Electronic and Magnetic Properties of Metal-Atomic Clusters
Abstract
The electronic and magnetic properties of metal -atomic clusters have been studied using cluster beam methods, including mass spectrometry, photoionization spectroscopy and magnetic deflection measurements. The ionization potentials of cold Na_ {rm N} clusters (N = 19-80), generated in a new, liquid nitrogen cooled laser ablation source, are found to be in excellent agreement with the spherical jellium background model, when spheroidal perturbations are included. In certain size ranges, a strong even-odd variation is observed in the ionization potentials (as a function of N), an electron pairing effect that cannot be explained by existing theories. Measured photoionization thresholds of Group IIIA metal clusters, Al_{rm N} and In_{rm N} (N = 1-70), exhibit similar electronic shell structure, with deviations from the spherical jellium model resulting from two separate effects. In the small size range, N < 40 atoms, the electronic s and p bands (derived from the corresponding atomic orbitals) hybridize, causing higher ionization potentials than predicted by the spherical models. Deviations in the ordering of levels in the electronic shell structure, is interpreted as an effect of symmetry breaking, the Z = 3 ionic lattice is a strong perturbation on the spherical potential assumed in the jellium model. Mass spectrometry on larger Al_ {rm N} clusters, to N = 430 atoms, reveal a continued quantized electronic level structure for n_{rm e} = 100 -1300 electrons. Calculations based on the spherical jellium model show that this level structure, continued to unexpectedly large cluster sizes, is due to a periodic (in E ^{1/2} or momentum) modulation of the electronic density of states. The deviations in level ordering observed for smaller clusters, persist to N = 250 atoms. The level structure for N > 250 atoms is in excellent agreement with the spherical jellium prediction. Magnetic deflection measurements by the Stern -Gerlach method, reveal a paramagnetic response for Fe _{rm N} (N = 14-70) and Co _{rm N} (N = 9-82) clusters, while no deflection is observed for Al _{rm N} clusters (N = 15 -70). The magnetic moment per atom of Fe_ {rm N} and Co_{ rm N} increases approximately linearly with N, for N > 20, but remains well below the bulk values. As has been previously reported, the magnetic moments exhibit an anomalous temperature dependence, the measured magnetic moments increase with increasing cluster source temperature (110 K and 295 K source temperatures), but remain constant when the source temperature is increased to 395 K. Finally, the effect of electric field ionization on time-of-flight mass spectra is investigated, through observations of the ionization dynamics of the Al atom in an external electric field.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- February 1991
- Bibcode:
- 1991PhDT........61P
- Keywords:
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- SUPERPARAMAGNETISM;
- MOLECULAR BEAM;
- CLUSTER BEAM;
- Physics: Condensed Matter; Physics: Molecular; Chemistry: Physical