Knowledge Bank

The interaction between levels of an electronically-excited singlet state and those of a neighboring triplet state establishes a possible channel for intersystem crossing. An interaction of this type can be detected experimentally in the vapor state by observing the effect of an external Zeeman field on individual rotational lines of the singlet-singlet absorption which carries the oscillator strength in transitions from the ground state. Matrix elements of the Hamiltonian for coupled singlet and triplet states of an asymmetric rotor in an external field have been derived, and the results are used in analyses of magnetically sensitive perturbations observed in the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A2\leftarrow 1A1$ system of $CH2O$ and the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A\prime \prime \leftarrow 1A\prime$ band system of $CH2\cdot CHO$ (propynal). Line profiles in the Zeeman field are calculated and the results used to obtain an improved understanding of magnetic rotation spectra induced in singlet-singlet transitions by singlet-triplet mixing.