CO2-laser induced photodissociation studies of size-selected small benzene clusters

Abstract

The infrared photodissociation of size-selected, small benzene clusters has been investigated in the region of the ν18 CH in-plane bend using a pulsed CO2 laser. By scattering the cluster beam with a secondary Ne beam and observing off-axis the effect of the laser irradiation with a rotatable mass spectrometer, cluster-specific spectroscopy is performed. The dependence of IR absorption and subsequent dissociation of (C6H6)n clusters has been investigated as a function of laser frequency and laser fluence for n=2, 3, and 4. The absorption profiles are structureless and show only little variation with cluster size. If, instead of He, Ne is used as carrier gas, the absorption profiles are distinctively narrower. This effect is attributed to a lower internal temperature achieved with Ne. In contrast to the benzene dimer and tetramer, the fluence dependence for the trimer dissociation is stronger than linear suggesting that more than one photon is needed to dissociate this cluster. In a computational approach, the structures of the benzene dimer, trimer, and tetramer have been calculated employing an energy minimization program. For the trimer a cyclic ring structure is determined. The computational results are in perfect agreement with the experimental findings.