Knowledge Bank

“In earlier experimental $work,1$ rotational fine structure in the singlet-triplet $(3A2\leftarrow 1A1)$ absorption spectrum of formaldehyde was resolved by using low gas pressures in a multiple-pass absorption cell at -- $78\circ C$. Because of the large number of lines in each band and the borderline nature of the resolution $(0.12cm-1)$, a number of nearly equivalent assignments in the low J range could be made. Their indistinguishability affected the rotational constant only slightly but presented difficulties in dealing with the spin splittings. The recent $avavilability2$ of very accurate ground state energy levels for formaldehyde has removed much if the ambiguity in the analysis, and rotational constants and spin splittings have now been obtained for the $0+$ and $1+$ inversion levels of the triplet state. The main results of the analysis are: (1) the weak transition dipole lies along the CO axis; (2) transitions to triplet state levels for which $J=N\pm S$ are much stronger than those for which J = N; (3) the observed spin splittings are well described by the theory of Van $Yleck3$ for an approximately symmetric rotor; and (4) the molecule is highly nonplanar in the triplet state.”