Ionization potentials of cobalt-sodium bimetallic clusters (Co _nNa_m)

Cobalt-sodium bimetallic clusters (Co_nNa_m, n = 3-48) were produced by a two independent laser-vaporization method. Ionization potentials of the Co_nNa_m clusters were measured up to m = 3 using a tunable ultraviolet laser combined with a time-of-flight (TOF) mass spectrometer. In general, the ionization potentials decrease monotonically with the number of sodium atoms, and the ionization potentials of Co _nNa_m+1 decrease by 0.2-0.8 eV compared to those of the corresponding Co_nNa_m cluster. However, the amount of IP decrement by the Na doping is relatively large at n ≤ 17 whereas it is small and constant at n ≥ 18. This feature can be explained by a geometric rearrangement; at small n, the Na doping induces a large geometric rearrangement of the cluster, but at large n, the geometric change is small. Reactivity of Co_nNa_m cluster toward H₂ was also measured and the effect of the Na doping was studied. The reactivity also suggests the geometric change induced by the Na doping. Moreover, we examined the anticorrelation between IP and reactivity of the Co_nNa_m clusters, and no anticorrelation between them could be revealed.