Knowledge Bank

The magnetic splitting of the singlet-triplet absorption bands of benzoquinone and of pyrazine single crystals has been observed at $4\circ K$ and in fields ranging from 70 to 210 kilogauss. The splittings are symmetrical with g-factors of two, as expected. The intensity of the $m=0$ relative to $m=\pm 1$ component is a function of the orientation of the magnetic field with respect to the molecular axes. These data show directly the type of singlet-triplet mixing responsible for the transition. Only one sharp absorption line of quinone showed a Zeeman splitting. Another one earlier attributed to the triplet state did not split. In pyrazine, each member of the series of vibronic bands shows the same splitting pattern, but the origin band shows a different pattern. The vibronic splitting pattern leads to the same results for the symmetry of the triplet state as does the polarized phosphorescence technique.