PHOTODISSOCIATION SPECTROSCOPY OF BENZENE CLUSTER IONS - (C6H6)2+ AND (C6H6)3+

The photodissociation of size-selected benzene cluster ions, (C6H6)m+ + h-omega --> (C6H6)n+ + (m - n)C6H6, has been investigated in the 410-750 nm wavelength range using tunable dye laser radiation. The measurements were performed using a tandem mass spectrometer [a linear time-of-flight (TOF)/reflectron TOF] combined with multiphoton ionization (MPI) for ion preparation. Only C6H6+ was detected as a photofragment of (C6H6)2+ and (C6H6)3+, while both C6H6+ and (C6H6)2+ fragments were observed in the case of (C6H6)4+ photodissociation. Photodissociation spectra, i.e., photofragment yield spectra as a function of wavelength, of (C6H6)m+ (m = 2,3) were obtained. Two local excitation bands of (C6H6)m+ were seen in this region and assigned to the C(A2u) <-- X(E1g) and the B(E2g) <-- X(E1g) transitions of a C6H6+ unit in the clusters. The origin of the B <-- X transition of (C6H6)2+ and (C6H6)3+ was redshifted relative to that of C6H6+ by about 1400 and 2400 cm-1, respectively, while the C <-- X bands of (C6H6)2+ and (C6H6)3+ were seen at the same wavelengths of 440 nm. Possible structures for the cluster ions are discussed based on the spectral shifts.