Photodissociation studies of C n+ at 193 nm ( n = 5−19) 1

Abstract

Carbon cluster cations from C n + to C 19 - were generated by laser ablation/ ionization of graphite in the Fourier-transform ICR ion trapping mass spectrometer, and dissociated by excimer laser pulses with photon energy 6.43 eV. Branching ratios and relative apparent cross-sections were determined. Further mechanistic and thermochemical information was sought by several additional approaches, including aging experiments in which dissociation was induced at varying times after ion formation; pump-probe experiments in which an initial pumping laser pulse preceded the photodissociation pulse; and time-resolved photodissociation (TRPD) experiments. The Northwestern strain model for systematizing the energetics of carbon cluster ions was extended and modified to give agreement with experimental and theoretical dissociation energetics of these cluster ions. This was combined with a loose-transition state RRKM-theory picture of dissociation to give a proposed overall interpretation of the dissociation chemistry. The observation of metastable ion dissociation via C 1 loss for C 11 + and C 12 + remains an outstanding puzzle, although a partially satisfactory interpretation is proposed.