Photodissociation spectroscopy of benzene cluster ions: (C6H6)+2 and (C6H6)+3

The photodissociation of size-selected benzene cluster ions, (C6H6)+m+hω→(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 C6H+6 was detected as a photofragment of (C6H6)+2 and (C6H6)+3, while both C6H+6 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 C6H+6 unit in the clusters. The origin of the B←X transition of (C6H6)+2 and (C6H6)+3 was redshifted relative to that of C6H+6 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.