Highly dispersed, unsupported Pt, Pd, Rh, and bimetallic R h P t and P t P d clusters were produced by laser ablation using a KrF excimer laser operating at power densities up to 1.9 GW/cm2. By the evaporation of two metals simultaneously, bimetallic clusters with variable stoichiometric composition were obtained, depending on buffer gas, buffer gas pressure, and the deposited laser energy. The ablation was carried out in a glass cell under a continuous gas flow or in a UHV system with a static gas atmosphere. The pressure ranged from 5 to 100 Torr. Emission spectra of the plasma plume were taken for detection of ions and highly excited atoms in order to study the temporal evolution of the ablation process. The expansion of the plasma plume was imaged with a CCD camera at different times after the laser pulse. In the first few nanoseconds, the expansion velocity reached values up to 3.5 x lo6 c d s . Characterization of the metal clusters by XPS showed that samples produced in the glass reactor are highly contaminated with carbon up to 60 atom %. However, by using a UHV system, the carbon content was reduced to 14 atom %. Alloy formation in bimetallic Rh/Pt particles with a l-to-1 atomic ratio of rhodium to platinum was verified by EXAFS spectroscopy. The coordination environment around platinum consists of both platinum and rhodium neighbors, 56 and 44 atom %, respectively, while rhodium appears to be coordinated predominantly to other rhodium atoms. These results are consistent with a bimetallic Rh/Pt cluster model whereby the cluster core is composed of rhodium atoms and the outer surface is formed by platinum atoms. Particle size and size distribution were analyzed by XRD, TEM, and BET isotherms. The particle diameters ranged from 2 to 20 nm, showing a linear dependence on buffer gas pressure and laser energy.

Production and Characterization of Noble Metal Clusters by LaserAblation.-Highly dispersed unsupported clusters of Pt, Pd, and Rh, as well as bimetallic Rh/Pt and Pt/Pd clusters are produced by laser ablation ( KrF excimer laser, ≤ 1.9 GW/cm2) carried out either in a glass cell under a continuous gas flow or in an UHV system with a static gas atmosphere (5-100 torr). The clusters exhibit particle sizes of 2-20 nm with a narrow size distribution. The particle size can be controlled by varying the laser energy, buffer gas, and buffer gas pressure. The samples produced in the glass reactor contain up to 60 at% carbon. The carbon contamination can be reduced to 14 at% by using the UHV system. The simultaneous ablation of two metal targets can be used to produce bimetallic clusters with variable stoichiometry. The bimetallic powders consist of nm-sized alloy clusters. -(LILLICH, H.; WOLFRUM, J.; ZUMBACH, V.; ALEANDRI, L. E.; JONES, D. J.; ROZIERE, J.; ALBERS, P.; SEIBOLD, K.; FREUND, A.; J.