An anomalous drop in the intensity distribution of cluster ions Ar lX +, X = Xe, N 2, CO 2 and CH 3OH

Abstract

Intensity distribution in the mass spectra of binary cluster ions Ar l X +, where X = Xe, N 2, CO 2 and CH 3OH, has been investigated by producing the ions through the exchange reaction of Ar clusters with X and subsequent electron-impact ionization. An abrupt intensity drop is found between l = 12 and 13 in the sequence of Ar l X + peaks for each X. The magnitude of the drop becomes larger as the ionization potential of X decreases. From these observations, it is inferred that the binary cluster ion settles down to a structure in which 12 Ar atoms are disposed around the ion X + making a stable “shell”. The binding energy of Ar l X + is calculated for spherical X by assuming the ArAr and Ar—X + interactions, and it is shown that the stabilization of the ion due to successive clustering is larger for l ⩽ 12 than for l ⩾ 13. Thus the observed intensity anomaly is explainable by a supposition that the population of the ions is proportional to the energy difference between Ar l−1 X + and Ar l X +.