The T-1 absorption and phosphorescence spectra of alpha-phase and gamma-phase p-dichlorobenzene crystals

The T-1 absorption and emission spectra of the recently discovered gamma-phase of p-dichlorobenzene are presented and analyzed. New high resolution spectra of the a-phase am also reported and the analysis of the T, absorption spectrum is revised in tight of results for the gamma-phase. The frequency of the b(g) mode, involving out-or-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 to the 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 alpha-phase shows detailed fine structure due to both multiple site emission and to splittings of some of the ground state vibrational levels.