J. Conceicao

UPS of Buckminsterfullerene and other large clusters of carbon

Abstract Ultraviolet photoelectron spectra (UPS) are reported for mass-selected negative carbon clusters extracted from a pulsed supersonic beam. In the size range from 48 to 84 atoms, three clusters were found to be closed-shell species with appreciable HOMO-LUMO gaps: C 50 (0.3–0.6 eV), C 60 (1.5–2.0 eV), and C 70 (0.7–1.2 eV). UPS data for all other clusters revealed no appreciable HOMO-LUMO gap, indicating they are either open-shell species, or closed-shell species with small HOMO-LUMO gaps. Buckminsterfullerene (C 60 ) was found to have the lowest electron affinity (2.6–2.8 eV) of any cluster. Agreement between these UPS data and electronic structure calculations strongly support the spheroidal shell model for C 60 .

UPS of 2–30-atom carbon clusters: Chains and rings

Abstract Ultraviolet photoelectron spectra (UPS) of negative carbon clusters are reported in the size range from 2 through 29 atoms. The clusters were prepared in a supersonic beam by laser vaporization, and a F 2 excimer laser (7.9 eV) was used for photodetachment. The resultant UPS data indicate that carbon clusters in the 2–9-atom size range take the form of linear chains: the even-numbered chains having open shell electronic structures with high electron affinity, the odd chains having closed shell singlet ground states (for the neutral) and substantially lower electron affinity. Clusters in the 10–29-atom range give UPS patterns consistent with a monocyclic ring structure.

Threshold photodetachment of cold C−60

Abstract Threshold photodetachment of cold C − 60 has been investigated with a tunable dye laser from 2.55 to 2.80 eV. The cold C − 60 was produced through laser vaporization of a purified C 60 target with a specially designed nozzle. The threshold behavior of the detachment cross section as a function of energy was found to be well described by Wigners threshold law plus a dominating polarization term. The electron affinity of C 60 was measured to be 2.650±0.050 eV. Two unique effects, which are characteristic of bulk materials, were observed in this nano-sized molecule, i.e. the thermionic emission and the inelastic electron—“phonon” scattering effects, with the former strongly dependent on the laser fluences and the later on both the internal temperature of C − 60 and the photoelectron kinetic energy.

XPS probes of carbon-caged metals

Abstract X-ray photoemission spectral probes of endohedral lanthanum—fullerene complexes, La@C n , show that the central La atom is in a formal charge state close to +3, and has been effectively protected from reaction with water an oxygen by the enclosing fullerene cage. Similar results were obtained with endohedral yttrium complexes, Y @C n , and the first carbon-caged metal cluster, Y 2 @C 82 . EPR spectral results are also reported for Y @C 82 which is found to be an S = 1 2 spin system with a 0.48 G hyperfine splitting (at 9.216 GHz) centered at g =1.9999(1).

Ultraviolet photoelectron spectroscopy of copper clusters

Using a new magnetically focused time‐of‐flight photoelectron spectrometer, the ultraviolet photoelectron spectra (UPS) of mass‐selected negative copper clusters have been measured at photon energy of 4.66 eV for all clusters in the range from 6 through 41 copper atoms. These UPS data provide the first detailed probe of the 4s valence band structure of such medium size negative copper clusters, and extend previous approximate measures of the electron affinity of the corresponding neutral species. The results are in accord with the predictions of the ellipsoidal shell model for monovalent metal clusters. In particular, clusters 8, 20, and 40 (which correspond to spherical shell closings in this simple model) are found to have unusually low electron affinities and large HOMO–LUMO gaps. Subshell closings at 14 and 34 also appear special in this new UPS data.

Fullerene triplet state production and decay: R2PI probes of C60 and C70 in a supersonic beam

Abstract Lifetimes of the lowest triplet state of the two most stable fullerenes, C 60 and C 70 , were measured in a supersonic beam by two-color resonant two-photon ionization. When prepared by intersystem crossing from the singlet manifold, excited at 4.03 eV, these triplet states were found to have lifetimes of 42 and 41 μs, respectively. The energies of these triplet states (1.7 and 1.6 eV, respectively) were measured by photoelectron spectroscopy of the corresponding negative ions.

Carbon ARC Generation of C60

Generation of C 60 at a rate of more than 10 grams per day has been accomplished by operation of a carbon arc in an atmosphere of helium. Optimum yield of 15% was found to occur near 100–200 torr, but yields greater than 3% were found throughout the range between 50 and 760 torr. A model is proposed to explain the observed behavior based on competition between annealing of graphitic sheets to curve so that they minimize dangling bonds, and further rapid growth of these sheets in the gas phase to form giant fullerenes. In agreement with predictions of this model, laser vaporization of graphite targets was found to produce macroscopic quantities of C 60 only when performed in an oven above 1000 C.

Ultraviolet photoelectron spectra of gallium arsenide clusters

Abstract The ultraviolet photoelectron spectra (UPS) of mass-selected negative gallium arsenide cluster ions in the (2–50) atom size range was measured with a photon energy of 7.9 eV. The measured photodetachment thresholds displayed a strong even/odd oscillation through the largest clusters in this range, suggesting the presence of a substantial HOMO—LUMO gap in the corresponding neutral clusters which evolves to the band gap of bulk GaAs crystals.

Vibrational autodetachment spectroscopy of Au−6 : Image‐charge‐bound states of a gold ring

Spectral experiments on mass‐selected negative cluster ions of gold and silver were performed in the wavelength range near the threshold for one‐photon photodetachment of the extra electron. The Au−6 cluster ion displayed a uniquely well resolved spectrum consisting of a progression in a single vibrational mode. Details of this threshold photodetachment spectrum and the associated photoelectron energy distribution suggest an explanation based on autodetachment from totally symmetric vibrational levels of very weakly bound excited electronic state (bound by image charge forces) of the Au−6 cluster in the form of a planar, six‐fold symmetric, gold ring.