Knowledge Bank

“A rotational analysis of the $\lambda 3970$ singlet-triplet electronic system of the nearly symmetric rotor $CH2O$ has been carried out. In the triplet state the N-levels within each rotational sublevel behave, except for asymmetry splitting, as do the levels of a diatomic molecule in a 3$\Sigma$ electronic state. The spin-spin and spin-molecular rotation interaction constants could not be determined precisely because of the relatively low resolution ($0.1cm-1$) available in the optical region of the spectrum, but upper limits to these values showed them to be no larger than those in the ground state of the isoelectronic molecule, $O2$. Transitions to levels where $J=N$ are extremely weak. For $J=K\pm S$, the observed ground state differences for all the unblended lines of moderate intensity agree to within $0.03cm-1$ with the calculated values. Rotational constants in the $1+$ vibrational level are $A=8.7cm-1$, $B=1.148cm-1$, and $C=1.033cm-1$. The B and C constants are about 2 per cent larger than in the singlet state. The analysis shows the $40cm-1$ “doubling” in these bands to be a result of head formation in the $J=N\pm S$ subbranches, and not, as was suggested earlier, to be due to unquenched electron orbital angular momentum. In reality the analysis shows the triplet state to be a well-behaved example of case-b coupling between spin and molecular rotation. A discussion of the rotational subband structure will be included as well as remarks concerning the vibrational levels in the triplet states of $H2CO,HDCO$, and $D2CO$.”