PHOTODISSOCIATION AND PHOTODETACHMENT OF SMALL CARBON CLUSTER ANIONS

Carbon cluster anions as large as C33- were produced by laser desorption-ionization of graphite and trapped in the ion cyclotron resonance (ICR) ion trap, although after a few laser shots a stable cluster distribution terminating with C-13- was reached. Under irradiation at 308 nm both dissociation and photodetachment processes were observed, the latter via CCl4 electron scavenging. Electron photodetachment was the dominant photoreaction for all ions between C4- and C-13-. For ions from C-8- to C-13- dissociation competed with detachment; loss of one or more C3 units was observed, and in some cases also loss of C5. For C4- to C-11-, photodetachment showed linear intensity dependence at low intensities, indicating a one-photon process and showing that the adiabatic electron affinities are less than about 4.0 eV for the photodetaching anions. All dissociations showed quadratic or stronger intensity dependence at onset, indicating processes needing two or more photons. The photodetachment cross sections mirrored the reported electron affinities, with the highest cross sections for the ions of lowest electron affinity. It was considered quite likely that the ions from C-10- upwards were mixed populations of linear and cyclic isomers, with the cyclic isomers giving most or all of the photodetachment. It also seemed likely that the C-8- population had a cyclic component, but the structures of the C-8- and C-9- ions could not be assigned with confidence.