $CO-H2$.