Decomposition of ionized clusters in short supersonic pulses of argon

Abstract

The decay rate of argon cluster ions produced by electron impact ionization has been studied in short supersonic pulses (≈0.5 ms) as a function of the stagnation pressure (1–7 atm) at different distances from the nozzle and at different electron energies in the flight-time range of 12–26 µs. Divergence of the cluster flow, caused by decomposition of the clusters, was used for these measurements. The mass distribution of the cluster ions has been studied by the retarding potential method. It has been found that the pulses consist of two fractions of clusters, light and heavy, which are also differently localized in space. The cluster stability is not homogeneous along the pulses as a consequence of the nonhomogeneous mass distribution. The stability of the cluster ions reveals a great variety of dependences on the stagnation pressure and on the flight-time when the distance from the nozzle and the electron energy are changed. It has been shown that, in the studied region of large clusters, the stability of the cluster ions is mainly an increasing function of the size, in contrast to the small cluster region. The varied behavior of the cluster ion stability is explained by competition between two contributions to the stability: sublimation energy and the internal temperature of the cluster ions, depending on the experimental conditions.