Knowledge Bank

The phosphorescence of naphthalene in p-dibromobenzene, $\alpha$-phase p-dichlorobenzene, and $\gamma$-phase p-dichlorobenzene host crystals has been studied at high resolution and $4.6\circ K$ . The $T1$ absorption of naphthalene in a p-dibromobenzene host crystal at $4.6\circ K$ was recorded by the use of excitation spectroscopy. With the exception of the $b3g$ $\nu 17$ fundamental (which is nearly degenerate with the $ag\nu 1$ fundamental and probably borrows intensity from it) only vibrations of $ag$ symmetry are active in the naphthalene $T1$ absorption and emission. This vibronic activity indicates that the external spin-orbit coupling mechanism operative in the heavy-atom-containing chemically mixed crystal is different from the internal spin-orbit coupling mechanism active in the $T1$ of naphthalene in a deuteronaphthalene crystal.