Knowledge Bank

In the singlet $\pi \ast \leftarrow n$ transitions of $CH2O$, some 30 lines of the $202401$ vibronic band show an observable Zeeman effect. Each magnetically-sensitive line lies close to the resonance, of crossing point, of a group of mild perturbations observed in the zero-field spectrum in sub-bands with $K\prime =0-6$. The pattern denotes an upper state perturbation in which the resonance shifts progressively from $J\prime =12$ at $K\prime =0$ to $J\prime =16$ at $K\prime =6$, and is considered to result from the interaction $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A2\leftarrow p\to 3A2$. The perturbed singlet state $2241(30658cm-1$) is a vibronic $B2$ state. The nature of the perturbation ($\Delta K=0$) proves that the perturbing triplet state has vibronic species $B2$ or $B1$. A rotational analysis of this triplet vibronic state has been carried out, based upon `extra’ lines in the spectrum. The coupling scheme will be discussed.