Excimer fluorescence XII. The pyrene crystal excimer interaction potential

The excimer fluorescence spectra of pure single pyrene crystals were observed from 4 to 353 $^\circ$K. The data are analysed to determine the vibrational zero-point energy of the excimer ( = 90 cm$^{-l}$), the force constants of the excited state ( = 1.93 x 10$^5$ dyn/cm) and ground state ( = 3.25 x 10$^5$ dyn/cm) of the dimer, and the equilibrium intermolecular separation ( = 3.34 $\overset{\circ}{\mathrm A}$) of the excimer. The repulsive intermolecular potential, the excimer energy, and the attractive excimer interaction potential, $V' = 51330 -\frac{1.136 x 10^6}{r^3} (cm^{-1}$ are determined as functions of the intermolecular separation r ($\overset{\circ}{\mathrm A}$). V' is shown to be consistent with an exciton resonance state originating from the $^1$B$_a$ state of the pyrene molecule.