The experimental excitation function for the 7α de-excitation of 28 Si nuclei excited to high excitation energies in the collisions of 35 MeV/nucleon 28 Si with 12 C reveals resonance structures. The possibility that these structures may indicate the population of toroidal high-spin isomers such as those predicted by a number of recent theoretical calculations is discussed and the need for further investigations is emphasized.
The excitation function for the 7 alpha de-excitation of 28 Si nuclei excited to high excitation energies in the collisions of 35 MeV/nucleon 28 Si with 12 C reveals resonance structures that may indicate the population of toroidal high-spin isomers such as those predicted by a number of recent theoretical calculations. This interpretation is supported by extended theoretical analyses.
Ligation and decomposition of 1,6-hexanedithiol on copper clusters have been studied by means of temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Copper cluster anions were first made via magnetron sputtering, then size selected and soft landed into a frozen matrix of 1,6-hexandithiol on highly ordered pyrolytic graphite (HOPG) maintained at 100 K. After warming up to 298 K, a combination of TPD and XPS were performed to characterize the newly deposited sample. TPD data shed light upon the adsorption and decomposition pathways of 1,6-hexanedithiol molecules on copper clusters. Based on the TPD data, two different binding motifs are proposed: the dangling motif is with one sulfur atom binding to a copper cluster, and the bidentate motif is with both sulfur atoms binding to a copper cluster. Different decomposition products were observed for each binding motif. A series of hydrogen atom titration experiments were designed to provide further evidence for the proposed decomposition mechanism. XPS measurements at varied temperatures agree well with the TPD profile by confirming the formation of dithiol ligated copper clusters through Cu–S bond formation, and the decomposition of them via C–S bond scission. How well the dithiol ligand can protect the copper clusters from being oxidized is discussed, and the ligand number per cluster is estimated.
Room temperature decomposition and thermal decomposition of dimethyl methylphosphonate (DMMP), a chemical warfare agent (CWA) simulant, on size-selected copper clusters have been studied via combined X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). Cu100 and (CuO)80, which have the same nominal masses, were chosen to present a direct comparison between the reactivity of metallic copper and that of cupric oxide with DMMP. Room temperature XPS results have shown that most of the DMMP molecules decompose completely and reductively into atomic phosphorus on Cu100, while almost all the DMMP molecules are only dissociatively adsorbed on (CuO)80 as methyl methylphosphonate (MMP). XPS and TPD have been carried out to analyze the thermal decomposition of adsorbed DMMP by identifying the surface species after annealing to certain temperatures and the gaseous products evolved during linear temperature ramps, respectively. Methanol, formaldehyde, and methane are the three most significant gaseous products for DMMP decomposition on both Cu100 and (CuO)80. Methanol and formaldehyde, which evolve in the low temperature region, are believed to originate from surface methoxy species. Methanol, formaldehyde, and methane evolved in the high temperature region are related to further decomposition of the phosphorus-containing surface species. A set of methanol-probed TPD experiments have also been carried out, which suggest that methane evolution originates from the methyl group within DMMP instead of the surface methoxy species.
In recent years, zirconium hydroxide powder and zirconium-based metal organic frameworks have found promising applications as chemical warfare agent (CWA) decomposition materials. While bulk zirconium oxide (ZrO2) has proven to be relatively inactive for such purposes, well-controlled fundamental studies investigating the potential CWA decomposition propensity of subnanoscale zirconium oxide, in which undercoordinated metal centers abound, are still severely lacking. Herein, the adsorption and decomposition of the nerve agent simulant dimethyl methylphosphonate (DMMP) on size-selected zirconium oxide trimer, that is, (ZrO2)3, clusters supported on highly oriented pyrolytic graphite (HOPG), have been investigated via the combination of X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption/reaction (TPD/R). XPS measurements acquired for the DMMP-adsorbed, HOPG-supported clusters at a preparation temperature of 298 K, and also after annealing to several successively higher temperatures of 473, 573, and 673 K, elucidated the uptake of DMMP to the (ZrO2)3 clusters, with one DMMP molecule adsorbed per cluster and virtually no thermal molecular desorption observed up to 673 K. These measurements also showed dissociative adsorption of DMMP at room temperature on some clusters, likely via scission of a P–OCH3 bond in DMMP, with further decomposition accompanying an increase in temperature above 473 K. TPD/R experiments showed the evolution of methanol as a major reaction product via two distinct pathways, with desorption peaks centered around 410 and 575 K. Evolution of dimethyl ether and formaldehyde as minor reaction products was also observed with desorption peaks centered around 560 and 620 K, respectively. A second TPD/R cycle following cluster-induced DMMP decomposition resulted in no detected decomposition chemistry, showing DMMP decomposition on the (ZrO2)3 clusters to be stochiometric and non-catalytic, whereby the remaining P-containing species poisoned the clusters.
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