ROTATIONAL ANALYSIS OF THE SINGLET-TRIPLET TRANSITION IN FORMALDEHYDE

Author Institution: Department of Chemistry, The Johns Hopkins University“A rotational analysis of the $\lambda 3970$ singlet-triplet electronic system of the nearly symmetric rotor $CH_{2} O$ 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 ($0.1 cm^{-1}$) 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, $O_{2}$. 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 $0.03 cm^{-1}$ with the calculated values. Rotational constants in the $1^{+}$ vibrational level are $A = 8.7 cm^{-1}$, $B = 1.148 cm^{-1}$, and $C = 1.033 cm^{-1}$. The B and C constants are about 2 per cent larger than in the singlet state. The analysis shows the $40 cm^{-1}$ “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 $H_{2}CO, HDCO$, and $D_{2}CO$.