C.L. Pettiette

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.

Ultraviolet photoelectron spectroscopy of semiconductor clusters: Silicon and germanium

Abstract Ultraviolet photoelectron spectra (UPS) are reported here for semiconductor clusters prepared in a supersonic beam. Using a new magnetically focused time-of-flight photoelectron spectrometer, UPS spectra were obtained for mass-selected negative cluster ions of silicon and germanium in the 3-12-atom size range. An ArF excimer laser (6.4 eV) was used for photodetachment, enabling the first 3–4 eV of the valence band structure of the clusters to be probed. With few exceptions, the UPS data for corresponding clusters of the two semiconductors were remarkably similar. The spectra suggest that clusters 4, 6, 7, and 10 of silicon and 4, 6 and 7 of germanium are closed-shell species with band gaps of 1 to 1.5 eV.

Magnetic time‐of‐flight photoelectron spectrometer for mass‐selected negative cluster ions

Design considerations and initial results are presented for a new type of time‐of‐flight photoelectron spectrometer which is particularly suited to the study of cold metal and semiconductor cluster anions prepared in a supersonic molecular beam. The desired cluster is extracted from the molecular beam, mass‐selected after an initial time‐of‐flight, and decelerated as it enters the photoelectron spectrometer. Photoelectrons ejected from the cluster by an ArF excimer laser are collected with >98% efficiency in an intense pulsed magnetic field of carefully controlled divergence. This divergent field parallelizes the photoelectron trajectories and maps smoothly onto a low, uniform magnetic field which guides the electrons along a 234‐cm flight tube leading to a microchannel‐plate detector. The strong magnetic fields and simple, open design provide excellent rejection of stray photoelectrons in a clean, ultrahigh‐vacuum environment. The UPS spectrum of Si20− is given as an example.

Metal cluster ion photofragmentation

Photofragmentation studies are described for mass‐selected transition metal cluster ions cooled in a supersonic expansion. Examples of the spectral utility of this technique are presented in the cases of Ni+2 and Nb+2 for which well resolved vibronic bands have been recorded by monitoring fragment yield as a function of dissociation laser wavelength. For larger, more complicated metal clusters the absorption spectrum is far too congested and perturbed for analysis, but photodissociation can reveal the thresholds and time scales of various fragmentation pathways. The order of the absorption event leading to dissociation is found from the extent of fragmentation as a function of laser fluence. Using this method, the one‐photon dissociation threshold of Fe+2 is found to lie between 2.43 and 2.92 eV, which determines the binding energy of this transition metal dimer cation. Combined with previous ionization potential measurements this places the binding energy of the Fe2 neutral between 0.83 and 1.32 eV. Simi...

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.

Ups of negative aluminum clusters

Abstract Ultraviolet photoelectron spectroscopy (UPS) of negative aluminum clusters in the size range of 3–32 atoms is presented. The clusters are prepared in a supersonic beam by laser vaporization, and an ArF excimer laser (6.42 eV) is used for photodetachment. The electron affinities (EA) of the neutral clusters are peaked at cluster sizes of 6, 13, 19, and 23, which is consistent with a shell model of electronic structure. Additionally, a size-dependent increase in the photoelectron yield at 5 eV binding energy in the UPS data can be interpreted as the merging of the 3s and 3p valence bands.

Photodetachment studies of metal clusters: electron affinity measurements for Cux

Laser photodetachment studies have been performed on silver and copper cluster negative ion beams extracted from a supersonic metal cluster source. This source involves the use of laser vaporization within a pulsed nozzle to prepare the neutral cluster jet, followed by ArF excimer irradiation of the nozzle orifice as the copper clusters emerge into the expansion. Photoelectrons ejected from the aluminum nozzle by the excimer radiation are entrained in the supersonic flow and attach to the neutral copper clusters, producing negative ions which stabilize and extensively cool in the subsequent supersonic expansion. Laser photolysis of mass‐selected negative copper cluster ions extracted from this source reveals efficient photodetachment to produce a free electron in the absence of measurable fragmentation. The laser fluence dependence of the photodetachment process at different photon energy permits a rough experimental determination of the electron affinities of copper metal clusters as a function of cluste...

Charge dependence of chemisorption patterns for transition metal clusters

A method is presented for the measurement of the chemisorptive reactivity of transition metal cluster ions at near room temperature. Similar to a technique introduced previously for neutral clusters [Rev. Sci. Instrum. 56, 2123 (1985)], this cluster ion method utilizes a fast‐flow reactor attached to a supersonic, laser vaporization metal cluster source, followed by time‐of‐flight mass spectral analysis of the cluster ions as a function of reactant concentration. Results are presented for clusters of cobalt and niobium in the 1–22 atom size range for their chemisorptive reactions with CO, CO2, and N2. Both Nb+n and Co+n clusters displayed chemical reactivity that is remarkably similar to that of the corresponding neutral clusters. For both charge states of each metal, CO was found to chemisorb with a rate which varied in a slow, monotonically increasing fashion with cluster size. The chemisorption rate of N2 and CO2, on the other hand, was found to be significantly slower than that of CO and sharply depen...

Formation and photodetachment of cold metal cluster negative ions

A general method is described for the formation of cold metal cluser negative ion beams which serve as excellent sources for photodetachment experiments. The method involves the pulsed laser vaporization of a metal target at the throat of a pulsed supersonic helium expansion. By the optimization of source conditions, intense beams (greater than 105 ions/pulse) of both positive and negative ions are produced routinely. Ionization of the metal cluster molecules, either during vaporization or by irradiation with 193 nm light, occurs prior to supersonic expansion and produces a cold plasma entrained in the neural flow that is renitent to stray electric and magnetic fields, unlike photoions produced in the collisionless downstream molecular beam. The enhancement of the negative ion flux by 193 nm irradiation is believed to be evidence for efficient electron attachment of low energy photoelectrons generated in the nozzle region. This attachment process, however, is apparently not effective for molecules contain...