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This article reports the use of the resonant two-photon ionization technique to selectively excite a molecular ion within a cluster and observe the dynamical outcome. We have excited clusters containing up to 14 benzene molecules to energies of 10.00 or 12.84 eV and measured the probability that an initially formed C6H6+ attacks a neighboring benzene unit of the cluster according to the vapor-phase reaction C6H6+ + C6H6 -* C7H7+ + C5H5, AH = 0.63 eV. The C5H5 radical is expelled from the cluster. At either energy excitation proceeds through an XH1 2 vibrational level of benzene (X = 6 or 8) but in the latter case the benzene cation is also produced electronically excited. Accordingly, at low-energy excitation the above pathway is entirely absent, while the 12.84-eV excitation leads to reaction with a probability increasing with cluster size, as predicted by solvation models. This result makes it appear quite likely that >12-eV excitation of condensed benzene will lead to transient tropylium ion centers for conduction electrons, accompanied by variable trapping of C5H5.

Photoabsorption spectra of benzene clusters BN containing up to N=60 molecules have been obtained on an N-resolved basis over the B2u region of the spectrum (λ=265–245 nm). The resonant enhanced two-photon ionization of a supercooled cluster beam yields distinctive spectra for each value of N. The energy of the 610 resonance is found to decrease monotonically with increasing N, but at N=50 is still far from condensed-phase values.