Photodetachment of fullerene monoanions and dianions in a Penning trap: Probes of delayed electron emission and associated activation barriers

Singly and doubly charged fullerene anions were introduced into gas phase by means of electrospray starting from a solution containing a very efficient electron donor compound. Following isolation and trapping, mass selected anions were irradiated with a ns-pulse from a Nd:YAG laser at the third harmonic (hν = 3.5 eV) in order to study (delayed) photoemission. We present results on the laser-fluence dependence of photoemission for singly charged C76− and C84− as well as for doubly charged C762−. Results are discussed in terms of statistical electron emission as opposed to direct detachment. They are further analyzed to give information on the number of photons necessary to induce delayed electron emission on the experimental time scale. The averaged absolute absorption cross-sections obtained were about 0.6–0.8 Å2 for all species probed. The agreement between experimental data and statistical modelling shows that electronic excitations which rapidly relax to vibrationally hot ions provide the dominant contributions to the 355 nm absorption cross-sections. As a corollary, we show that direct single-electron detachment is a minor process for the singly charged fullerene anions having a cross-section <0.02 Å2. For C762−, electron emission follows an absorption law in which single-photon excitation is the leading contribution. From the data we deduce a Coulomb barrier height for C762− of 1.20 eV.