Paul S. Bechthold

Electronic shells or molecular orbitals: Photoelectron spectra of Ag−n clusters

Photoelectron spectra of Ag−n clusters with n=1–21 recorded at different photon energies (hν=4.025, 4.66, 5.0, and 6.424 eV) are presented. Various features in the spectra of Ag−2–Ag−9 can be assigned to electronic transitions predicted from quantum chemical ab initio calculations. While this comparison with the quantum chemical calculations yields a detailed and quantitative understanding of the electronic structure of each individual cluster, a discussion in terms of the shell model is able to explain trends and dominant patterns in the entire series of spectra up to Ag−21.

A comparison of photoelectron spectroscopy and two‐photon ionization spectroscopy: Excited states of Au2, Au3, and Au4

Photoelectron spectra of Au−n with n=2–4 are reported. Due to the relatively high photon energy used in our experiment (hν=6.424 eV) and the energy resolution of about 50 meV, various transitions into excited states of the neutral clusters are resolved. It is demonstrated that photoelectron spectra can serve as a map of the electronic states of a cluster, while the high resolution of the resonant two‐photon ionization (R2PI) method gains information about the symmetry of the states. The comparison with similar data of Ag−n clusters indicates the influence of relativistic effects and the large spin–orbit splitting for Au.

Photoelectron spectra of Nbn− clusters: Correlation between electronic structure and hydrogen chemisorption

Photoelectron spectra of mass-separated Nbn− clusters reveal an even/odd alternation for n=6–17, indicating a closed electronic shell of the neutral even-numbered clusters. The HOMO–LUMO gap of Nb8, Nb10, and Nb16 is found to be larger than that of the other even-numbered clusters, which correlates with the low H2 reactivities of these species. The spectrum of Nb15− is different from all other clusters in this size range, which might be an indication for a geometric bcc shell closing. The influence of the electronic structure of the clusters on the reactivity is discussed.

La@C60: A metallic endohedral fullerene

We have produced an endohedrally doped fullerene that shows a metal-like density of states at the Fermi level. Individual La@C60 clusters deposited onto graphite exhibit a zero band gap as observed by scanning tunneling spectroscopy on single clusters at room temperature. Moreover, we find that an isolated La@C60 cluster on graphite shows a reversible opening of a band gap at a transition temperature of ∼28 K. The transition is associated with a freezing of the vibrational motion of the La atom inside the fullerene cage. The metallic behavior of La@C60 is attributed to the presence of a dynamical dipole in the single cluster.

Size-dependent hot-electron dynamics in small Pdn−-clusters

Using time-resolved photoelectron spectroscopy we show that electron relaxation processes via inelastic electron–electron scattering are efficient energy dissipation channels not only in bulk metals but also in extremely small transition metal clusters. The photoelectron spectra of optically excited Pd3−, Pd4−, and Pd7− reveal effective electron relaxation times of less than 100 fs. Moreover the relaxation times vary with cluster size. In comparison to simple metal clusters the bulklike inelastic scattering rates in open d-shell transition metal clusters are attributed to the larger valence electron level density. An energy transfer to the vibrational degrees of freedom occurs within 10 ps.

Delocalized electronic states in small clusters : comparison of Nan, Cun, Agn, and Aun clusters

Photoelectron spectra of Na; , Cu; , Ag; , and Au; clusters reveal the electronic structure of these particles. The experimental results are compared to the predictions of quantum chemical calculations and of the shell model. The spectra of Ag; allow for a stringent test of both approaches, because most of the observed features are assigned to s-derived orbitals and they also display much sharper features than the alkali data. A qualitative equivalence of the electronic shell model with high-level quantum chemical calculations in terms of the symmetries of the involved single particle orbitals is found for the delocalized states derived from the atomic s-electrons.

Chemisorption of benzene on metal dimer anions: A study by photoelectron detachment spectroscopy

Photoelectron detachment spectra of M2(C6H6)− (M=Pt, Pd, Pb) have been measured in the gas phase using photon energies of a Nd:YAG laser. The vibrationally resolved ground state transition from the anion to the neutral reveals an adiabatic electron affinity of (2.01±0.05) eV and (0.88±0.05) eV for Pt2(C6H6) and Pd2(C6H6), respectively. A ground state vibrational energy of (24.2±1) meV has been resolved for Pt2(C6H6). The corresponding vibrational energy of Pt2(C6H6)− amounts to (19.0±1.0) meV. The ground state vibrational energies of Pd2(C6H6) and Pd2(C6H6)− are (20.3±1.0) meV and (18.0±2.0) meV, respectively. The small vibrational frequencies suggest a perpendicular coordination (C6v-symmetry) of the benzene-adsorbed transition metal dimers. Pb2, on the other hand, is bound parallel to the benzene plane (C2v-symmetry). A closed shell ground state electron configuration is postulated for Pb2(C6H6) in contrast to the triplet ground state of unreacted Pb2. The vertical electron affinity of Pb2(C6H6) is (1.9...

Mass spectra of metal-doped carbon and fullerene clusters

We present a systematic study of the abundancies of metal-doped cluster cations MxCn+ (M=Ca,Sc, Y, La, Ce, Gd; x=1,2) produced in a laser vaporization source. The mass spectra of MCn+ and M2Cn+ reveal the onset of endohedrally doped fullerenes from which the smallest possible cage sizes can be deduced. Y2Cn and La2Cn show a distinct transition from alternation odd–even to even–odd at n=69 and n=71, respectively, which can be explained by a substantial change in the doped fullerene structure, i.e., from networked to bi-endohedral systems. A metal induced cluster formation pathway is briefly discussed.

VIBRATIONALLY RESOLVED PHOTOELECTRON SPECTRA OF ANNEALED

Photoelectron spectra of annealed carbon-cluster anions n=5–70, are presented which show vibrational fine structure. From the vibrational frequencies, theoretical results, and ion mobility measurements, the geometry of the clusters can be determined: clusters with n=5, 7, and 9 are linear chains; n=10, 12, 14, 16, and 18 build monocyclic rings; for n=20, 24, and 28 the vibrational features correspond best to bicyclic rings; and for even n>30 fullerenes are found. In contrast to carbon, the mass distribution of silicon-cluster anions (up to n≈30) does not change dramatically upon annealing. An enhanced abundance of typical fragmentation products with n=4, 6, 7, and 10 is found. The vibrational fine structure for agrees with that of a pentagonal bipyramid.