Multiple ionization of benzene clusters by ultraviolet radiation

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Abstract
Efficient multiple ionization of large benzene clusters has been observed following irradiation of a supersonic cluster beam by ArF-laser pulses at low fluences. The mechanism of multiple ionization is shown to be distinct from that believed to predominate in electron impact or synchrotron ionization experiments. The fluence dependence is instead interpreted by considering independent ionization of molecules within the cluster. The efficiency of multiple ionization over a range of laser fluence can be predicted quantitatively using a recently proposed exciton annihilation model, and explains the 106 enhancement of the double ionization rate over the molecular value. The critical sizes N*(z) for stability of a z-charged drop with respect to Coulomb fission are N*(2)=23, N*(3)=52, and N*(4)=92 and can be accounted for by liquid drop models. However, the fission of doubly charged clusters could not be detected in dynamical mass spectrometry experiments on the 10−6 to 10−4 s time scale. The surprising critical aspect of the fission rate is discussed, and arguments are made in favor of the Gay–Berne dynamical mechanism.