PHOTOELECTRON IMAGING OF METAL CLUSTERS AND THEORETICAL STUDIES OF METALLOCARBOHEDRYNES
Open Access
- Author:
- Sobhy, Mohamed Abdelmaboud
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 28, 2007
- Committee Members:
- Albert Welford Castleman Jr., Committee Chair/Co-Chair
James Bernhard Anderson, Committee Member
Thomas E Mallouk, Committee Member
Robert John Santoro, Committee Member
Jorge Osvaldo Sofo, Committee Member - Keywords:
- metallocarbohedrynes
photoelectron spectroscopy
photoelectron imaging
cluster anions - Abstract:
- Abstract Clusters have been the subject of extensive experimental and theoretical research because of their fundamental and practical importance as an intermediate state of matter between atoms and their bulk counterpart. The present work is comprised of two main phases. First, the experimental phase is concerned with the study of the electronic structure of negatively charged metal clusters through photodetachment and photoelectron imaging. Second, the theoretical phase is devoted to the investigation of the properties of a particularly stable class of transition-metal carbide clusters known as the Met-Cars. The design and implementation of a new velocity-map photoelectron imaging spectrometer for mass-selected negative ions are discussed in detail in the experimental setup. The photoelectron imaging technique has proved to be of great value in investigating the electronic structure of cluster anions in the gas-phase. Among the major advantages of the imaging technique over regular photoelectron spectroscopy techniques is the direct determination of the angular momentum distributions of the electrons and thus the partial-wave function composition of the orbitals through which the detachment occurs. The experimental study is mainly comprised of the photoelectron imaging investigations of noble-metal ions attached to multiple ligands, metal ions solvated in small ammonia clusters and bimetallic clusters. The results of photoelectron imaging experiments of Cu and Ag mono- and diamine anions are reported. The photoelectron images were recorded at two photon energies, 800 and 527 nm. The vertical detachment energies of CuNH2- and AgNH2- are lower than those of the respective atomic metal ion and are measured to be 1.11¡À0.05 and 1.23¡À0.05 eV, respectively. By contrast, the electron detachment energies for Cu(NH2)2- and Ag(NH2)2- are higher than those of the corresponding metal ion and are determined to be 1.48¡À0.05 and 1.85¡À0.05 eV, respectively. Energy-dependent photoelectron anisotropy parameters are also reported. The photodetachment of the Cu and Ag mono- and diamine anions exhibit a cos2 ¦È angular dependence relative to the direction of the laser polarization. The nature of the chemical bonding and the symmetry of the highest occupied molecular orbitals (HOMO) are discussed in relevance to the measured anisotropy parameters. Photoelectron imaging study of bismuth and lead metal ions associated with small ammonia clusters is presented as an example of solvated metal-ion systems. The clusters show similar electronic transitions to the metal-ion chromophore; however, the spectral features are shifted towards the higher electron binding energy as expected in the case of solvation. The remarkable feature in the photoelectron images of the solvated clusters is the significant change of the angular distributions of the electronic bands compared to those of the respective metal ion. The stronger binding between the solvent molecules and the core metal ion in the case of solvated lead anions results in changes in the character of the highest occupied molecular orbital of the metal-ion, which correspondingly affects the angular distributions of the observed electronic bands. Main-group metal clusters are of wide interest from both fundamental and technological aspects. The results of photoelectron imaging experiments on small semiconductor-like clusters of gallium and bismuth, groups (III, V) elements, are presented. Also, the results of photoelectron imaging of small Pbn- clusters n¡Ü4 are reported for the first time. The theoretical phase of this work is concerned with the investigating of the atomic structure, electronic, magnetic and vibrational properties of the different Ti8C12 metallocarbohedryne clusters. The density-functional theory (DFT) calculations are performed with the all-electron projector augmented-wave method and generalized gradient approximation for the exchange-correlation functional. The seven low-energy isomers of the Ti8C12 metallocarbohedrynes are studied using spin-polarized DFT, where a correlation is found to exist between the number of rotated carbon dimers and the cohesive energy of the structure. The electronic density of states (eDOS) show that C3v, D*3d, and D3d isomers are spin polarized. The partial eDOS shows that, depending on the dimer orientation, carbon atoms and a subgroup of the metal atoms form a covalent framework while other metal atoms are bonded to this framework more ionically. This picture is further supported by the charge density of the different structures, where the Ti atoms with higher charge density show less contribution to the covalent bonding of the Ti-C framework. The vibrational spectra of the different structures are calculated using the frozen-vibration method. Also, the vibrational spectra of the C3v and C2v structures are calculated using molecular-dynamics simulations at two different temperatures. The results of the simulations demonstrate the local stability of the structures beyond the harmonic limit explored by the frozen-vibration method. The other part of the theoretical studies is an investigation for the effect of the substitution of transition-metal atoms on the reactivity of the resulting metallocarbohedryne cluster. The substitution of two Ti atoms with other transition-metals with fewer valence electrons as Scandium or Yttrium results in more stable metallocarbohedryne clusters with less chemical reactivity. This is clearly seen from the electronic density of states of the substituted metallocarbohedrynes compared to those of the ground state Ti8C12 cluster of the C3v symmetry. These preliminary results can be regarded as a stepping stone for the formation of an extended lattice from the metallocarbohedryne clusters.