Guided ion-beam studies of the kinetic-energy-dependent reactions of Co_n~+(n=2-16)with D_2:Cobalt cluster-deuteride bond energies

摘要

The kinetic-energy-dependent cross sections for the reactions of Co_n~+(n = 2-16)with D_2 are measured as a function of kinetic energy over a range of 0-8 eV in a guided ion-beam tandem mass spectrometer.The observed products are Co_n D~+ for all clusters and Co_nD_2~+ for n=4,5,9-16.Reactions for the formation of Co_nD~+(n = 2-16)and Co_9D_2~+ are observed to exhibit thresholds,whereas cross sections for the formation of Co_nD_2~+(n=4,5,10-16)exhibit exothermic reaction behavior.The Co_n~+-D bond energies as a function of cluster size are derived from the threshold analysis of the kinetic-energy dependence of the endothermic reactions and are compared to previously determined metal-metal bond energies,D_0(Co_n~+-Co).The bond energies of Co_n~+-D generally increase as the cluster size increases,and roughly parallel those for Co_n~+-Co for clusters n>=4.These trends are explained in terms of electronic and geometric structures for the Co_n~+ clusters.The bond energies of Co_n~+-D for larger clusters(n>= 10)are found to be very close to the value for chemisorption of atomic hydrogen on bulk-phase cobalt.The rate constants for D_2 chemisorption on the cationic clusters are compared with the results from previous work on cationic and neutral cobalt clusters.