Analysis of some singlet-triplet perturbations in the 1A2 state of formaldehyde

The ${\text{2ν}}_2{\mathrm{\prime +\nu }}_4{\text{′(2}}_0^2{4}_0^1)$ band of the singlet $A_2{\mathrm{\leftarrow \; A}}_1$ electronic system of CH₂O shows a number of perturbations involving energy displacements ranging from a small value up to about 0.5 cm⁻¹. These perturbations are of the resonance type, characterized by an intersection of the rotational manifold of the perturbed state by that of a neighboring state with different rotational constants. Near each intersection, lines of the perturbed singlet‐singlet transition are broadened or shifted by a Zeeman field, indicating that the perturbing state has a large magnetic moment. We interpret the fact that every magnetically sensitive transition in the band is also energetically perturbed as definite physical evidence that the perturbation is singlet‐triplet in nature. Since the spin‐orbit matrix element vanishes for coupling between ¹A₂ and ³A₂ states, the perturbations must be due to second or higher order singlet‐triplet interaction. An analysis of energy shifts shows that two vibrational levels of the ³A₂ state, 1¹2²4¹ and 1¹2², are probably involved. The 1¹2²4¹ level has the same vibronic symmetry, B₂, as the singlet 2²4¹ vibrational level which carries the oscillator strength in transitions from the ground state. This perturbation obeys the selection rules $\text{Δ N=Δ K=0}$ , the interaction occurring by intersystem spin‐rotation coupling. Rotational constants of the perturbing state, as well as the intersystem spin‐rotation coupling constants for the interaction, are determined from the analysis. The perturbations caused by the 1¹2² triplet state level, which are less well characterized, are assigned to an intersystem vibronic spin‐orbit mechanism involving admixture of singlet or triplet electronic states of B₂ symmetry. Energy displacements due to the spin‐rotation and vibronic spin‐orbit coupling are of the same order of magnitude in this example.