THE MAGNETIC ROTATION SPECTRUM OF FORMALDEHYDE: SINGLET-TRIPLET PERTURBATIONS IN THE $4^{1}$ and $4^{3}$ LEVELS OF THE $A^{1} A_{2}$ STATE OF $H_{2}CO$

Abstract

Magnetic rotation spectra of the $4^{1}{0}$ and $4^{3}{0}$ bands of the $\tilde{A}^{1}A_{2} - \tilde{X}^{1}A_{1}$ system of formaldehyde have been photographed under high resolution . The $^{r}R$ and $^{p}P$ branches of these bands are found to be active and it is noted that the intensities appear to increase more rapidly with increasing $K_{a}$ in the MRS than in the absorption spectrum. The $4^{3}$ and $6^{1}$ levels are in Coriolis interaction at higher $K_{a}$ values, and some $^{r}R$ branches of the $6^{1}{0}$ band are identified in the MRS. Several strong lines in the MRS are shown to be associated with $^{r}{R}, ^{r}{P}, ^{p}{R}$ and $^{p}{P}$ lines in the absorption spectrum and to reveal the presence of singlet- triplet perturbations. Only one perturbed rotational level is found in the $4^{1}{0}$ band , but for the $4^{3}{0}$ band several rotational levels are found to exhibit local perturbations, e.g., ${19}{0}, 19, {19}{1}, 19, {17}{1}, 16, {10}{2,8,} {10}{2,9,} {11}{2,9,} {17}{3,14,} {17}{3,15,} {18}{3,15,}$ and ${18}{3,16}$. These perturbations are produced by $\Delta J = \Delta N = \Delta K{a} = 0$ interaction with the $4^{15}16^{1}$ level of the $a^{3}A_{2}$ state and by $\Delta J = \Delta N =0, \Delta K_{a} = \pm 1$ interactions with the $5^{1}6^{1}$ level. The matrix elements are of the order of $0.01 – 0.10cm^{-1}$.