The t1 absorption and phosphorescence spectra of α-phase and γ-phase p-dichlorobenzene crystals

The T₁ absorption and emission spectra of the recently discovered α-phase of p-dichlorobenzene are presented and analyzed. New high resolution spectra of the α-phase are also reported and the analysis of the T₁ absorption spectrum is revised in light of results for the γ-phase. The frequency of the b_2g mode, involving out-of-plane chlorine motions, is found to drop by 210 cm^?1 upon excitation and, as a result, it is active as a progression-forming mode in both the absorption and emission spectra. Its activity, relative :othe a_g modes, is quite different in the two phases of p-dichlorobenzene and is not the same in the absorption and emission spectra. The high resolution phosphorescence spectrum of the α-phase shows detailed fine structure due to both multiple site emission and to splittings of some of the ground state vibrational levels.