Knowledge Bank

“The absorption spectra of single crystals of hexachlorobenzene and of 1,3,5-trichlorobenzene in the near ultraviolet have been investigated. The experiments were performed with polarized light at $77\circ K$ and at $4.2\circ K$. No Davydov splitting has been observed in either spectrum. In both spectra the first absorption is relatively weak and strongly polarized, over 90% of the intensity being in the b-axis direction. These bands are assigned the $0-0$ transitions. Aside from the first band, the hexachlorobenzene spectrum has approximately equal intensity in the two polarization directions observed. The trichlorobenzene spectrum, on the other hand, is strongly polarized with about $70-80$ of the intensity in the direction perpendicular to the b-axis. In both crystals, the b-axis polarized spectrum is much sharper than the other component. In both crystals the b-axis can be assumed to be close to the molecular normal. The major parts of the transitions observed are therefore concluded to have an induced transition moment in the molecular planes but this is not the case for the $0-0$ band. The electronic transitions observed here are forbidden in the free molecule. The $0-0$ band may appear due to either crystal field perturbation or due to small molecular distortions or both. The first of these possibilities cannot be ruled out but it seems unlikely that the crystal field can induce a $0-0$ band polarized in a direction close to the molecular normal.”