Fragmentation of size-selected Xe clusters: Why does the monomer ion channel dominate the Xen and KrnKrn ionization?

The fragmentation of the small Xenn=2−5n=2−5 clusters following 70 eV electron impact ionization has been investigated in a size selective experiment and simulated using non-adiabatic dynamics. The experimental results show that the clusters strongly fragment to yield monomer Xe+ (more than 90%) and dimer Xe2+Xe2+ fragments (the remaining few percent). Trimer Xe3+Xe3+ fragments first occur from the neutral pentamers Xe5 in a very low yield of approximately 0.3%. The present results are compared with the previous ones for Kr and Ar clusters. It is shown that the Xe and Kr clusters exhibit a qualitatively similar behavior with a strong propensity for monomer fragments, while in the Ar case dimers prevail. The theoretical calculations also reveal a strong fragmentation to the dimer and monomer fragments. However, the dimer Rg2+Rg2+ is predicted to be the major product for all rare gases (Rg ≡≡ Ar, Kr, Xe). Possible reasons for the discrepancy between theory and experiment are discussed.