Kinetic energy release distributions from dissociative photoionization of weakly bound trimers at 14-27 eV

The formation of the intriguing ions C4H6O+, C6H6Cl+, and C6H6O+, by dissociative ionization of heterotrimers of butadiene/sulfur dioxide, benzene/hydrogen chloride and benzene/oxygen by 14-27 eV photons, illustrates the possibility that VUV irradiation of clusters comprised of three or more molecules could provide a route to make ions containing bonds not previously accessible. Kinetic energy release distributions were measured in an attempt to understand the formation of these ions and why clusters larger than dimers are needed. Standard theory was applied to find whether more complicated theoretical treatments are needed to understand the data. It was found that all of the above ions were most likely produced by essentially the same mechanism: excitation of one moiety, transfer of its excitation energy to the moiety that dissociates, followed by slow decay of the remaining excited ion into the unexcited moiety as the "solvent" plus the ion with the new bond. The very low reaction probabilities to produce these ions, combined with very low target densities in the presence of many orders of magnitude higher densities of other molecules, precluded the usual imaging techniques. However, we found that the retarding-potential method can give useful data. Also, at present laser photon energies higher than 15 eV provide significantly smaller average intensities than are needed.