Ori Cheshnovsky

Ultraviolet photoelectron spectra of coinage metal clusters

Ultraviolet photoelectron spectra (UPS) were recorded for mass‐selected negative clusters of copper (1–411 atoms), silver (1–60 atoms), and gold (1–233 atoms), using photodetachment lasers at 6.4 and 7.9 eV photon energy. The results provide a direct estimate of the vertical electron affinity (EA) of these clusters and information on the evolution of the d bands of copper and gold as a function of cluster size. The large even/odd alternation of EA in small clusters of these metals in earlier work is found to largely disappear as the cluster size exceeds 40 atoms. The ellipsoidal shell model is shown to be consistent with the observed EA behavior of all three metals, the predicted spherical shell closing at cluster 58 being evident for silver and gold. The UPS data show a smooth evolution of the d band toward that of the bulk metal.

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.

PHOTOELECTRON SPECTROSCOPY OF CL-, BR-, AND I- SOLVATED IN WATER CLUSTERS

We present the photoelectron‐spectra of Cl−, Br−, and I−, solvated in water clusters‐(H2O)n, where n is 1–7, 1–16, and 1–60, respectively, taken with 7.1 eV photon energy. The vertical binding energies of the solvated anions are used to extract the solvent electrostatic stabilization energies of the anion. The photoelectron spectra of the solvated I− indicate the formation of the first solvation layer with a coordination number of six. Ab initio calculations support solvation shell closure at n=6. This conclusion is not born‐out by current molecular dynamics calculations. These calculations favor structures with a surface solvated anion (coordination number of 3–4) and reproduce (within 0.2 eV) our vertical binding energies. The fitting of the experimental binding energies of large I−(H2O)n to the models of classical electrostatic solvation is consistent with surface solvation. In the size range n=34–40 we have detected special cluster structures, with very low electrostatic stabilization.

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.

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.

Detection of heating in current-carrying molecular junctions by Raman scattering

As the scaling of electronic components continues, local heating will have an increasing influence on the stability and performance of nanoscale electronic devices. In particular, the low heat capacity of molecular junctions means that it will be essential to understand local heating and heat conduction in these junctions. Here we report a method for directly monitoring the effective temperature of current-carrying junctions with surface enhanced Raman spectroscopy (SERS) that involves measuring both the Stokes and anti-Stokes components of the Raman scattering. All the Raman-active modes in our system show similar heating as a function of bias at room temperature, which suggests fast vibrational relaxation processes inside the junctions. These results demonstrate the power of direct spectroscopic probing of heating and cooling processes in nanostructures.

Proton transfer in neutral gas‐phase clusters: α‐Naphthol⋅(NH3)n

Efficient excited‐state proton transfer in neutral acid–base clusters α‐naphthol⋅Bn has been detected and studied by a combination of laser spectroscopic techniques (resonant two‐photon ionization, fluorescence excitation, and emission spectroscopy). S1 state proton transfer was observed for B=NH3 and n≥4, as evidenced by several criteria: (a) large red shift and substantial broadening of the R2PI spectra of the n≥4 clusters relative to those of the bare α‐naphthol and smaller clusters; (b) very large Stokes shift (∼8000 cm−1) of the emission spectra of the n≥4 clusters; (c) complete broadening of the fluorescence emission band for the n≥4 clusters; and (d) a striking similarity of the emission band position and width of the latter spectra to the emission spectrum of the α‐naphtholate anion in basic aqueous solution. No proton‐transfer reaction was observed for small solvent clusters with B=NH3 and n≤3, nor for any of the pair complexes studied, which involve a single base partner [B=triethylamine, 3‐dime...

Photoelectron spectroscopy of the `missing' hydrated electron clusters (H2O)−n, n=3, 5, 8 and 9: Isomers and continuity with the dominant clusters n=6, 7 and ⩾11

Abstract We report photoelectron spectra for the small anionic water clusters, W − n ( n =2–11, excluding the tetramer), to explore the nature of the trace species surrounding the intense parents appearing at magic numbers n =2, 6, 7 and ⩾11. Two isomeric forms are observed for most of these anions, which are distinguishable by their vertical electron detachment energies (VDE). While the band corresponding to the more strongly bound form continuously evolves from the band characteristic of the larger ( n ⩾11) clusters, the smaller clusters display a dramatically smaller sequential solvation shift, resulting in a sharp break in the cluster radius (∝ n 1/3 ) dependence of the VDEs.