Dissociation energies of the benzene dimer and dimer cation

The dissociation energy of the benzene dimer was measured indirectly via a precision measurement of its ionization potential, using tunable synchrotron radiation and an argon-benzene nozzle beam optimized for (C6H6)2. This ionization potential, 8.690 +/- 0.023 eV (1427 +/- 4A), was combined with other data from the literature to obtain D0((C6H6)2) = 2.4 +/- 0.4 kcal mol and D0((C6H6)2 ) = 15.3 +/- 0.9 kcal mol . The corresponding heats of formation are H/sub f/ ((C6H6)2) = 30.4 +/- 0.4 kcal mol and H/sub f/ 0((C6H6)2 ) = 230.8 +/- 0.9 kcal mol . The threshold portion of the photoionization efficiency function of (C6H6)2 shows no sign of autoionizing Rydberg structure, so direct ionization is the process that determines the threshold function shape. This shape also indicates that the geometry of (C6H6)2 is significantly different from that of (C6H6)2 , which is thought to have a sandwich structure, but is still sufficiently similar to permit the threshold to be distinctly observed.