Cooperative Excitons in a Crystal with Two Molecules per Unit Cell

The cooperative exciton formalism of Jortner and Rice is extended to crystals with two molecules per unit cell. Both degenerate and nondegenerate crystal states are considered. A numerical estimate is made of the double-to-single-excitation-probability ratio in a specific energy level of crystalline naphthalene. The previously unassigned 32122-cm−1 level observed at low temperatures by many spectroscopists may be interpreted as a double excitation.