Knowledge Bank

Using a technique described previously for $cetaldehyde,1,2$ benzaldehyde, and $acetophenone,1,3$ it has been possible to obtain sufficient emission intensity of formaldehyde vapor so that the spectrum could be photographed at relatively high dispersion. Introduction of benzene as a discharge-sustaining agent decreased the excitation conditions to a point where there was a minimum of dissociation of the formaldehyde and a complete absence of spectral impurities such as CO bands. Plates were obtained using the second order of a 21-foot 15,000-line-per-inch grating in a Wadsworth mounting and the second order of a 21-foot 30,000-line-per-inch grating in a Paschen mounting. The reciprocal dispersions were approximately 2.5 A/mm and 0.6 A/mm, respectively. Exposure times were less than one hour on the smaller instrument and about eight hours on the larger one. The rotational lines were quite sharp and for the most part resolved except near the band origins. Because of the relatively large difference in rotational constants between upper and lower states and the near superposition of some vibrational bands, the structure is quite complex. A rotational analysis of the bands is being attempted and from the rotational constants and band origins it is hoped that an unambiguous vibrational assignment can be gained.