Coupling of Optical Phonons to the T1 ← S Transition in p-Dibromobenzene Crystals

High resolution spectra of the $T_1{\mathrm{\leftarrow \; S}}_0$ transition in p‐dibromobenzene crystals have revealed that the coupling of optical phonons to the transition is much weaker than believed previously. By accounting for phonon coupling, an upper limit of about 15 cm⁻¹ has been established for the total electronic band width of the T₁ state. The zero phonon lines are very sharp and are more intense than the phonon addition bands. In fact, the zero phonon line of the (0, 0) transition is so sharp $\text{(<1\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{fw\; at\; hh})$ and intense that it can be observed in a crystal $\text{∼ 0.1\hspace{0.17em}}\mathrm{mm}$ thick. The major difference between this work and previous work is the spectroscopic resolution. At lower resolution where the spectral linewidth is narrower than the slit width, crystals of several millimeters thickness are required to observe the transition, and the spectrum is distorted in that the broad phonon lines are relatively enhanced. Also, the previously reported temperature dependence of the zero phonon lines can be ascribed to line broadening and not to intensity enhancement with increasing temperature. The order of magnitude increase in sensitivity obtained by going to high resolution may be a quite general phenomenon for triplet states of aromatic molecules making them much more accessible to spectroscopic study than believed previously.