Knowledge Bank

High resolution spectra of the $T1\leftarrow SO$ 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 lining of about $15cm-1$ 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 fw at hh) and intense that it can be observed in a crystal mm thick. The major difference between this work and previous work is the spectroscopic resolution. At lower resolution where the spectral line width 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 phonomenon for triplet states of aromatic molecules making them much more accessible to spectroscopic study than believed previously.