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Copper Clusters - The initial test and characterization of the source
Copper Dimer
Once the source had been developed to the point of being reliable and stable, copper dimer was chosen to be studied in an effort to characterize and test the source. Working with D. Powers, S. Hansen and C. Puiu, we investigated the absorption spectrum of Cu2 in the 4480-4660-A region by using two-color resonant two-photon ionization. In addition to the known B <-- X band system, a new system was found in the same spectral range which was designated as the C state.
From the information obtained during this experiment the Cu cluster beam was found to have translational, rotational, and vibrational temperatures of <5, <10, and 50-100 K, respectively. Showing that the extensive cooling normally observed in a supersonic expansion was preserved and that laser vaporization was a viable technique for producing cooled cluster beams.
Note: C. Puiu's postdoctoral term ended and D. Michalopoulos having completed his Ph.D. with Dr. Elliot R. Bernstein at Colorado State University, joined the group as her replacement.
Working with D. Powers, S. Hansen and D. Michalopoulos, we carried out an extensive survey of the ultraviolet absorption spectrum of the copper dimer. Resonance two-photon ionization (R2PI) with mass selective detection allowed for the detection of an additional five new electronic band systems in the region between 2690 and 3200 A, for each of the three naturally occurring isotopic forms of Cu2.
In the process of scanning the R2PI spectrum of these new electronic states, the ionization potential of the copper dimer was determined to be 7.894 eV.
Bracketing Ionization Potentials
Copper Clusters ranging in size from 1 to 29 atoms were produced and using a number of fixed frequency outputs of an excimer laser, the threshold behavior of the photoionization crosss section was monitored as a function of cluster size. The two main observations are:
1) There is a pronounced even/odd alternation present up to the largest cluster studied ( 29 atoms).
2) The ionization potential has not converged to the bulk for a cluster containing 29 copper atoms.
The 7.9 eV photon energy of the F2 excimer laser was found to be above the ionization potential of all dusters, and the photoion mass spectrum thus produced showed the copper cluster concentration in the beam to follow a monotonically decreasing function of cluster size.
The 6.4 eV ArF excimer laser photon energy was found to be above the photoionization threshold of clusters with three or more atoms in the case of odd-numbered clusters, but only for clusters with eight or more atoms for even-numbered clusters. Extending out to clusters as large as 29 atoms laser photoionization at 6.4eV produced a time-of-flight mass distribution with a pronounced even/odd alternation in cluster photoion intensity. This alternation in ionization threshold behavior was attributed to an even/odd alternation in the electronic structure of the copper clusters with the highest occupied moleculer orbital (HOMO) of the even clusters being considerably more strongly bonding than it is in the clusters with an odd number of copper atoms.
The 4.98 eV photon energy of the KrF excimer laser was found to be below the ionization threshold of all clusters in the 1 to 29 atom range.